Proceedings of the American Society for Composites — Thirty-third Technical Conference
3D Progressive Failure Modeling of Drop-Weight Impact on Composite Laminates
Composite laminates are susceptible to out-of-plane impact loads due to the lack of reinforcement in the through-thickness direction. Unlike the localized damage induced by a high velocity impact where the incident energy is dissipated near a contact area, low velocity impact damage involves multiple failure mechanisms such as matrix cracking, fiber breakage, and widespread interface delaminations. Depending on the extent of damage, significant reduction in the load-bearing capability of the structure has been observed. The prediction of composite impact damage resistance by a reliable progressive damage analysis tool is essential to reduce intensive and expensive certification tests at structural level. In this work, an enhanced explicit 3D damage model is implemented via VUMAT in Abaqus to perform a dropweight impact simulation of a [454/04/-454/904]s Hexply AS4/8552 composite laminate. The impact-induced damage and its extent are captured by a 3D Continuum Damage Model (CDM) coupled with an energy driven failure mechanism. The developed module provides a unified solution process for the impact response prediction followed by the residual strength prediction under compression within an explicit solver. Two examples are selected to demonstrate the capability of the progressive failure analysis under dynamic and static loading: 1) a drop-weight test; and 2) an open-hole tension test. Numerical predictions from the developed VUMAT are compared with the test data and predictions using the open source CompDam code developed by NASA.
Low Velocity Impact Simulation of CFRP Laminates Considering Microscopic Damage Interaction
This study is intended to be the basis for developing more advanced strategy of finite element (FE) modeling for low velocity impact (LVI) simulation. In this study, the induction of delamination by matrix crack was numerically observed by microscopic analysis of a representative unit cell (RUC) model. The RUC model implements intralaminar and interlaminar cohesive elements for describing matrix crack and delamination, respectively. The induction under various situations were appropriately observed by localization analyses; implement periodic boundary conditions on the RUC model, and apply macroscopic strain and curvature predicted by LVI simulation. Periodic boundary conditions were formulated based on the “key degrees of freedomâ€ method in order to apply macroscopic quantities to the RUC model efficiently. Intralaminar cohesive elements were degraded for reproducing matrix crack in the LVI simulation prior to the localization analysis. Matrix crack and delamination predicted by the analyses were qualitatively in good agreement with those in the impacted specimen observed by X-ray CT. Therefore, it can be expected that the interaction between matrix crack and delamination will be derived numerically.
Prediction of Delamination Area of Laminated Composite Under Low Velocity Impact Based on Experimentally Validated Finite Element Modeling and Machine Learning Methods
Laminated composite materials are usually vulnerable to impact loading. Low velocity impact (LVI) events can cause extensive internal damage including fiber breakage, matrix splitting and delamination. The internal damage is detrimental to the post-impact response of the composite material. Predicting internal damage due to LVI numerically with sufficient accuracy and detail tends to take several days on computer clusters with tens of CPUs. This paper aims at establishing a predictive model for the macroscopic extent of delamination of a composite subjected to LVI based on an FE model and an artificial neural network (ANN), which is a machine arameters. The input for the ANN model is the stacking sequence of the laminate and the output is the total area of delaminations in the composite. First, an experimentally validated numerical model for LVI based on the finite element method (FEM) is established. Then, a database is built using the FEM model for various stacking sequences. Finally, an ANN model is trained based on the database and a test set is predicted with the ANN and compared against the results obtained with the FEM model. With the help of the Experiment-FEM-ANN technology, predictions of LVI damage, and a parametric study to the full extent, and optimization of the design of a laminated composite against LVI, are all made possible.
Deployable Structures Constructed from Composite Origami
The goal of this paper is to construct lightweight, deployable, yet stiff structures made from composite origami. In particular, the deployable origami structures manufactured in this study are based on the Tachi-Miura-Polyhedron (TMP). The TMP is a bellows-like structure that can be stored in a flat state and deployed to a finite volume with defined axial stiffness. The stiffness is highly tunable and depends on several geometric properties as well as the material properties of the crease-lines. The structure can exhibit auxetic behavior among other interesting traits when tuned properly. This study will focus on the manufacturing of this origami utilizing composite materials which can offer a lower weight and higher stiffness solution for constructing the TMP for real world engineering applications. Prototypes are constructed with dry glass fibers to form the crease-lines. Polyurethane resin is then infused to bond the structure together and protect the resin. This research can contribute to the design of origami-based deployable structures in industry and research. The manufacturing process used is relatively simple since it does not require a complex system of moving parts or mechanical connections. Additionally, this structure is hollow and therefore offers a space filling feature which gives it great potential as a space structure or a disaster relief shelter.
Computationally Efficient Damage and Residual Strength Predictions using Progressive Damage Failure Analysis (PDFA) with an Enriched Shell Element
An important aspect of the design of composite structures is certification for damage tolerance. Current certification involves tests for low velocity impact (LVI) damage, and the residual compression strength after impact (CSAI). In this work, an adaptive fidelity shell (AFS) model is used to simulate this mutli-step LVI/CSAI process, improving previous LVI results. The two-step process is simulated to ensure that the damage is transferred correctly from one step (LVI) to the next (CSAI) correctly. Use of this model will enable efficient large-scale simulations of full structures with accurate estimations of damage and strength. The AFS model was used to simulate LVI and CSAI for two laminate stacking sequences from the ONR high fidelity database. The model shows promise, but further development is needed to fully capture damage processes seen in LVI/CSAI.
Computationally Efficient Interface Modeling in Fiber- Reinforced Composites Through Displacement-Based Component-Wise Approach
The paper presents a computationally efficient numerical tool for interface modeling in fiber-reinforced composite structures. The proposed numerical tool is part of computational platform built for virtual testing of composites developed within the scheme of the Carrera Unified Formulation (CUF), a unified hierarchical formulation to generate refined structural theories through a variable kinematic description. In this work, Lagrange-type polynomials (LE) are used to interpolate the displacement field over the cross section leading to a purely displacement-based refined one-dimensional model. The Component-Wise modeling (CW), an approach that stems out of LE models, is utilized to model various components of composite structures across scales, e.g., fiber, matrix, laminae and laminates. A class of higher-order cohesive elements is implemented within the CW modeling framework for simulating interfacial fracture mechanics problems. Zero thickness cohesive cross-section elements are introduced along interface of various components of composite materials and structures. A dissipation-based arc-length scheme is implemented within the CUF framework to trace the equilibrium path. The arc-length constraint introduced is based on the energy dissipated during the delamination process. The numerical example demonstrates the effectiveness of the higher-order beam models.
Progressive Failure Mechanism of FRP Composite Laminates with Discrete Element Method
This paper presents a novel approach to investigate the mechanism of progressive failure of fiber reinforced polymer composite (FRP) using a 3D model based on Discrete Element Method. The elasticity of this model has been validated by classic laminate theory and Finite Element Method with a very good agreement for lamina, cross-ply laminate and angle-ply laminate. Thereafter, the Tasi-Hill failure criterion is adopted in the model for predicting progressive failure of cross-ply laminates under tension. The progressive behavior of failure could be captured by the model and in the future the crack density of the failure obtained from the model will be validated by the experimental data.
NASA’s 2040 Vision Roadmap Study: A Framework for Integrated Computational Materials Engineering (ICME)
Over the last few decades, advances in high-performance computing, new materials characterization methods, and, more recently, an emphasis on integrated computational materials engineering (ICME) and additive manufacturing have been a catalyst for multiscale modeling and simulation-based design of materials and structures in the aerospace industry. While these advances have driven significant progress in the development of aerospace components and systems, that progress has been limited by persistent technology and infrastructure challenges that must be overcome to realize the full potential of integrated materials and systems design and simulation modeling throughout the supply chain. As a result, NASA’s Transformational Tools and Technology (TTT) Project sponsored an industry led study to define the potential 25-year future state required for integrated multiscale modeling of materials and systems (e.g., load-bearing structures) to accelerate the pace and reduce the expense of innovation in future aerospace and aeronautical systems. Herein the overall findings of this 2040 Vision study will be briefly reviewed, with an emphasis toward those applicable to ICME of composites. These findings for example include the 2040 vision state; the required interdependent core technical work areas defined as Key Elements (KE); associated critical gaps and actions to close those gaps; and major recommendations.
Prepreg Platelet Molded Composites Process and Performance Analysis
Prepreg platelet molding compounds (PPMCs) are utilized in a growing number of composite applications where parts with complex geometry require both a moderate degree of processability and performance. In collaboration with Boeing Commercial Airplanes, a research group at Purdue University has developed a comprehensive framework for the analysis of process-structure-property-performance relationship of PPMC material systems. The research team focused on non-destructive inspection, multi-scale performance analysis, and molding flow simulation utilizing a complementary set of commercial and research tools that aid in design and understanding of PPMC materials and structures. The lessons learned from each approach about required degree of orientation information, variability in fiber orientation state, variability in performance, and best practices for modeling PPMC systems should be utilized wherever possible and added to as inspection and simulation technologies improve.
Effects of Manufacturing-induced Residual Stress on the Strength of an L-Shaped Textile Composite Flange
Polymer Matrix Composites (PMC) use is increasing in several industries due to their attractiveness relative to weight savings. Fabrication of this material system type requires a cure cycle, performed at elevated temperatures, that induces residual stresses at post-cure due to significant mismatch of fiber and matrix material properties. An integrated has been developed encompassing heat transfer analysis, a viscoelastic constitutive law, and cure kinetics to predict the residual stress distribution and corresponding geometric change after demolding. This paper summarizes efforts performed toward enhanced understanding of these residual thermal stress effects on the delamination type of failure for an angled composite flange under 4-point bending. This study paves the way for fully coupling the composite manufacturing process with structural performance through an Integrated Computational Materials Engineering (ICME) framework.
Characterizing Fiber Reinforced Polymer Composites Shear Behavior with Digital Image Correlation
The interlaminar and intralaminar shear properties, such as modulus and strength, are important for the design of fiber reinforced polymer (FRP) composite materials. Short beam strength is often used as a screening tool for interlaminar strength, whereas the V-Notched beam methods are considered valid for shear modulus and strength measurements. The advent of optical strain measurement techniques such as digital image correlation (DIC), provide new opportunities to generate high resolution maps of the shear strain field as a function of the globally applied strain. In this work, we show that non-contact measurements of the strain field are an important enhancement to measuring the shear properties of FRP composites. The V-Notched rail shear method is used to measure the shear modulus and strength of two different FRP composites that vary in reinforcement type (glass or carbon), matrix type (epoxy chemistry) and angle of the principle fiber axis to the shear loading axis. DIC was used to image the V-Notchrail- method samples during shear measurement to track local strains and evaluate the shear strain across the gauge length and near the notch root in these specimens. The results are compared to analytical predictions of shear modulus derived from tensile properties of the epoxy polymer matrix and the reinforcing fiber.
Characterization of Mode I Interlaminar Fracture Toughness in Composite Materials Using Wedge Loaded DCB Specimens
A simple and accurate data reduction scheme for wedge loaded DCB specimens used for mode I fracture toughness tests was introduced in this study. The effects of axial loading applied to the specimen by wedges were considered in the data reduction scheme. The presented method was verified by CFRP interlaminar fracture toughness tests. A relationship between strain on the surface of a specimen and opening load (strain rate) was experimentally and theoretically obtained to compare the deformation of the specimens during the wedge loaded DCB tests. Strain rate obtained by experiments well fitted the theoretical values. R-curves obtained by the wedge loaded DCB tests were compared with standard DCB tests and other data reduction methods. mode I fracture toughness both at the onset of crack growth and during crack propagation obtained by the wedge loaded DCB and standard DCB tests corresponded reasonably by using the presented data reduction method. Fracture toughness obtained by the modified compliance calibration method (MCCM) and modified beam theory (MBT) were lower than that obtained by the presented method. In addition, in order to clarify the failure mechanisms of CFRP laminates and adhesively bonded CFRP joints, in-situ observation around the crack tip with an optical microscope during fracture toughness tests was carried out. Sequential photographs were taken during the fracture toughness tests. Normal strain in crack opening direction was analyzed by the digital image correlation (DIC) method.
Micro Punch Shear Testing of Unidirectional Composites: A New Test Method
Punch shear damage of unidirectional (UD) composite is an important damage mode during the penetration and perforation of fiber-reinforced composites. A rate dependent progressive composite continuum damage model MAT162 in LS-Dyna® requires the punch shear strength and progressive damage softening parameters as input. A micro punch shear experimental test method has been developed for UD composite at sub-millimeter length scale for determining the punch shear properties and parameters. S Glass/DER353 epoxy UD composite ribbon specimens have been fabricated using the VARTM process. The samples have an nominal cross-sectional dimensions of 1000 × 57~70 , with about six (6) through-thickness fibers. The UD ribbon is adhesively bonded onto the specimen holder and a transverse load is applied across the width of the sample. Localized transverse shear loading is created within a narrow gap (153)between the punch and fixture support. Punch shear load-displacement data is reduced to engineering punch shear stress-strain. A large number of replicates are tested to quantify the stochastic non-linear progressive damage behavior.
Molecular Dynamics Simulations of Fiber-Sizing Interphase
In this study, the structure of the Glycidoxypropyl-trimethoxy silane (GPS) based sizing and its interaction with glass fiber surface and an epoxy film former (FF) are studied using all-atom molecular dynamics (MD) methods. Sizing is formulated with 0-30% wt. concentration of GPS in a GPS-Film Former (FF) mixture. The silica-sizing system is first equilibrated with non-reactive General AMBER Force Field (GAFF) to identify the spatial distribution of silane molecules within the simulation volume. Condensation reactions are simulated using a crosslink algorithm where Si-O-Si linkages form between GPS molecules as well as between GPS and the glass surface. Simulations results show that silane molecules agglomerate as macromolecules and create an inhomogeneous sizing structure. GPS molecules near the glass surface are attracted to the glass surface and form a monolayer of Si-O bonds. Simulations show that a continuous cross-linked silane network is not formed even at the highest GPS concentration levels. However, the atomic structure that evolves during the sizing of the fiber provides insight into the mechanisms of interphase formation that develops during composite processing when the curing agent present in the matrix (e.g. Jeffamine in the case of epoxy matrix) diffuses into the sizing and reacts with both the epoxy FF and the silane through epoxide-amine reactions.
Atomistic scale simulation for the inter-diffusion of Epon 828 and Jeffamine
The understanding of inter-diffusion between fiber sizing and a matrix is an important mechanism governing interphase formation during composite processing. Diffusion governs the concentration gradients, interphase thickness and the resulting atomic structure and properties of the interphase. Epoxy film former and Jeffamine curing agents are two main constituents of the matrix and sizing phases, respectively. In this study, the inter-diffusion between an Epoxy film former (Epon) and Jeffamine curing agents are investigated by performing a series of molecular dynamic simulations using general AMBER force field (GAFF) classical interatomic potential. In the simulations, Epon and Jeffamine phases are initially combined to form a discrete boundary followed by relaxation, tracking of molecules and the creation of concentration profiles. All simulations are performed over a wide range of temperatures. Simulation results indicate that inter-diffusion is Fickian type and temperature dependent inter-diffusion follows Arrhenius law. Inter-diffusion coefficients at room temperature are on the order of 10-8 cm2/s. The use of short term data to predict long time scale diffusion and interphase morphology is assessed.
Molecular Dynamics for the Prediction of the Interfacial Shear Stress and Interface Dielectric Properties of Carbon Fiber Epoxy Composites
The thermoset epoxy resin Diglycidyl ether of Bisphenol F (EPON 862), crosslinked with the Diethylene Toluene Diamine (DETDA) hardening agent, are utilized as the polymer matrix component in many graphite (carbon fiber) composites. Since it is difficult to experimentally characterize the interfacial region, computational molecular modeling is a necessary tool for understanding the influence of the interfacial molecular structure on bulk-level material properties. The purpose of this research is to evaluate and compare the interfacial shear stress and dipole moment for the pristine carbon fiber composite and the one with the moisture content at the interface. Molecular models are established for Carbon fiber reinforced EPON 862-DETDA polymer with and without the moisture content at the interface. Interatomic interactions are defined by Reactive Force Field (ReaxFF). Material characteristics such as polymer mass-density and dipole moment are investigated near the polymer/fiber interface. It is determined that a region exists near the carbon fiber surface in which the polymer mass density and dipole moment are different than that of the bulk values. It can further be seen that material having larger values of dipole moment in interface region have comparatively lesser values of interfacial shear stress.
A Vision for the Next Generation Composites
The field of structural composites has reached a point of stagnation. This paper describes a vision for the next generation of composites with three dimensional microstructures that are inspired by Nature. There are specific natural structural materials that exhibit unprecedented damage tolerance and incredible multi- functionality. These biological composites can provide inspiration and ignite “game changingâ€ strategies that overcome the existing road blocks characteristic of conventional laminated systems. A multi-disciplinary program is described that is aimed at developing the next-generation of Fiber Reinforced Plastics (FRPs) through an integration of activities, including: i) an identification of microstructures that can provide the greatest potential for high specific stiffness and damage tolerant FRPs, ii) a collaborative academic-industry effort to introduce new fiber and matrix materials, with complimentary modeling, to achieve multi-functionality and manufacturability, iii) the use of multi-scale modeling to guide design of the hierarchical microstructures from the nano- to the meso-scale, and to optimize the mechanical and physical properties, and iv) a transformation in additive manufacturing processes to enable production of FRP materials with hierarchical 3D reinforcement distributions and functionally graded microstructures. The proposed efforts could transform the future design and manufacturing processes used for developing FRP components, as well as cultivate a new class of composite materials with unprecedented damage tolerance and multi-functional performance.
Novel Engineered Composite Materials for Protection Inspired by Natural Dermal Armors
To achieve protection from potential environmental and physical threats, fish, mammals and reptiles have developed flexible dermal armors. Engineers and scientists have been inspired by the design of these biological materials and their potential for guiding the development of new structural composites for protection. In this study, we examine the scales from carp fish and find aspects of the microstructure that can serve as a valuable model for the design of a new generation of lightweight, bioinspired armors. Elasmoid fish scales possess a unique balance of stiffness, strength and light weight to provide protection against predators while not interfering with locomotion. When compared to a puncture resistant rubber the scales provide up to 4X improvement in load required for puncture. In evaluation of the scale microstructure, the outer mineralized layer plays a major role in puncture resistance and the external elasmodine and internal elasmodine layer has a hierarchical structure which join the hard, mineralized layer with collagen fibers. The unique structure of elasmoid fish scales provides flexibility without sacrificing strength and toughness. Expanding on these results, novel synthetic soft and hard composite armors were designed and tested. The biomimetic composite material surpasses other synthetic materials in many aspects such as high specific strength and low specific flexural stiffness. Further work is underway to improve on these designs for specific applications that require puncture and penetration resistance.
Long-term Stress Rupture Limitations of Unidirectionasl High Strain Composites in Bending
The efficiency of any deployable structure for aerospace applications is directly correlated to the mass/stiffness ratio and the volume compaction ratio. While High Strain Composite (HSC) materials have a large mass/stiffness ratio the volume compaction ratio is a direct function of the maximum safe stowage strain-limit of the HSC. Previous works have shown that HSC materials are capable of achieving surface strains in excess of the tensile and compressive allowable strains for the constituent fiber. However, it has also been noted that large surface strains exhibit a time-dependent failure behavior. The purpose of this paper was to begin an investigation of the stressstrain- time dependent behavior of an IM7 unidirectional TP-HSC utilizing a novel adaptation of the Column Bend Test method for Long-Term testing (CBT-LT). Ultimate failure curvature testing was conducted on the IM7 laminate to determine long-term test curvature levels corresponding to 80%, 85%, and 90% of the ultimate value. Out of the ten stress rupture/relaxation tests conducted, five were observed to reach stress rupture within six-months of accelerated effective time. All five of the ruptured samples were held at surface strains in excess of 1.7%, corresponding to 89.5% of the reported ultimate tensile failure strain of an IM7 fiber and 91.8% of the ultimate failure surface strain of the IM7 laminate determined in this study. While the initial stress relaxation/rupture trends observed using the CBT-LT method is promising, future work is needed to improve the fidelity of the test method.
Validation of Compression-after-Impact Experiments Using ABAQUS Simulations
Fiber-reinforced composites are increasingly being used in civilian and military aircraft applications due to better performance indices like high strength-to-weight ratio and ability to tailor mechanical properties. Since composites are made of multiple components, their failure mechanisms are complex and involve competing failure modes. This paper studies the experimental and numerical modeling of compression-after-impact (CAI) experiments of T650-5320 composite laminates. Experimental study is conducted at National Institute of Aviation Research (NIAR) laboratories in accordance with ASTM D7136 and ASTM D7137. The numerical finite element modeling is studied using ABAQUS software. Shell elements as well as solid elements are used to model the composite layup. The impact and the CAI experiments are modeled using ABAQUS/Explicit and ABAQUS/Standard solver respectively. Results are compared with the experiments to show a good match. Shell modeling is demonstrated as a simpler approach to model CAI to get coupon-level results. Solid modeling is proposed to get detailed results to determine damage levels within each ply.
Evaluation of Compression Strength After Low Velocity Impact
Damage due to impacts is a large concern for composites use in the aerospace industry. Low Velocity Impact (LVI) damage can significantly reduce the in-plane strength of laminated composites, and understanding how they behave after such damage occurs is important for predicting the failure of these composites. LVI will cause damage to a structure called Barely Visible Impact Damage (BVID) due to the internal delaminations and transverse cracks that form in the laminate. BVID can result in a loss of up to 40% of the in-plane residual strength of a composite. This paper explores the effects of the different stacking sequences and LVI energies on the Compression Strength After Impact (CSAI) strength of the composite, along with producing high fidelity data to map the deformation and damage and compare to progressive damage and failure analysis methods.
A Material Model Development and Validation for Dynamic Response of a Composite Intrusion Beam
As a part of ongoing efforts in lowering the weight of GM vehicles to improve fuel economy and reduce emissions while meeting several safety requirements, the applications of advance lightweight materials, and in particular composite materials, are becoming more essential. Utilizing such materials instead of more traditional metallic materials, structural automotive components become lighter and more efficient. The focus of this study as a part of ongoing Ultra Lightweight Composite Door Design (ULCDD) Project is to develop computational models enabling the design of a composite intrusion beam subjected to a lateral impact. A relatively new physics-based material model in LS-DYNA was developed at a coupon-level using quasi-static flexural test results and subsequently employed to validate the sled impact test results at a component-level. The correlations of the force-displacement results as well as damage progression obtained from the simulation and experiment showed good agreement, demonstrating the capability of the present material model in predicting the dynamic response of composite materials. The above mentioned validated model is particularly important for conducting a multi-variable optimization study for the next phase of this project.
Verification and Validation of a Generalized Orthotropic Material Model MAT213 Implemented in LS-DYNA
A general orthotropic material model has been developed which is suitable for impact analysis. The material model is driven by tabulated data rather than point-wise material properties data, and has been implemented into LS-DYNA. The stress-strain behavior of a material is one of the input tabulated data used to drive the material model which can be obtained either from experiment or virtual testing or a combination of both. The material model consists of three sub-models – deformation, damage and failure. The implementation of the sub-models and some of the salient features are presented in this paper. The second part of the paper presents some of the validation tests carried out using a unidirectional fiber reinforced composite, T800/F3900.
In-Situ Co-Extrusion: Additive Manufacturing of Continuous Reinforced Thermoplastic Composites
A novel co-extrusion system for continuous fiber reinforced thermoplastic composites was designed, fabricated, and tested. This process, called In-Situ Co- Extrusion, is a combination of pultrusion and extrusion that impregnates continuous dry fiber reinforcement tows in-situ with thermoplastic for applications ranging from additive manufacturing using robotic manipulation to automated fiber placement. The performance goal was to design a co-extrusion system that directly uses raw materials (thermoplastic pellets and rolls of carbon fiber tow) instead of ‘prepreg’ tow in an effort to streamline and cut costs in advanced composites manufacturing and deliver fully customizable fiber orientation. The working coextrusion system has been tested with multiple exit geometries and degrees of fiber spreading. Microscopy techniques were employed to estimate the degree of impregnation and fiber volume fraction based on cross sections of the resulting composite tow. FEA modeling was conducted to evaluate the temperature uniformity of the extruder die design and the placement of cartridge heaters. Future work includes testing the extrusion system with different material systems and deposition using a robotic or CNC platform to demonstrate feasibility for additive manufacturing.
Producibility Considerations for Carbon Fiber/Epoxy Prepregs for Use in Aerospace Propulsion Structures
This paper presents an overview of the types of data and information needed to use a composite prepreg in commercial aircraft propulsion structures applications. The intention of this overview is to aid the composites community in developing a more complete understanding of the types of characterization needed to implement new prepreg materials on next generation composite propulsion structures. A review of typical production-level composite acoustic panel fabrication, and the resulting producibility aspects requiring characterization, will first be given for a typical 350°F autoclave-cure prepreg. A general test methodology for a selection of mechanical properties, thermal properties, compatibility verifications, and producibility aspects will be presented along with examples for one or more composite prepreg forms. The intention of this discussion is to provide an understanding of current propulsion concerns, and the often heavily experimental methods of addressing them, in order to provide background for further innovation in the field of composites testing and characterization by the composites community.
A Multi-Scale Viscoelastic Processing Model for Predicting Residual Stress Buildup in Thermoset Composites
The focus of this paper is to develop an experimentally validated viscoelastic processing model to predict the residual stress buildup and stress relaxation in fiber reinforced composites in the curing process. Providing elastic properties of carbon fiber and cure-dependent viscoelastic properties of the resin, the effective viscoelastic properties of the composite are calculated based on the correspondence principle of viscoelasticity, and the Rule of Mixtures (ROM) is used to compute the composite properties in the frequency domain. The computed composite properties are then incorporated in a viscoelastic constitutive law to capture the cure-induced residual stress. The accuracy of our proposed processing model is assessed by comparing the deformed shape of the composite part predicted by the finite element model to that of a laminate fabricated using Vacuum Assisted Resin Transfer Molding (VARTM) technique. Compared with our previous elastic models, the proposed viscoelastic processing model is able to provide more accurate predictions on the cure-induced warpage of the composite part after demolding.
Effect of Edge Distance to Diameter Ratio on Progressive Failure of Bolted Joints in Laminated Composites
The paper presents the progressive failure analyses of composite laminate single bolt joints for three different laminate ply stacking sequences and four edge distances. Failure of composite bolted joint is a multiscale, multimode failure event. Designs and/or load cases that lead to competing failure in multiple modes can be more difficult to predict and can be used as test cases to study predictability of failure and the ability to capture the failure mode interactions. The goal of the analyses was to identify bolted joint designs where the bearing failure mode and shear failure mode compete and collaborate with each other. Progressive failure analyses performed uses the Hashin’s failure criteria to define onset of damage and an energy-based continuum damage model with a smeared crack representation for material stiffness degradation as function of damage. For short edge distances since shear stresses dominate a nonlinear material model was used for the in-plane shear behavior. The progressive damage model was implemented as a user subroutine for explicit analysis (VUMAT) in ABAQUSTM finite element software. The model is validated from experimental pin bearing tests performed for two different materials. The results indicate by varying edge distance to diameter ratio and the proportion of 0, 45 and 90-degree plies in a laminates lead to design that exhibit multi-mode failure with interaction in the failure modes and resulting load paths. These designs with multimode failure and interactive failure can be more useful to choose for validation tests to assess accuracy of prediction models.
A Blended Damage and Fracture Mechanics Model for Progressive Damage Analysis of Notched Composite Structures
This paper presents a blended continuum damage and fracture mechanics finite element model for the progressive damage analysis of composite structures subjected to quasi-static loading. Continuum damage modeling, utilizing the failure criterion of Puck, was used to predict the damage initiation of matrix cracks and fibre failures, while fracture mechanics was used to model onset and propagation of delamination and the propagation of matrix cracks. Numerical implementation was performed in the commercial software ABAQUS, extended with user subroutines for material constitutive behaviour. The numerical framework was validated using experimental results found in literature of open-hole specimens subjected to tensile loading. As the main focus was given on the capability of the model to predict different failure patterns, the ply-block effect was investigated using a [45n/90n/-45n/0n]s with n=1,2,4,8 lay-up made of IM7/8552 composite prepreg. The proposed model was benchmarked against a model that uses only continuum damage approach and it was found that it can predict accurately the failure load the damage pattern especially when delamination is the dominant damage mechanism and interacts with matrix cracking, while the other model fails to predict even the failure load.
Conjugate Stress/Strain Pair Approach for Anisotropic Materials
The conjugate pair approach, when derived from an upper-triangular decomposition of the deformation gradient, allows constitutive relations to be constructed without a need for invariant theory. Constitutive equations are derived from three sets of independent conjugate pairs introduced by the authors. Each conjugate pair describes a different stress/strain response, each with a different experimental response. The conjugate pair model has some advantages over the widely used fiber model of Spencer, e.g., the model presented here has one less model parameter than the classical fiber model of Spencer. In this study the two-dimensional conjugate pair approach has been reviewed. Polyvinyl-ester (stiff) and silicone (compliant) composite materials, the latter with a fiber volume fraction of 18%, have been used to corroborate our linear, anisotropic, conjugate-pair approach for both small and large deformations.
CT Data Based Multiscale Virtual Material Characterization of Textile Composites in ESI Virtual Performance Solution (VPS)
The substitution of traditional engineering materials with state of the art composites often fails due to the uncertainties about the material properties. Especially when using textile materials, the effective material behavior is highly dependent on the local reinforcement architecture. Experimental tests on composite parts have shown that the local material structure can vary significantly since it is influenced by the manufacturing process conditions and the handling of the unconsolidated material.
Material Simulation’s Advantage: An illustration with 3D Woven
In the research of light weighting solutions, the use of CFRP has dramatically increased during the last two decades both in aerospace and automotive industries. However, designers are still facing the challenge to accelerate the insertion of new materials for applications. Virtual Material Testing helps to shorten the design cycle of a new component, expand the design space at reduced costs and provides ways to understand the complex mechanisms appearing at different scales and their effect on material macroscopic behavior. The present paper will expose some of the benefits an engineer can get by using a virtual material testing tool. The methodologies available to conduct such work will be described as well. Finally, the benefits of such tool will be illustrated on the example of 3D woven composite development.
Computational Study for Size Effect in Composites and Nanocomposites
An extensive parametric study sweeping both material and geometrical parameters is performed using size effect law (SEL) and Cohesive Zone Modeling (CZM). These simulations and recent experimental results suggest that the cohesive law of composites and nanocomposites is bi-linear and that the Fracture Process Zone (FPZ) may be almost fully developed already for laboratory-sized specimens. As a consequence, the scaling of the fracturing behavior may be influenced significantly by not only the total fracture energy but also the parameters describing the shape of the bi-linear cohesive law. The goal of this work is to formulate a master size effect curve that can be used for quick, yet accurate, estimation of energy and cohesive law parameters that can be used for the design of composite aircraft structures.
A Double Compliances Method for Measuring the Mode I Interlaminar Fracture Toughness of Composite: Theory and Applications
Delamination is one of the most serious failure modes in laminated composite materials. From the material testing and characterization point of view, it is highly desirable to have a testing procedure which is easy to prepare, conduct and yields consistent Interlaminar fracture toughness. When the ASTM test standard D5528 is applied to measure the fracture toughness, the applied load and its associated displacement and crack length are required. However, it is tedious to measure the growing crack length and to record the corresponding load and displacement simultaneously, especially for unstable crack growth and DCB test in certain environment. Recently, we devised a double compliances method to measure the mode I fracture toughness for composite materials. Only the load and displacement recorded from the test machine are required for the determination of the fracture toughness. The theoretical foundation of this method will be briefly given in this paper. It is then used for two special cases, where the crack growth length is difficult to get access. One is a DCB test in a cold temperature environment. The other is a DCB test for a 2D plain weave woven textile composite. There are several finite crack growths and associated load drops in the load displacement curve in the DCB test for woven textile composites, which increase the difficulty in measuring the crack growth length, displacement and load simultaneously. Using the double compliances method, the finite crack length and static fracture toughness can be obtained from the loaddisplacement relation. It is demonstrated that the double compliances method is very useful for determining the mode I interlaminar fracture toughness of composites, especially for the case where the crack growth length is hard to obtain.
A Novel Test Method to Induce Bi-Axial Stress States in Thin-Ply Carbon Composites Under Combined Longitudinal Tension and Transverse Compression
A novel test configuration has been developed to induce combined stress-states of inplane longitudinal tension and transverse compression in unidirectional (UD) composite layers. Two different multi-directional laminates have been designed incorporating UD carbon/epoxy plies embedded in angle-ply blocks of the same material. The scissoring deformation of the angle-plies induces compression in the central UD layers when the composite is strained in the 0° fibre direction. The amount of transverse compressive stress is determined through an inverse identification method from the measured surface strains of the laminates. Despite the large in-plane transverse compressive strain generated, there is only about 9% drop in the failure strain of the laminates when compared to the baseline failure strain of the UD carbon/epoxy material.
Effect of Temperature and Strain Rate on Damage Accumulation Behavior of Unidirectional CFRP
Carbon Fiber reinforced plastics (CFRP) is consisted with the carbon fiber and matrix resin. It has reported to consider that the matrix resin’s properties also depends on CFRP’s mechanical properties in the recent research. Matrix resin’s considerable mechanical property is viscoelasticity that depends on time and temperature. In this study, we focused on the effect of temperature and strain rate on the damage accumulation behavior of unidirectional CFRP laminate, and tensile tests with acoustic emission technique were carried out at three different temperature with three different strain rate. As the results of tensile tests, lower temperature and higher strain rate showed higher tensile strength, that is, it is understood that tensile strength depended on the matrix’s viscoelastic properties. Crack initiation behavior, which was defined at the point of rapid increase in cumulative AE energy, was depended on the temperature and strain rate. To understand the details of AE information, the frequency analysis was done. In the frequency analysis, the resonant frequency of AE sensors effects on the frequency properties of the detected signals, so we tried to erase the effect of sensors by deconvolution of AE sensor’s frequency properties from detected signals. As the results of deconvolution, we could distinguished the failure modes as matrix cracking, fiber/matrix interfacial debonding and fiber breaking. Using these results, matrix cracking, fiber/matrix interfacial debonding were effected by temperature and strain rate, however, fiber breaking was not affected. Consequently, the crack accumulation behavior of each failure mode depended on the temperature and strain rate as the same tendency of matrix’s viscoelastic properties.
Reactive Molecular Dynamics Simulation of Accelerated Cross-linking and Disintegration of Bisphenol F/DETDA Polymer using ReaxFF
Molecular dynamics simulations of polymers can help in understanding the dependence of molecular structure, cross-linking and the chemistry of the polymer chain and resulting thermo-mechanical properties of the polymer. Apart from these thermo-mechanical properties, molecular dynamics is also a powerful tool to examine the durability of polymers with potential aerospace applications under harsh environmental conditions. Ultraviolent radiation from the sun results in dissociation of molecular oxygen into atomic oxygen (AO) which is abundant in lower earth orbit. Testing composites under AO impact requires an extensive experimental setup to simulate low earth orbit (LEO) conditions and is therefore expensive. Using a newly developed accelerated cross-linking methodology in the framework of ReaxFF, bisphenol F and diethyltoluenediamine epoxy polymer chains are manufactured virtually. This simulated polymer is virtually tested for modulus, glass transition temperature, density and is impacted by atomic oxygen at 8 km/s. Thermomechanical properties show good agreement between experiments and simulations. Simulations the polymer during AO impact using ReaxFF provides useful insight to the degradation mechanism in terms of polymer chemistry and thermal profile.
Atomistic Design of Carbon Nanotube Junctions of Arbitrary Junction Geometry
Creating any workable materials construct for any viable applications using carbon or any other nanotubes would invariable involve dispersion of the nanotube in either twodimensional spatial mesh or three-dimensional volumetric space. These dispersed nanotubes invariably are interconnected via overlap or junctions. It is known that the atomic configuration of these nanotube junctions critically influence the bulk properties (structural, thermal, electrical, dielectric). Thus, it is extremely important to pay a close attention to how optimally these junctions can be formed to attain the desirable properties. In all practical situations, experimentally synthesized junctions (either single CNT junctions or junctions in 2D and 3D CNT network structures) are expected to have random orientation of defect sites (non-hexagonal rings) around the junction. Such random nature of junctions’ topology and defect characteristics is expected to affect their strength and durability as well as have impact on associated mesoscopic and macroscopic properties. In this work, we present a generic framework on creating junctions between CNTs with arbitrary spatial (orientation and degree of overlap) and intrinsic (chirality) specifications, as well as to tune degree of topological defects around the junction via a variety of defect annihilation approaches. Our method makes use of the primal/dual meshing concept where the development and manipulation of the junction nodes occur using a triangular meshes (primal mesh), which is eventually converted to its dual (honeycomb mesh) to render a fully-covalently bonded CNT junction where each carbon atom has 3 bonded neighbors (mimicking sp¬2 hybridization). This design approach offers an opportunity to investigate the effect of topological arrangement of defects around the junction on mechanical, electrical and thermal properties. In addition, this junction design methodology is applied to a CNT-graphene junction and to study the effect of local carbon defects (pentagonal or heptagonal carbon ring versus the hexagonal) on junction strength. It is observed that a symmetrical distribution of carbon ring defects around the CNT-graphene junction yield higher strength that that of irregular defect distribution.
Molecular Dynamics Study for Self-Sensing/Self-Healing Materials to Simulate Damage Detection and Repair in Thermoset Polymer Matrix
The mechanochemical response of a novel cyclobutane-based mechanophore (force-sensitive molecule) covalently bonded with an epoxy-based thermoset polymer matrix is characterized by experiments and modeling. This new mechanophore consists of one cyclobutane (stress-sensitive molecular unit) and six amine groups (epoxy curing agents) that enable the cyclobutane to be connected to epoxy network chemically. Thereby the external force applied to mechanophores directly through the epoxy network, which makes the sensitivity of mechanophore to the external stimulation improved. Furthermore, the mechanical properties of epoxy can be preserved since mechanophores are connected to the host materials chemically, not embedded physically. The experimental results indicated that the yield strength slightly increased compared to the neat epoxy with improved optical response of mechanophore to the external mechanical loads. In parallel, molecular model of this new mechanophore embedded thermoset polymer system has been developed. Numerical simulations correctly capture the effect of the new mechanophore on the improvement in sensitivity of mechanophore activation and mechanical properties; the modeling framework is validated experimentally.
Effect of Consolidation Pressure on the Transverse Compressive Strength of UHMWPE Composites at High Strain-rates
UHMWPE (Ultra-High Molecular Weight Polyethylene) composites are commonly utilized as protective materials against fragments and projectiles. They are composed of UHMWPE fibers and a thermoplastic elastomeric matrix, typically Styrene- Isoprene-Styrene (SIS) or Thermoplastic Polyurethane (TPU). The dynamic mechanical performance of these composites is known to scale with consolidation pressure, but the underlying mechanisms are poorly understood. The majority of the experiments on this material described in the literature are performed at quasi-static strain-rates, even though the composite is loaded at ballistic strain-rates (10ô€µ¯ – 106 s-1) when used in protective applications. In this work, Dyneema® HB80 laminates consolidated at two different pressures are loaded in uniaxial out-of-plane compression at strain rates of the order of 103 s-1 in a Kolsky bar. Using high speed optical imaging, we are able to visualize the dynamic damage evolution in the composite as a function of the applied stress.
Length-Scale Effect on Fracture Behavior Of Nano-Composites
It has been recently observed that addition and dispersion of a few weight percent of nanoscale particles in polymer matrix composites have reduced brittleness and microcracking of polymer matrices and improve their strain to failure and fracture toughness without incurring weight penalty. This paper aims at using molecular dynamics to study length scale effects at the nanoscale, identifying the existence of a lower bound on flaw-size that marks the transition from brittle to ductile failure in nanocomposites, thereby causing deviations from linear elastic fracture mechanics (LEFM) predictions. Crack-tip bond-order based prediction of critical far-field stress and stress intensity factor is also addressed in this work. The MD predictions are observed to deviate from LEFM predictions below a certain length-scale. This study on nanoscale fracture of crystalline (graphene) lays the foundations for the future atomistic predictions of fracture in amorphous (polymer) nanocomposite systems.
Material Characterization and Finite Element Modeling for the Forming of Highly Oriented UHMWPE Thin-Film and Unidirectional Cross-ply Composites
Thermoforming is a cost-effective, high-volume production process used for the manufacture of complex shaped thermoplastic composite parts. Finite element modeling of the process offers a cost-effective and time-saving tool to explore how changes in the ply stack-up orientations and processing conditions can impact part quality and throughput. A credible finite element model of the process requires a robust constitutive model of the material system and a comprehensive associated material characterization program to develop the set of material properties to go into that model. In this paper, some of the technical challenges associated with the charaterizaton of the in-plane shear mechanical behavior of two UHMWPE compsosite formats; a unidirectional fiber/matrix cross-ply (Dyneema® HB210) and a highly-oriented extruded film (DuPontTM TensylonTM HSBD 30A) and the subsequent finite element modeling of the shear response are explored. Picture frame shear testing of each material system is conducted to examine the mechanical behavior at room temperature and a processing temperature of 100°C. Trilinear and ploynomial empirical fits of the evolution of the in-plane shear stiffness as a function of the state of in-plane shear are derived from these experimental data. The curves are then used as material inputs for two material models in LS-DYNA, i.e. the built-in *MAT214 (*MAT_DRY_FABRIC) and *MAT41 (user-defined material model), respectively. The two material models are first used to investigate their respective abilities to replicate the material characterization shear-frame tests for Dyneema® HB210 and TensylonTM HSBD 30A and subsequently to model an in-plane shear test, which is a variation of the bias-extension test. Both material models correlate very well with the shear-frame test data for the two material systems and for the in-plane shear test of Dyneema® HB210. However, both material models underpredict the load deformation response for TensylonTM HSBD 30A. Future work is to understand what enhancements need to be added to the material models such that the in-plane shear response of TensylonTM HSBD 30A is better predicted by the models.
Optimization and Polynomial Chaos-Based Uncertainty Analysis of Additively Manufactured Polymer Composites
Additive manufacturing (AM) of composite structures includes various sources of uncertainties at multiple levels from design to manufacturing stage. This affects adversely by the formation of void contents, weak bead bonding in AM parts, and reducing structural properties, which often hinders the widespread application of AM components. Fused deposition modeling (FDM) is the most widely used AM process because of its simplicity and affordability. In this paper, a multiscale experimental and uncertainty quantification framework have been developed considering various FDM process parameters to achieve optimum structural properties. Filament level extrusion process was first studied by experimental investigation of different fiber distribution (0-10 wt. %) and then by genetic algorithm based optimizer. At the manufacturing level, an experimental scheme was designed for bead, lamina and laminate level properties. The adjusted process parameters for 5 wt. % carbon fiber reinforced polylactic acid (PLA) composite shows minimum variability and maximum structural property at a print temperature of 220℃, platform temperature of 80℃, and print speed of 20 /. The dimensional accuracy and void contents support this results. Physics based surrogate models were also developed at each manufacturing stages to improve the computational efficiency and accurately model the uncertainties. A nonintrusive polynomial chaos based uncertainty analysis was carried out on the surrogate models by considering voids, material variations and imperfect bonding between the beads. The uncertainty model further revealed the distribution of different categories of voids (intra-bead, inter-bead and interfacial-bead), which paved the way for reduced design phase prior to manufacturing. The different types of voids relating to independent uncertainty sources can be encountered by addressing improved fibermatrix bonding for intra-bead voids, modified nozzle geometry for inter-bead voids and optimized printing speed for the interfacial-bead void.
Shadowed Delamination Area Estimation in UT C-Scans of Impacted Composites Validated by X-Ray CT
Although ultrasonic C-scanning (UT) is a mature non-destructive evaluation technique for imaging internal damage in composite structures, a major impediment of obtaining a full characterization of the internal damage state is delamination shadowing effects. Specifically, shadowing refers to regions of interest that are behind other delamination planes or discontinuities with respect to the scanning surface. The delamination planes block ultrasonic wave transmission and the regions of interest are thus hidden (i.e., shadowed) from the scan. A methodology has been developed to expand UT scan data of impacted composites by utilizing damage morphology information that is well established in the composite impact research community, such as matrix cracks bounding delaminations, to estimate shadowed delamination information and matrix cracking. First, impacted flat composite plates were C-scanned by pulse-echo UT and the results were segmented by depth of damage to establish interface-by-interface delamination information. These delaminations were then fit by bounding lines representing the fiber/matrix crack directions defined by the ply orientations adjacent to each interface to estimate the shadowed portion of the delamination results. The area inside this boundary was added to the original UT delamination area to create an estimation of the full delamination state at each shadowed interface. Additionally, because this extension method is based on the interactions between delaminations and matrix cracking, this extension method provides an approximation of the matrix cracking of adjacent plies. Results were compared with X-ray computed tomography scans to assess the effectiveness of the extension method.
Visualization of Fiber/Matrix Interfacial Transverse Debonding
The aim of this study is to visualize the transverse debonding along the fiber/matrix interface under both quasi-static and dynamic loading conditions to understand debonding initiation and progression. S-2 glass fiber reinforcement was used with SC-15 epoxy matrix to manufacture a single fiber embedded within a cruciform specimen. A modified tension Kolsky bar integrated with a high-speed synchrotron X-ray phase contrast imaging (PCI) was used to record the high-speed images of the dynamic debonding event. On the other hand, a quasi-static load frame along with a high-speed camera was used to obtain the quasi-static debonding event. The specimens were pulled at loading velocities of 0.25 mm/s and 2.5 m/s. The results showed that the crack velocity revealed minor increase with increasing loading velocity.
Numerical Simulation of Failure Behavior under Impact Loading for Cylindrical Carbon Fiber Reinforced Polymer
This study presents a prediction for the dynamic failure behavior of the carbon fiber reinforced polymer (CFRP) under impact loading. In the experiment, the split Hopkinson pressure bar (SHPB) test was conducted by using two types of cylindrical CFRP specimens. The commercial finite element analysis (FEA) software Abaqus was used for the analysis. In this study, we considered two types of failure mechanisms, a problem that mainly involves matrix-dominated failure and does not involve fiber breakage and a problem that involves fiber-dominated failure. To predict the matrix-dominated failure, we adopted multi-scale analysis, which is a method that can carry out analysis at different scales. We considered the damage initiation criterion based on continuum damage mechanics (CDM). This damage initiation criterion was based on the Christensen failure criterion. We simulated interfacial failure by using cohesive zone modeling (CZM), which considers failure conditions in mixed mode. In micro-scale analysis, a two-dimensional periodic unit cell (PUC) was used and we considered CDM and CZM. Macro-scale analysis was conducted in the same way as in the SHPB experiments. To predict the fiber-dominated failure, simulation was carried out using the Hashin damage theory. Parameters used in macro-scale analysis were obtained from micro-scale analysis. Macro-scale analysis is currently conducted.
Quantifying the Delamination of L-Shaped Composite Laminates Under Low Velocity Impact Using X-Ray Computed Tomography
Low velocity impact response and failure behavior of L-shaped composite laminates were investigated. The composite laminates with variable thickness, stacking sequences, and corner radius were manufactured using autoclave manufacturing method. Tests were conducted on a custom low velocity impact test set-up. In order to investigate and quantify the internal damage of the samples, x-ray micro-computed tomography technique was used. The results showed that the deflection of cross-ply samples was higher than the unidirectional ones, and as the thickness decreases deflection increases for the same loading conditions. In addition, as the corner radius decreased, the deflection was more pronounced. Delamination migration was observed at the mid-plane of the corner in unidirectional samples while delamination planes largely remained separate and parallel to each other in the crossply samples.
Mechanics of Edge-Cracking and Toughness Determination for Strain Locking Composite Materials
In this paper, we describe the mechanics of edge cracking and methods for determining the fracture toughness of strain locking materials using homogenized constitutive models for strain locking materials. We implemented a thermodynamically consistent constitutive model for a strain locking material into a plane stress finite element model and determined the energy release rate for a single-edge cracked configuration. Using material parameters suitable for a copper-clad polymer flexible circuit board and for a biological material, we determined the relationship between the strain energy release rate and the crack length for an applied load history using crackadvance methodology. The change of total potential energy (ÐŸ = - (U-W)) as an edge crack propagates through a prismatic bar loaded in tension is determined. A polynomial is fitted to ÐŸ where U is the total strain energy stored and W is the work done by the external loads for the purpose of differentiating with respect to the crack length, a. The energy release rate, G, is derived from the slope Î as a function of crack length from these numerical results. Additionally, an additively manufactured strain locking composite material specimen is produced and tensile tested. The results are used to fit the material constants to a previously derived implicit nonlinear elastic model.
Investigating Flexural Failure in Carbon Fiber Reinforced Polymer Composites Interleaved with Carbon Nanotube Sheets
While carbon fiber reinforced polymer (CFRP) composites exhibit exceptional mechanical properties in tension, their relatively low compressive strength is often a limiting design factor. To mitigate this shortcoming, this study investigates effects of inserting aligned carbon nanotube (CNT) sheets at ply interfaces on flexural properties of CFRP composites. CNT sheets were placed between carbon fiber plain-weave fabrics prior to infusion with epoxy. Resin transfer molding (RTM) was subsequently used to fabricate the specimens. Composite specimens were examined after fabrication and similar quality for both CNT interleaved and reference samples were observed. Samples were tested for on-axis tensile and flexural properties where the aligned CNT sheets were parallel to the bending direction. As expected, on-axis tensile tests showed no change in stiffness due to CNT reinforcement. The addition of CNT sheets increased the flexural strength by 28.9% and improved the resistance to delamination compared with the reference samples. This study suggests an innovative, inexpensive and scalable solution for mitigating the low compressive and interlaminar properties of conventional composites while potentially improving their physical properties such as electromagnetic interference (EMI) shielding and electrical/thermal conductivities. Keywords: Carbon Fiber Polymer Composite, Carbon Nanotube Sheets, Compressive Strength, Flexural strength
Peridynamics for Progressive Failure Analysis of Composites
This study applies the Peridynamic Differential Operator (PDDO) to solve for the equilibrium equations of Classical Laminate Theory (CLT) for progressive failure analysis of composites without employing a stiffness degradation factor. The PD representation of the displacement derivatives and the transformed reduced stiffness matrix permits the evolution of fiber and matrix cracking during deformation, and enables the modeling of progressive failure. In the derivation of equilibrium equations of CLT, the transformed reduced stiffness matrix is considered as spatially varying unlike the common assumption of its uniform variation. The PD representation of these equations enables the modeling of progressive failure during the deformation through the removal of PD interactions (bonds). The stiffness degradation is natural, and it is achieved by removing the PD bonds. The numerical results concern unidirectional laminates and a symmetric cross-ply laminate with a through-the-thickness crack under tension, and a nonsymmetric cross-ply laminate with a crack only in the bottom ply subjected to a uniform distributed load.
Multiscale Failure Analysis for Prediction of Matrix Crack Formation in Polymer-Matrix Composites
The matrix phase of fiber-reinforced polymer-matrix composites typically exhibits nonlinear viscoelastic/viscoplastic behavior with damage evolution. For the structural safety evaluation, these polymer characteristics should be appropriately modeled to predict crack initiation on the fiber-diameter scale accurately. In this study, the matrix modeling effect on multiscale prediction of nonlinear response and crack initiation was investigated using a multiscale approach that consists of two finite-element analysis on different length scale. On the macroscopic scale, laminate-scale finiteelement analysis assuming to be a homogeneous orthotropic lamina was conducted to obtain strain histories at failure-expected points. On the microscopic scale, periodic unit-cell (PUC) analysis considering heterogeneous material structure was performed to predict crack initiation in the matrix phase of composite laminates, based on strain histories obtained from a macroscopic laminate analysis. Two constitutive models and four sets of failure criteria were applied to the matrix phase of the unit cell for PUC analysis, and compared with each other to evaluate important factors in multiscale prediction of polymer-matrix composites.
Prediction for Stiffness Reduction and Progressive Damage of Composite Laminate Including Ply Cracks
Fiber-reinforced laminates have a specific fracture mode such as ply cracking (cracking in a direction parallel to the fiber), delamination, and breakage of the fiber. Of these fracture modes, ply cracking occurs in the earliest stage, and it can result in significant degradation in material stiffness. Furthermore, the initiation of ply cracking causes stress redistribution, resulting in more severe forms of damage such as delamination and breakage of the fiber due to the stress concentration at the crack tip. Therefore, it is important to clarify the mechanical behavior of laminates including ply cracking. This study focuses on the stiffness reduction of laminate due to ply cracking and the ply cracking progression in a ply. First, the continuum damage mechanics (CDM) based model for predicting the stiffness reduction of composite laminates is established. The effective compliance of laminate based on CDM and the classical laminate plate theory (CLPT) was used to formulate the thermo-elastic properties of laminate of arbitrary lay-up configurations as a function of ply crack density. The damage variable d2 is formulated analytically by three-dimensional local stress field (3-D LSF) model in a ply including ply cracking subjected to tensile loading. Next, ply cracking progression in laminates including 90° plies was investigated. The energy release rate associated with ply cracking is calculated using the 3-D LSF model, and then ply cracking progression in 90° plies of CFRP cross-ply laminates based on both strength and energy criteria is predicted using the Monte Carlo method. The models proposed in this study can predict the experiment results and finite element analysis results.
Spectral Stiffness Microplane Model for Unidirectional Composite
The use of unidirectional (UD) composites for primary and secondary structures is becoming broader and broader with applications including aeronautics and astronautics, automotive and bioengineering. However, the efficient use of these materials requires the development of proper computational tools for design. Leveraging the spectral stiffness decompostition of the stiffness tensor and a microplane formulation, the present contribution proposes a computational model to capture the main damage and fracturing mechanisms in UD composites. Preliminary simulations of uniaxial tests are compared to experimental data taken from the “World Wide Failure Exerciseâ€ to investigate the predictive capability of the model.
Modeling-Driven Damage Tolerant Design of Graphene Nanoplatelet/Carbon Fiber/Epoxy Hybrid Composite Panels for Full-Scale Aerospace Structures
The objective of this study is to design a new nano graphene/carbon fiber/polymer hybrid composite that can be used for the NASA SLS Composite Exploration Upper Stage (CEUS) forward skirt structure. The new material will improve the resistance to open-hole compression failure of the structure relative to traditional polymer fiber composites. The material is designed rapidly and with little cost using the Integrated Computational Materials Engineering (ICME) approach. Multiscale modeling and experiments are used to synergistically optimize the material design to yield improved properties and performance by controlling key processing parameters for manufacturing nano-enhanced materials. Specifically, the nanocomposite panel showed a 22% reduction in mass relative to the traditional composite panel, while both designs are equal in terms of ease of manufacture. This potential mass savings corresponds to an estimated 45% savings in materials and manufacturing costs.
Prediction of Fiber-reinforced Composite Material Properties Using Collaborative Filtering Techniques
The traditional structural design process follows a deductive, “bottom-upâ€ approach in which the manufacturing process determines the material structure (e.g., crystalline structure), which in turn determines the material properties, which in turn govern the ultimate structural performance. This causal chain of analyses is at the core of the integrated computational materials engineering (ICME). There are two key challenges in the implementation of ICME. First, accounting for the processingstructure- properties-performance relationship often requires very detailed computational simulations. These simulations can be very expensive to perform, which consequently makes an exploration of a material design space expensive, if not outright intractable. Second, for a given material, different processing methods and environmental conditions can give rise to dramatically different macro-level performance characteristics at the structural scale. A machine learning framework has been developed to address these challenges. The proposed framework reduces the amount of testing required to obtain material properties and avoids the need for microscale modeling. In this research, collaborative filtering (CF) is used to find missing material properties from similarities among material families at both the constituent level (i.e., fiber and matrix) and the laminate level. The properties obtained with CF can be used in the traditional deductive approach to determine performance at the part level. Several CF techniques were explored and implemented in a flexible workflow that may be used to find missing properties from an existing material database. These techniques include several machine learning algorithms, as well as an invariant-based approach to represent elastic properties recently developed by Stephen Tsai and Daniel Melo (2014) to enhance the material data set. Some of the techniques allow for stochastic prediction and provide statistical distributions of the missing properties instead of a single deterministic prediction. This information is then used to quantify performance uncertainty.
Microscale Analysis of Virtually Cured Polymer Matrix Composites Accounting for Uncertainty in Matrix Properties During Manufacturing
This paper proposes a study on mechanical properties of virtually cured microstructures accounting for uncertainty in matrix properties and fiber packing. Computations are carried out using the finite element method (FEM) using the commercial code Abaqus supplemented by user written subroutines. An instantaneous linear elastic model for the stress evolution during manufacturing is employed to model curing. Results are presented in terms of transverse stiffness and strength for cured microstructures of different sizes. Comparisons are proposed with models not including curing induced residual stresses.
An Efficient Multiscale Virtual Testing Platform for Composite Via Component-wise Models
The aim of the current work is to develop a multiscale framework based on higherorder 1D finite elements developed using the Carrera Unified Formulation (CUF). The multiscale framework consists of a macroscale model to describe the global structure, and a CUF micromechanical model described using the Component-Wise approach. Such an approach allows for the explicit modelling of the fiber and matrix at the microscale, resulting in a high-fidelity finite element model at both scales. The use of refined CUF elements result in a computationally efficient analysis, due to a reduction in the degrees of freedom at both scales, as well as the reduction in total computational time when compared to standard 3D finite element analysis. The parallel implementation of the multiscale framework results in additional savings in computational time.
Double-Bubble Fuselage Subcomponent Experimental Testing to Support the D8 Composite Fuselage Design
The design of the D8 wide fuselage consists of a vertically joined dual-lobe fuselage cross-section. Vertical structural members run down the centerline of the fuselage. Connecting these vertical structural members to the fuselage skin is a so-called “Yâ€-joint. The structural “Yâ€-joint of the double-bubble fuselage is composed of a co-cured fuselage skin with a 3-dimensional (3-D) woven composite preform. objective of the subcomponent test is to create characteristic stresses within the “Yâ€- joint and keel, similar to what will be observed in a full-barrel fuselage under cabin pressure loading. To this end, a custom self-reacting test fixture is manufactured to permit introduction of pressure loads into the subcomponent skin via pressure bladders. Experimental boundary conditions provide hoop skin loads and vertical tension rod attachment loading on the keel. Strain, force, and bladder pressure data are collected and compared to finite element predictions for multiple fuselage pressures. Aspects of manufacturing the co-cured skin, “Yâ€-joint preform, and hat stiffeners as well as the assembly of the subcomponent article are discussed. Lessons learned from the experimental phase are included and related to the full D8 fuselage design.
Time and Temperature Dependent Stress-Strain Behavior of Unidirectional Carbon Fiber/Polyimide Composites Under On-axis and Off-axis Tensile Loading
Phenylethynyl terminated polyimide “TriA-Xâ€ developed in Japan Aerospace Exploration Agency (JAXA) exhibits excellent mechanical properties with high glass transition temperature (> 350 °C), therefore it is suitable as matrix resin of high temperature composites. However, the time and temperature dependent mechanical properties have not been investigated. This study examined the mechanical behavior of unidirectional (UD) carbon fiber (CF)/ TriA-X polyimide composites by off-axis monotonic tensile tests and off-axis tensile creep tests. As results, it was revealed that one parameter plasticity model by Sun and Chen was effective to represent the nonlinear stress-strain behavior of UD-CF/ TriA-X composites. Furthermore, linear viscoelastic behavior was observed from the off-axis tensile creep tests of UD-CF/ TriA-X composites. On the other hand, residual strain was observed in creep recovery curves after unloading, which suggested that the creep and creep recovery behaviors of UDCF/ TriA-X composites are not affected only by viscoelastic and plastic deformation, but also by viscoplastic deformation.
3-D X-ray Tomography for In-Situ Characterization of Progressive Damage Response of Carbon Fiber Laminates Subject to Mechanical Loadings
Composite laminates possess heterogeneous microstructures which make characterization and modeling a great challenge, particularly in their failure response to different loadings. One of the most emerging research areas involves the development of a robust, high-fidelity, physics-based model to predict the progressive damage response of composites under mechanical loading. In the literature, many failure models have been developed with a view to predicting various failure modes observed in composites, including fiber breakage, fiber kinking, matrix cracking, and delamination between plies. Digital image correlation (DIC) techniques have been widely used to identify hot spots and failure evolution by tracking the surface strain histories. Although this method can capture crack propagation, the application is limited to determining surface intra-ply damage, and the resolution is generally not fine enough to capture the failure at the fiber level. The most viable approach to produce data of value for the formulation and validation of composite material models would need to be fully 3-D and in-situ. In this experiment, a proof of concept approach to study carbon fiber laminates with 3-D X-ray tomography and in-situ tensile loading is proposed and developed. Test results revealed information regarding through-ply cracking and its impact on catastrophic failure of the specimen. Based on the results of this experiment, the implementation of 3-D data correlation (digital volume correlation) can be evaluated as a way to quantify the load- and time-based material changes that lead to failure. Additionally, other types of loadings including temperature, compressive loading, and 3-point/4-point bending can be considered for future studies.
A Dielectric Resonant Cavity Method for Monitoring of Damage Progression in Moisture-Contaminated Composites
A method for monitoring of damage progression due to combined mechanical and hygroscopic loading in polymer composite materials is presented. Polymer-based materials have a tendency to absorb moisture from their operating environment. Dielectric properties of these materials are significantly affected by the total amount of absorbed moisture and the degree of its interaction with the host polymer. Bound water molecules which are restricted in their ability to rotate with an applied electromagnetic field contribute less to the bulk relative permittivity. 'Free' water molecules rotate without impediment and are therefore associated with a higher relative permittivity. The bulk relative permittivity as a function of total water content of a contaminated composite is a unique function of the internal physical and chemical characteristics of the specimen. Holding chemical contributions constant, physical characteristics dominate. Thus, relative permittivity provides insight into the physical state of composite, including amount of free space from processing-induced voids or, critically, the presence of physical damage such as cracks and voids across multiple length scales. Here, we demonstrate a method for leveraging this phenomenon to provide insight into the initiation and accumulation of physical damage in moisturecontaminated composites. This is accomplished using a split-post dielectric resonant technique operating in the low GHz frequency range, where dipolar contributions to relative permittivity dominate. Further, continuous and non-contact monitoring of relative permittivity is achieved by integrating a resonant cavity with a fatigue loading frame. Preliminary experimental assessment of this test method is supportive of its potential in damage tracking. Water-contaminated 12-ply bismaleimide (BMI) / quartz laminate specimens were tested in impact and flexural fatigue, while a 4-ply glass/epoxy laminate was tested in tensile fatigue while changes in relative permittivity were recorded. The results show a distinct rise in relative permittivity consistent with the expected magnitude and progression of damage in all cases.
From Addition Reactions to Cross-Linked Network Formation
An Atomic-level model that can analyze the influence of the synthesis conditions (Molar ration, catalyst et al.) on the mechanical properties of phenol formaldehyde (PF) resins has been developed. This model clarified the relationship between synthesis conditions, the structure formation, and the structure-depended mechanical properties by introducing a comprehensive reaction model that includes both addition and condensation reactions. We validated the effectiveness of the model by verifying the influence of primary synthetic index, molar ratio, on the mechanical properties such as glass transition temperature (Tg) of resol resins. The computing cost has also been reduced since we adopted a multi-scale model which combined the Quantum chemistry calculation (QM), Monte Carlo (MC), and Molecular Dynamics (MD) method. This model will be helpful to reduce the cost of attempts at synthetic PF resins and more efficiently to find the suitable synthesis conditions for the desired material properties.
The Effect of Chirality and Strain Rate on Mechanical Properties of Carbon Nanotube (CNT) and CNT/Epoxy Composite
The effect of strain rate and chirality on mechanical behavior of single walled carbon nanotube (SWCNT) and SWCNT/epoxy composite under tensile loadings was investigated using a computational molecular dynamics simulation and the Reactive Force Field. The analysis comprises the methodology employed to develop the molecular structures of SWCNTs and SWCNT/epoxy composites, the techniques for relaxing the materials systems, and the method utilized to characterize tensile parameters of the structures. The simulated results showed that the chirality of the SWCNTs as well as rate of deformation affect the Young’s modulus of both the SWCNTs and the SWCNT/epoxy composite.
Changes in Micro-Phase Separation of Di-Block Copolymer Melts Induced by a Circle Fiber
Using self-consistent field theory (SCFT), we computed phase-separated structure of di-block copolymer to apply this structure for the matrix of a composite. For this purpose, we combined SCFT with finite element method, which enables us to consider the effect of a smoothed boundary of a carbon fiber on the phase separation. In the result, a circularly shaped carbon fiber disturbs the phase separated structure: changes from lamella phase to cylinder phase are induced by a circle fiber and this induction is emphasized with increasing the fiber radius. Furthermore, we found that four parameters, block-ratio ( f ), polymer length ( N ) and Flory-Huggins parameter ( ï£ ), volume fraction of fiber (VM) are independently important to determine the equilibrium phase structures in a matrix penetrated by a circle fiber, even if only f and Nï£ are essential in the bulk phase.
Scaling of Fatigue Crack Growth in Pristine Epoxy
This work proposes an investigation on the scaling of fatigue crack growth in pristine epoxy. Towards this end, fatigue fracture tests on geometrically scaled Single Edge Notch Bending (SENB) specimens were conducted. It is shown that Paris-Erdogan law exhibits strong size effect on both slope and threshold for pure epoxy. This indicates that the fatigue Fracture Process Zone (FPZ) size is significantly larger compared to the quasi-static one. In fact, fracture tests conducted by Mefford et al. [1] and a comprehensive literature study [2] showed that the FPZ has a negligible effect on the scaling of the fracturing behavior of pure epoxy under quasi-static loading condition. By introducing a fatigue size effect model based on the energetic-equivalence framework, the thresholds in the Paris-Erdogan curves representing geomaticallyscaled specimens can be successfully adjusted whereas the slopes still exhibit size effect. This latter aspect needs to be studied further to enable the application of the Paris-Erdogan law to quasi-brittle structures of different sizes and geometries.
Improvement of Durability Property by Using Low Diameter Glass Chopped Strands
The diameter of glass fiber chopped strands used for injection molding is usually from 10μm to 15μm. On the other hand, some reports have stated that the mechanical property is improved by slimming the diameter of the glass fiber chopped strands. By increasing the surface area between the glass fiber and matrix resin, in other words, an increase in the interface to bear the load is the main factor of this improvement. However, their report focused on the short-term load, so the effect on the durability strength has not been reported. This study was made to investigate the effect on the durability property by using low diameter fibers. In addition, we investigated the effect of using high strength and high modulus glass composition fibers on the durability property.
Stiffness Degradation Model for Fatigue Life Prediction of GFRPs under Random Ocean Current Loading
A new phenomenological fatigue model based on stiffness degradation is proposed for life prediction of Glass Fiber Reinforced Plastics (GFRPs) under random ocean current loading. The model used Weibull cumulative density function to account for a shift in failure modes and corresponding change in the rate of damage accumulation. The effects of stress ratio and stress amplitude on the model parameters were characterized using the results from constant amplitude tests. The fatigue life of GFRP coupons under random ocean current loadings was predicted both by the proposed model and by the conventional life prediction methodologies using Palmgren-Miner’s rule. The fatigue life was predicted within a factor of two by the proposed model, which was more accurate than the conventional methods.
Modelling of Lightning Strike-Induced Shock Wave Damage in CFRP Composites
Composite structures made of carbon fibre reinforced plastics (CFRPs) are getting increasing uses in modern aerospace, green energy, marine, and transportation industries due to their high strength-weight ratio and high stiffness-weight ratio. However, the inclusion of CFRP composites in mainframes of modern engineered structures presents special problems regarding lightning strike threat. During a lightning strike, a large amount of energy is delivered in a very short time, inducing the ionised channel to expand with a supersonic speed. The sudden high-temperature plasma expansion creates a shock wave propagating in the CFRP composites, which may cause the severe deformation of structures and even mechanical damage. This work develops a finite element (FE) model to predict shock wave-induced damage in a CFRP laminate with full electrical-thermal protection. A surface plasma expansion model was introduced assuming partial energy transfer from the lightening current to the plasma pulse. The FE model was validated by an existing experiment, considering the failure behaviour such as intra-laminar damage and interlaminar delamination of the CFRP laminate.
Impact Performance and Flexural Behavior of Composite Sandwich Structures in Low Temperature Arctic Conditions
Advanced composite materials play vital structural roles in automotive, aerospace and marine industries. Recently, reduction in arctic sea ice region over the last three decades has opened new sailing routes which are more efficient and economical. This has resulted in the increased use of marine and naval vessels in extreme low temperature arctic conditions. The fundamental challenge of operating in such cold and harsh environment lies in the understanding of how materials and structures behave and perform in extreme low temperature. Composite sandwich structures far exceeds classical composite laminates in terms of flexural capability and performance. In this study, we experimentally investigate the impact and post-impact bending response of Divinycell H-100 foam core sandwich panel with woven carbon fiber reinforced polymer (CFRP) facesheets. Specimens were conditioned and impacted over a temperature range (from room temperature down to -70°C). Results show that exposure to low temperature generally causes more severe damage in the specimens. Post-mortem inspection using x-ray micro-computed tomography revealed complex failure mechanisms in the composite facesheets (such as matrix crack, delamination and fiber breakage) and foam core (core crushing, core shearing and interfacial debonding).
Dynamic Impact Behavior of Syntactic Foam Core Sandwich Composites
Sandwich composites and syntactic foams have historically been used in many engineering applications to meet the needs of a system. However, there has been minimal effort to take advantage of the weight saving ability of syntactic foams in the cores of sandwich composites, especially with respect to the impact response of the structure. The goal of this experimental study is to investigate the mechanical response and damage mechanisms associated with sandwich composites with syntactic foam cores. The core was manufactured using epoxy resin as the matrix and cenospheres as the reinforcement with varying volume fractions of 0%, 20%, 40%, and 60%. The sandwich composites were manufactured with the vacuum assisted resin transfer molding (VARTM) process. Impact tests were performed on the specimens according to ASTM D7766 at two energy levels: 80J and 160J. The data from the tests was post-processed to gain quantitative understanding of the damage mechanisms present in the specimens. A qualitative understanding was obtained through MicroCT scanning imaging. The analysis showed that increasing the volume fraction of cenospheres in the syntactic foam made the damage mechanism more desirable, even at high energy levels.
Dynamic Behavior of Carbon Fiber Reinforced Polymer (CFRP) Composites at Higher Strain Rates
Carbon Fiber Reinforced Polymer (CFRP) composites are known to have highly variable modulus and strength based on fiber direction. This presents significant challenges when attempting to identify their mechanical properties. In particular, the composite strength and failure envelope in multi-axial loading is expected to have a complex nature due to anisotropy. Furthermore the heterogeneity of CFRP composites makes it even more difficult to model their failure modes and behavior. These intricacies become more pronounced at higher strain rates. In this study specimens with varying layup, geometry, and fiber volume fractions were tested in different loading conditions. Fiber volume fractions of the samples have been determined using thermogravimetric analysis (TGA) in nitrogen gas environment burnout tests. High strain rate response of CFRP composites are of scientific and technological interest. They are used extensively in aerospace (due to their high specific strength and stiffness) which necessitates their characterization for high velocity impact. The polymeric resins are of course expected to demonstrate rate dependence. Therefore split Hopkinson pressure bar (SHPB) experiments were used to determine the high strain rate response of CFRP composites in this study. The dependence of failure stress and strain on the strain rate was examined and summarized based on different loading conditions, geometries and layups. The failure stress is not very sensitive to strain rate in the range of this study, however comparisons with quasi-static data is done to further analyze this effect. The failure strains are higher when bidirectional specimens are loaded in the transverse direction (normal to the plane of fibers) compared to the axial loading of the unidirectional specimens. Meanwhile it was observed that the failure stresses of both unidirectional and bi-directional fiber specimens are close to each other. This has led to proposing a resin strength dominated failure mode for CFRP composites.
Examination of Pre-gelation Behavior in AS4/8552 Prepreg Composites
Recent work on the topic of manufacturing defects in composite parts has focussed on understanding the changes between initial uncured state and final cured state of the material. Increasingly it is clear that understanding the early state, or pre-gelation behaviour, of a material becomes essential in controlling popular manufacturing techniques such as automated fibre placement (AFP) and forming. It is therefore timely to re-examine the behaviour of materials during the pre-gelation regime. This paper presents a method for characterizing the pre-gelation regime of unidirectional Hexcel AS4/8552 prepreg material system using the Digital Image Correlation (DIC) technique and microscopy. Through experimental observations in this study, we show that the pre-gelation regime can be broken into two phases. In the first phase, the material behaviour is dominated by surface tension of the liquid resin. In the second phase, behaviour is dominated by tool response via tool-part interaction. The findings in this study contribute to our understanding of how processing is influenced by the initial material state and will inform the simulation of processes such as AFP and forming.
Development of Composite Leaf Springs Made by 4D Printing
4D printing is a manufacturing process which combines 3D printing with reconfiguration of the structure into a more complex form. The normal 4D printing would involve the deposition of polymeric materials with special properties to make flat layers. These flat layers are then subjected to some activation mechanism such as heat, light, water absorption etc. The materials in the flat layers then react to the activation mechanism to change the shape of the structure. 4D printing work was started in 2013 by Tibbits, and has received increasing attention. Most of the materials used in 4D printing have low mechanical properties. The modulus of these materials in only about 5 MPa, and these need to have special properties which can be expensive, and may not be widely available. 4D printing of composites is similar to the 4D printing mentioned above, except that the materials are regular composite materials that have been used to make structures such as airframes. These materials are light and stiff (modulus along fiber direction in order of 180 GPa), and strong (strength along fiber direction in the order of 1500 MPa). Flat layers of the composite are laid using either Hand Lay Up (HLU) or Automated Fiber Placement (AFP). The layers have different orientations to make unsymmetric laminates. Upon curing, the interaction of layer of different orientations will make the structure to be curved. This technique can be used to make structures of different curvatures, without the need to use complex molds. While the shape can be obtained, the question that remains is whether the structure is strong and stiff enough for engineering applications. This paper presents the formulation to determine the curvature, and the stiffness of the composite curved beams, intended for spring applications, that are made using 4D printing method.
Non-crimp fabric (NCF) preforms are an attractive alternative to traditional preimpregnated tapes due to their low manufacturing cost and high efficiency. The orientation of the fibers in each layer can be tailored, independent of the other layers, to optimize the required load carrying capacity in that particular direction, making them capable of improved performance. Stitches help to keep the fiber tows in the NCF fabric straight during handling; however, the stitches prevent the fibers from reorienting easily to accommodate complex shapes without wrinkling. In order for NCF fabrics to be used to create complex geometric shapes, their draping behavior needs to be understood with respect to different fabric variables so that the draping performance can be maximized. To date, the draping behavior of NCF fabrics has been only sparsely investigated in contrast to the amount of research reported on woven fabrics. This paper presents an investigation on the role of fabric architecture in the formability of NCF fabrics. This study is a subset of a broad study conducted under the purview of a Department of Energy project funded to General Motors for developing state of the art computational tools for integrated manufacturing and structural performance prediction of carbon fiber composites. For modeling the draping behavior, fabric characterization tests such as bending and bias-extension evaluations were conducted for NCF fabrics with varying areal weights and construction. Taking advantage of ESI’s PAM-FORM material model, asymmetric shear behavior was included in conjunction with different membrane and bending behavior to model the draping behavior of these fabrics. For this study, the fabric characterization data was first used to calibrate the draping models in simple shear and bending tests. Later these models were validated against the deformation of the fabrics when they were formed using a truncated pyramid tool designed at the General Motors Research Labs to assess the drapeability of the dry fabrics.
Characterization of Cohesive Zone Laws Using Digital Image Correlation
Delamination is a major failure mode in composite structures. This work addresses lack of understanding of delamination cohesive laws which is a fundamental challenge of strength and fatigue progressive damage and failure analysis (PDFA) of composite structures. Typically such laws are based on mathematical assumptions which are fit to simplify convergence of the analysis tools and are not based on physics or measurement. The simplifying assumptions may cause wrong cohesive material properties as they fit to a specified simulation task. This work presents the results of a feasibility study focused on developing experimental methods and analysis techniques for capturing material cohesive properties and increasing confidence in material input data for PDFA. A direct approach is proposed to measure cohesive laws in static Double Cantilever Beam (DCB) and Thick Adherend (TA) shear specimens that have been pre-cracked under fatigue load. High resolution Digital Image Correlation techniques are used to measure crack tip separation in these specimens, supported by X-ray Computed Tomography (CT) reconstructions to locate the crack front. Additionally, by J-integral and VCCT calculations, the precise shapes of the cohesive damage laws in mode I and mode II are characterized. These cohesive laws were verified by direct comparison of load-displacement curves between numerical Finite Element models and experimental data of the DCB and TA test. The specimen compliance is accurately reproduced, and maximum strength prediction is within 4%. It is noteworthy that accurate crack location has been essential for consistent results.
Direct Numerical Simulation of 3D Woven Textile Composites Subjected to Compressive Loading: A Multiscale Approach
Results for the compressive response of hybrid 3D woven textile composites (H3DWTCs) that consist of carbon, glass and kevlar fiber tows and a polymer matrix material are presented, using a combination of experiments and a multi-scale analysis of the full laboratory scale coupon for a class of H3DWTCs. The coupon model considers macro, meso and micro scale based modeling, where the notched gage area is explicitly modeled with fiber tows embedded in the matrix and the grip areas are homogenized at the macro scale. The meso scale fiber tows are considered homogenized entities of fibers and matrix, and are related to the constituents through an analytical micromechanics model. The experimental results suggest that carbon fiber compressive strength dictates the initiation of kink banding failure in carbon tows, while glass fiber tow compressive strength dictates the maximum load attainable. These experimental observations are seen to be captured by the 3-scale modeling strategy developed here and hence, seems to be a computationally efficient tool for the progressive damage and failure analysis of textile composites.
Experimental Characterization of Mode I and Mode II Peridynamic Critical Stretch Parameter
The focus of this study is to determine the critical stretch parameters for laminated composites when using peridynamic approach for modeling interlaminar delaminations in idealized modes I and II with and without z-pin reinforcements. The doublecantilever beam test and the end-notch flexure test were used to evaluate the normal (Mode I) and shear (Mode II) bond critical stretch in the absence of z-pins. Subsequently, the interlaminar delaminations with z-pins were simulated in order to evaluate the corresponding critical stretch.
Effects of Out of Plane Stress on Progressive Kinking in Internal Zero Plies
Compressive strength and failure are a common benchmark to qualify the performance of a composite material for many applications. Standard test procedures typically involve compressive test of unidirectional or quasi-isotropic composites from where the properties are back calculated for a single composite ply to obtain the compressive strength of a ply and the laminate. In many applications, the composite material is under multi-axial stress states. In this paper, the influence of through-the-thickness stress on the compressive behavior is studied, with specific intent of replicating loading conditions seen in a bolted joint. Plane strain model of a laminated composite with explicit modeling of fiber and matrix is used in a layered stack-up with different boundary conditions to study the changes in compressive strength as well as residual (post-peak) strength.
3D Continuum Damage Mechanics Model with Permanent Strain
Most currently available material models for composites do not allow for permanent strain. For applications such as ballistic containment and energy absorption applications, this results in under predicting material performance. This study extends the Matzenmiller, Lubliner and Taylor (MLT) model, a continuum damage mechanics (CDM) based constitutive model for unidirectional composites, to 3D solid elements, and enhances it with permanent strain capability. The model was implemented into the commercially available finite element code LS-Dyna. The model was validated with 3-point bend experiment. It was then used to simulate ballistic impact tests of an aluminum projectile against a glass fiber composite plate. As shown in the results below, this provides significant improvement in prediction of material performance
Multi-Scale Analysis of Joints in Hybrid Metal/Composite Structures in ESI Virtual Performance Solution (VPS)
The lack of maturity of crash simulation of structures made of hybrid materials is a key issue for lightweight engineering in automotive industry. Tailoring local behavior and mixing materials while accounting for this mix and joining problematics in the simulation methodology are required to optimize weight and costs, without the need of a real prototype. Current lightweight vehicle programs use local strengthening of structural body components through hot or warm formed high strength steels joined with spot-welds. New metals or composites parts with new joining techniques are progressively introduced for the next stage of weight saving. Local reinforcement by thermo-plastic composites is also considered to offset costs trade-offs.
Powering NASTRAN with SwiftComp for Multiscale Modeling of Composites
MSC.Nastran has been powered to analyze composite structures using a novel multiscale constitutive modeling approach by integrating SwiftCompTM, which is developed based on mechanics of structure genome (MSG). The multiscale constitutive modeling can provide a lower cost alternative or complement for predicting effective constitutive information in the preliminary design phase. Three dimensional (3D) orthogonal woven composite plate and beam structures are analyzed using CQUAD4 and CBEAM shell and beam elements in MSC.Nastran, which take the constitutive information as input from the subscale computed by MSG plate and beam models. Besides, MSG has been extended to predict the thermoelastic constitutive information considering the pointwise thermal loads. A 3D woven plate with pointwise temperature distribution along the thickness is analyzed to show the accuracy and efficiency of the MSG-based thermoelastic analysis. A graphical user interface (GUI) has been developed for MSC.Nastran to incorporate the constitutive modeling and to enable the automation of the information passing between constitutive modeling and structural analysis.
A Method for Rapid Determination of Fiber Orientation in Reinforced Composites at Lab and Component Scale
Thermal and mechanical response of the fiber reinforced polymeric, metallic, and ceramic composites are strongly dependent on reinforcement fiber orientation and matrix resin rich zones within a composite. In many manufacturing techniques the final orientation is an outcome of the process and thus, necessitates quantification after formation to validate the material performance to design specifications. Current orientation measurement techniques are laborious, destructively invasive, expensive to implement, and are limited to small spatial domains. This creates a significant limitation in the ability to validate performance at the part scale where predictive capability is challenging and a barrier to market entry, limiting the use of light weight, energy efficient glass and carbon fiber reinforced composite materials for structural applications. Hence, there is an immediate need for rapid, non-destructive fiber or fiber bundle orientation evaluation for large spatial domains, including whole parts, for expeditious prototyping, composite engineering, and quality assessment to bring novel fiber reinforced composites to wind, compressed gas storage, infrastructure and automotive markets. A novel method is developed by the authors using a custom Thermal Digital Image Correlation (TDIC) method with which one can image a fiber reinforced composite materials including epoxy, vinyl ester, polyester, phenolic, or thermoplastic resin with carbon fiber, glass fiber, basalt fiber or natural fiber reinforcements.
Modeling Turf Through Discrete Element Analysis
Composite materials play a considerable role in sports. Artificial turf is a composite which attempts to advantageously replicate a natural grass/soil playing surface. Despite numerous advantages, the technology is correlated with an increase in injuries. The concept behind this research is to create and validate a discrete element model (DEM) for the ground/foot interaction through the use of open source software. Related models could investigate footwear and interactions with other granular materials, e.g. sand on a beach or climbing on gravelly hills. After validation, the goal is to analyze an arrangement of studs to determine what role stud geometry plays on torque [N-mm]. The validated data was provided through laboratory experiments. Three football studs attached to a rigid bracket were turned at 1 degree per second in rubber infill and artificial grass+infill on a servo hydraulic load frame for 60 seconds. Torque was sampled at 100Hz. A trial and error approach was employed to calibrate the elastic modulus, Poisson's ratio, and density until it had a similar output to the validation data provided by the load frame. Through this approach, the DEM model was able to yield results within a 2% difference for average torque relative to the validation data. DEM simulations demonstrated stud geometry appears to play a significant role in torque and force generation.
Contamination Transfer from Processing Aid Materials to Prepreg
A lot of amount of processing aid materials (e.g. parting film, bag film, etc.) are used for fabrication of composite parts. Quality of the processing aid materials tends to be treated as unimportant, however, contamination transfer from processing aid materials to composite material could cause reduction of interlaminar strength. (Figure 1) Therefore, it is important to understand phenomenon of the contamination transfer in order to select clean processing aid materials and process which cause less contamination transfer.
Pseudo-ductility of Unidirectional Thin Ply Hybrid Composites in Longitudinal Compression
The compression response of unidirectional thin-ply carbon (C) standard thickness S-glass (SG) hybrid composites is presented. Gradual failure up to high strain has been demonstrated for [SG1/(C1/SG1)17] and [SG1/(C2/SG1)17] hybrid configurations. The favourable pseudo-ductile behaviour is due to progressive fragmentation and dispersed delamination of the thin ply carbon/epoxy layers. For the [SG1/(C3/SG1)17] configuration, sudden failure occurs at a lower strain, but still higher than the compression failure strain of the carbon fibres calculated from the manufacturer’s data sheet. The energy release rate concept can be used to qualitatively explain the behaviour of the different thickness hybrid laminates under compression.
Integrated AFP Manufacturing and Stress Analysis/Design Process
Design of structures manufactured with automated fiber placement machines presents a unique set of challenges when compared to traditional hand layups. However, it has a significant advantage in simulation of the manufacturing process. Fiber is placed by a robot, providing an accurate digital model of the manufactured part. In this paper, a process is presented to map this manufacturing data from Vericut Composite Programming (VCP) to the stress analysis performed in HyperSizer. Such a process makes it possible to incorporate manufacturing constraints in the design process, thereby streamlining the laminate design and potentially reducing the weight of the structure.
Predicting the Influence of Manufacturing Parameters on Curing Generated Deformations Using Thermo-Mechanical Modelling
The present study is related to the prediction of residual stresses and associated cure-induced distortions that commonly occur during the manufacturing of fiberreinforced thermosets. RTM-6 is used as the matrix material, as abundant literature data is available for both the cure kinetics and the mechanical response as function of different cure degrees. After the final shape has been virtually demolded from the component, it is compared to the as-designed shape, i.e. the surface of the mold. This allows to numerically investigate the effect that changing manufacturing and design parameters, such as applied thermal profile, lay-up, part thickness and shape, can have on the cure-induced deformations. To this end, the simulations will be compared to experiments performed from the literature using C- and L- shaped parts.
Symmetrical and Antisymmetrical Sequenced Fibers with Epoxy Resin on Rectangular Reinforced Structures Under Axial Loading
In this study, Finite element Method (FEM) evaluation is performed for the compressive failure of reinforced structures with layered composite shells under axial loading. In addition, embedded delamination between the reinforcing layered composite shells and the core is considered as a defect. The layered composite shells are made of 12 plies of equal thickness of Kevlar, CFC, and E-Glass with epoxy resin. Considering the orientation and laminate, three different layered composite shells, (0°/90°/0°/90°/0°/90°), (45°/-45°/0°/90°/60°/-30°), and (60°/-30°/90°/0°/30°/90°), are considered for symmetrical and antisymmetrical sequences. These results are obtained through ABAQUS simulations and subsequent analysis. The results show that symmetrical and antisymmetrical sequences can be used as an index for quality control and as a safety factor of composite shells produced by the hand lay-up technique in certain industrial processes. The delamination growth is also investigated with the help of cohesive elements. Buckling phenomenon occurred abruptly due to the fast propagation of delamination, having face/core debond.
A Numerical Model to Simulate Void Dynamics During Processing of Honeycomb Core Sandwich Structures with Prepreg Face-Sheets
Composite sandwich structures are fabricated by placing partially-cured thermoset prepreg sheets and an adhesive layer on either side of a honeycomb core enclosed by a vacuum bag in an autoclave and co-cured under a pressure and temperature ramp. It is imperative that the adhesive layer forms a good bond between the core and the prepreg face sheets. One of the complications is the bubbles/voids could form due to outgassing and grow in and around the bond-line during the manufacturing process. In this work, a model of bubble growth is incorporated in a simulation that provides the distribution and size of voids within the adhesive layer as a function of the process, material, and geometric parameters. This understanding should prove useful in reducing or eliminating voids from the bond line during processing of such sandwich structures.
A Cohesive Fatigue Model based on the S-N Diagram
The relationship between fatigue life and fatigue crack propagation rate is explored with a new cohesive damage model. The parameters of the model are obtained from idealizations of S-N diagrams used in engineering design. The model is based on the hypothesis that both stable tearing damage and damage due to cyclic loading are representations of a density of microcracks and, therefore, a single damage variable can describe the state of damage. This assumption implies that the quasi-static cohesive law that describes tearing is also the envelope of the fatigue damage. Fatigue damage within the cohesive envelope is assumed to accumulate at a rate that depends on the displacement jumps. The fatigue model was implemented as a UMAT subroutine for Abaqus cohesive elements by adding fatigue damage accumulation to a cohesive model based on the Turon quasi-static cohesive law. The analyses were conducted using a simplified cyclic loading procedure in which the maximum applied load is kept constant and the computational expense of cycling the load is avoided. The predicted propagation rates for a double cantilever beam (DCB) specimen were compared to experimental results for IM7/8552 graphite/epoxy tape.
Multi-axial Fatigue Behavior of Notched Composite Structures
This work proposes an investigation on the failure behavior of Glass Fiber Reinforced Polymer (GFRP) laminates with an open hole or a center crack under both multi-axial quasi-static and fatigue loading. To this end, several multi-axial stress states were applied by means of a modified Arcan rig, specifically designed for fatigue tests. It was shown that the stress-strain behavior of notched [+45/90/ - 45/0]s and [0/90]2s layups under multi-axial quasi-static loading was characterized by a significant nonlinearity. This phenomenon becomes more and more significant under shear-dominated loading conditons. It was also shown that, according to Digital Image Correlation (DIC) analysis, the damage of all the investigated laminate layups under fatigue loading is generally much more diffused compared to the one under quasi-static loading. More importantly, the damage mechanisms of investigated specimen configurations under multi-axial quasi-static loading were different compared to multi-axial fatigue loading. This is a challenge for the development of physics-based computational models, since the damage initiation and progression can be substantially different depending not only on the multi-axiality ratio but also the loading condition (quasi-static versus fatigue).
Effect of Manufacturing-Induced Voids on the Fatigue Performances of Multidirectional Laminates
In the present work, an overview is presented on the recent work carried out by the authors on the influence of manufacturing-induced porosity on the long-term performances of composite materials. The effect of voids was first studied in terms of fatigue damage mechanisms at the micro-scale. An extensive experimental campaign was then carried out to assess the effect of porosity on the life to crack initiation (S-N curves), crack propagation (Paris curve), crack density evolution and global stiffness drop of [0/902]S and [0/452/0/-452]S laminates. Finally, a model that allows to predict the life to crack initiation from the behaviour of the void-free material was developed, showing good accordance with the experimental results.
Damage Evolution in U-Shaped Composite Beams Loaded in Fatigue
U-shaped laminated carbon-epoxy composite beams were subjected to fatigue loading. The fatigue load caused a well-defined bending moment and shear load to be applied to the curved section of the beam, resulting in a multiaxial state of stress in this region of the specimen. Stacking sequences were varied from one specimen to the next, and consequently the magnitude and orientation of the stresses induced in the curved section are expected to vary from one specimen to the next. During testing tension-tension fatigue loads were applied for 200,000 cycles, resulting in an increase in measured specimen compliance. The increase in compliance was due to matrix cracking that occurred in the curved section of the beam. Images of the damaged region obtained using computed x-ray tomography (CT-Scan) are presented.
Integrated In-Process Monitoring of High-Rate Production CFRP Structures for Material Quality Assurance
SIP (Cross-ministerial Strategic Innovation Promotion Program) - SM4I (Structural Materials for Innovation) was established by the Council for Science, Technology and Innovation (CSTI) of the Japanese Cabinet Office, as one of the national R&D subjects to realize scientific and technological innovation strategically under its initiative. Under this SIP - SM4I, our project, "Innovative Aircraft Polymer Matrix Composites (iAPMC)" started in 2014 as a five-year project. The main purpose of this project is to develop high-rate production aircraft CFRP products and quality assurance technology for next-generation CFRP aircraft structures. This project consists of five research units, (1) OoA CFRP (Airframe) Unit, (2) Low-cost Autoclave CFRP (Airframe) Unit, (3) CFRTP (Engine) Unit, (4) High-Temperature CFRP (Engine) Unit, and (5) Academic Support and Material Evaluation Unit. This presentation provides a summary of recent results in this project. Especially, optical fiber sensor based integrated in-process process monitoring methodology is presented with some successful examples which cannot be provided by conventional material characterization methods. Precise in-process material property data are obtained and fed into the process simulation code for better prediction of CFRP structures to avoid many trials and errors in the development of new CFRP materials for high-rate production.
Multiscale Modeling of the Impact Response of Triaxially Braided Polymer Matrix Composites, Including Effects of Adiabatic Heating
Significant local temperature rises often accompany the high rate deformation of polymer matrix composites. In the case of impact loading, heat is generated locally within the polymer matrix due to plastic dissipation, but the rapid nature of the loading precludes significant heat transfer from occurring; ballistic impact loading can therefore be regarded as fully adiabatic. In this paper, the development of a synergistic multiscale approach to simulate the architecturally dependent impact response of polymer matrix composites with complex fiber tow architectures is presented and applied to a representative triaxially braided composite material system. To approximate the heterogeneity of the composite braid architecture at the highest analysis length scale, a subcell-based approach is utilized whereby the mesoscale repeating unit cell of the material is discretized in-plane into an assemblage of laminated composite subcell regions, with stacking sequences determined from the braid architecture. Each unidirectional layer of the laminated composite subcells are modeled with the generalized method of cells micromechanics theory, where a nonisothermal viscoplastic constitutive model is employed to model the rate, temperature, and pressure dependent polymer matrix. Matrix temperature rises due to inelastic deformation are computed. in matrix elastic properties are determined from neat resin dynamic mechanical analysis data. The commercial transient dynamic finite element code LS-DYNA is utilized to conduct simulations of quasi-static coupon tests and flat panel impact tests performed on a T700/PR520 [0°/60°/–60°] triaxially braided composite. Good agreement is found between simulations and experiments. It is expected that, once progressive damage and failure are incorporated into the multiscale scheme, the incorporation of adiabatic heating affects will greatly improve the predictive capability of current models.
Modeling Impact and Mechanical Response of Carbon-Fiber Reinforced Polymer Composites
Carbon fiber-reinforced composites typically do not perform as well as materials containing polymer and glass fibers in ballistic impact applications. This is due to the brittle nature of the carbon fibers, which cannot admit the larger deflections required for composite- and fabric-based armors to stop a projectile without breaking. That said, the high stiffness and strength and low density of carbon fiber composites makes them ideal for use in structural regions of vehicles, aircraft, and other assemblies that may be unintentionally subjected to high-rate impacts while in service. Thus, an understanding of the damage mechanisms active in carbon fiber composites during impact and how they differ from those that occur in polymer and glass fiber composites under similar conditions is of interest to anyone performing impact modeling of these materials. Tests of carbon fiber composites with two-dimensional non-crimp and three-dimensional woven architectures were devised in order to improve the understanding and modeling of these materials under impact conditions. Carbon fiber composite panels were subjected to ballistic impact to obtain both residual velocities of the projectiles and backface deflection measurements. Additionally, slow-rate mechanical tests of the composite were conducted to provide information about the material’s response under various stress states. Mesoscale models of the two- and three-dimensional composites were constructed. These models include individual elements for the carbon fiber tows, resin matrix, and interfaces between the two constituents. Results from the finite element computations were compared to both the ballistic impact and mechanical test data.
Dynamic Response and Validation of a Flexible Matrix Composite
Testing and predicting the dynamic response of flexible matrix composites in impact loading condition faces two primary challenges: (i) experimentally, existing techniques using existing instruments do not always provide high fidelity material data under simultaneous high strain and high strain rate loading conditions; and (ii) finite element simulations of a highly flexible material requires many material parameters and complex mathematical formulations. To address these limitations, this research investigation presents a technique originally developed in-house for modeling and validating hyper-viscoelatic materials and applies it toward the flexible matrix composite. Results from a simple low velocity impact (2 m/s) test on a 75 x 75 mm2 flexible matrix composite indicate that the critical material properties for the low strength, highly deformable matrix in conjunction with an updated constitutive model can accurately predict the dynamic behavior within 10% with respect to the force time history response using MATLAB and ABAQUS/Explicit. Finite element interrogation also shows full field stress response within the composite specimen not easily measured via sensors and deformation matching the behavior observed via high speed camera. Finally, on-going research in this arena indicates that the technique can be applied to higher rate loading mechanisms, such as a gas gun and Hopkinson bar apparatus, in order to obtain material parameters for even more devastating impact loading strain rates.
Delamination Resistance and Size Effect in Discontinuous Fiber Composites
In this paper, we investigate Mode-I and Mode-II delamination behavior of Discontinuous Fiber Composites (DFCs). Owing to the complex heterogeneous mesostructure in DFCs, conventional testing methodologies such as the double cantilever beam (DCB) and end-notched flexure (ENF) tests used to characterize Mode-I and Mode-II interlaminar failure may fail to characterize the non-linear behavior during delamination. This is because DCB and ENF tests based on Linear Elastic Fracture Mechanics (LEFM) models, fails to account for the quasi-brittleness of DFCs. As a result, this approach may not be able to capture the variation in the Fracture Process Zone (FPZ) which becomes large due the distributed damage in the platelets. Hence, there is a need to account for this non-linear behavior of the FPZ to effectively estimate the delamination fracture energy. This paper proposes an experimental investigation on the effects of the FPZ on the inter-laminar delamination of DFCs. To shed light on the role of the FPZ size versus the structure size and geometry, geometrically-scaled DCB and ENF specimens were tested. The results show a significant size effect. While for small sizes the specimens exhibit a limited strength reduction by the presence of the crack (which indicates a pseudo-ductile behaviour), the failure becomes more and more brittle for larger sizes. Future work will focus on the understanding of this phenomenon leveraging stochastic Finite Element modelling and quasi-brittle fracture mechanics.
Experimental Study of In-plane Shear Response of Interface Toughened Carbon Fiber Composites
Automated Construction and Insertion of Layer-by-Layer Finite Element Sub-Models of Damaged Composites
Finite element models of composite structures are generally shell-based and modeled at the laminate level. More detailed layer-by-layer lamina-level models are sometimes needed for representing joints or for modeling defect growth processes. We describe a method and toolkit for automating the creation and insertion of layerby- layer finite element sub-models of composite laminate. We focus in particular on representing damage captured from nondestructive evaluation (NDE) measurements. The method is based on scripting existing simulation and solid modeling tools (ABAQUS and ACIS). It works even on complicated, curved CAD models. The submodel location is identified by the intersection of a cylinder with the structure. We then execute a series of instructions to generate a new shell with the submodel region removed, generate the layer-by-layer submodel, and bond together the layers and models with desired boundary conditions and defects. The instructions represent the steps of lamination and bonding for creating the composite. The output of the method includes CAD models of the new shell and each lamina within the submodel, and a Python script for ABAQUS that will load the CAD models, bond them together, and apply the specified boundary conditions.
Validation of a Mesoscale Fiber Kinking Model through Test and Analysis of Double Edge Notch Compression Specimens
Tests and analyses were conducted on a series of geometrically-scaled double edge notch compression specimens to validate the capability of the fiber-kinking model in the NASA continuum damage mechanics code for progressive damage analysis, CompDam. The tests and analyses focused on the fiber-kinking damage mechanism, which is of critical importance for predicting accurately longitudinal compression failure in carbon fiber reinforced polymer laminates. The fiber-kinking model in CompDam was augmented with a new capability to represent a hardening response once a kink band has fully formed, enabling prediction of band broadening. Correlation of the test and analysis results showed agreement in stiffness and strength with less than 10% error. The analysis predicts the same sequence of events leading to ultimate failure that was found in the test. The overall excellent correlation in terms of stiffness, strength, and failure process validates the capability of the model for predicting longitudinal compression failure in notched laminates with cross-ply layups.
A Benchmark Example for Delamination Propagation Predictions Based on the Single Leg Bending Specimen Under Quasi-static and Fatigue Loading
Benchmark examples based on Single Leg Bending (SLB) specimens with equal and unequal bending arm thicknesses were used to assess the performance of delamination prediction capabilities in finite element codes. First, the development of the quasi-static benchmark cases using the Virtual Crack Closure Technique (VCCT) is discussed in detail. Second, based on the quasi-static benchmark results, additional benchmark cases to assess delamination propagation under fatigue loading are created. Third, the application is demonstrated for the commercial finite element code Abaqus Standard 2018. The benchmark cases are compared to results obtained from VCCTbased, automated quasi-static propagation analysis. A comparison with results from automated fatigue propagation analysis was not performed at this point since the current version of Abaqus does not include this capability under variable mixed-mode conditions. In general, good agreement between the results obtained from the quasistatic propagation analysis and the benchmark results were achieved. Overall, the benchmarking procedure proved valuable for analysis verification.
A Multiscale Two-Way Thermomechanically Coupled Micromechanics Analysis of the Impact Response of Thermo-Elastic-Viscoplastic Composites
A dynamic multiscale micromechanics model that accounts for two-way thermomechanical coupling in composites is presented. It predicts not only the standard deformation that results from a temperature change, but also the temperature change that results from mechanical deformation. In addition, viscoplastic material behavior is included at the scale of the fiber/matrix constituents in a local High-Fidelity Generalized Method of Cells analysis, which is conducted at each material point of a global analysis on a composite specimen. The global analysis of the composite considers time-dependent thermomechanical boundary conditions capable of simulating impact loading while solving the coupled transient thermal problem and the dynamic mechanical problem. Effective thermoelastic and physical properties, as well as homogenized inelastic strains are provided to the global model from the local scale. The multiscale model was employed to examine the impact response of a carbon/epoxy composite with emphasis on the temperature changes and inelastic strains induced by the impact loading.
Discrete Damage Modeling for a Transverse Compression Experiment of a Polymer Matrix Composite
Strengthening the fundamental understanding of micromechanical methods in continuity is a critical aspect in developing and designing future composite systems. Virtual testing has provided additional understanding of the behavior of materials on a microstructural scale. However, experiments must be executed to determine their validity. Modeling realistic microstructures under realistic loading conditions could help develop physically based micromechanical constitutive laws needed to predict the intrinsic failure. In this study, discrete damage modeling was performed on a microstructure of polymer matrix composite under transverse compressive loading. The discrete damage model utilized a Regularized eXtended Finite Element Methodology (RXFEM formulation to initiate cracks, a Cohesive Zone Methodology (CZM) was used to simulate crack propagation, as well as debonding between the fibers and matrix. The discrete damage model provides insight to the microstructural behavior under transverse loading and correlates well with experiment.
Failure in Unidirectional Composites with Nonuniform Fiber Distribution Under Combined Transverse Tension and Axial Shear
This study is continuation of the authors’ previous work on examining early failure events in unidirectional polymer matrix composites under transverse tension or under axial shear. This paper considers the combined effect of applying these two loading modes. The early failure events consist of brittle cavitation or yielding depending on the local stress states resulting from the applied loading. The criterion for brittle cavitation is attainment of the critical dilatational energy density when the stress state is hydrostatic tension (or nearly so), while that for initiation of yielding is attainment of the critical distortional energy density. As in the previous works, the nonuniform fiber distribution is described in terms of the degree of nonuniformity, which is implemented in a representative volume element (RVE). Several realizations of the RVE for different fiber volume fractions and different degrees of nonuniformity are analyzed using a finite element model. The results show that the occurrence of brittle cavitation versus yielding depends on the biaxiality ratio, i.e., the ratio of the axial shear stress to the transverse tension
Statistical Machine Learning and Sampling for Composite Fabrication and Performance
We apply manifold learning and sampling to the tasks of fabrication, manufacturing, and testing of composites. We specifically address the challenge associated with statistical inference on these tasks from a small size sample. Limitations on the sample size could emanate from constraints on computational resources as well as constraints on physical experiments. In either case, the analyst is typically presented with a short table that contains observations of environmental conditions and quantities of interest (QoI). In the case of numerical simulations, the QoIs can be at the discretion of the analyst while in a laboratory setting these are typically limited by access to sensing devices. We augment the statistical knowledge captured by the available dataset with knowledge of physics constraints (eg conservation laws) in order to enhance the predictive value of the dataset. Imposing these constraints typically requires additional experiments (either physical or numerical). We proceed differently as we discover, within the dataset, an intrinsic structure that is consistent with the manner in which the available data is interrelated. To that end, we rely on diffusion maps, a recent data-analytics procedure. This allows us to rapidly characterize feasible domains for complex phenomena involving multiscale and Multiphysics interactions. We augment the diffusion map procedure with a stochastic sampler guaranteed to sample on the manifold, thus allowing us to impute a very large sample that is consistent with the statistics of the original dataset and its learned intrinsic features.
Techno-Economic Model and Simulation for Wind Blade Manufacturing
The ongoing demand to reduce the LCOE (Levelized Cost of Electricity) drives the wind industry to explore new technologies that will advance the state-of-the-art for composite wind blade manufacturing. These new technologies span the range from new resins and fibers, to improved blade designs, to innovative manufacturing techniques. However, since the introduction and widespread adoption of vacuumassisted resin-infusion techniques for blade making, there has been no significant change in the basic labor-intensive manufacturing process for wind blade production. In the current research, a Techno-Economic Model (TEM) and a complementary simulation of a generic wind blade manufacturing facility are developed. The TEM is sufficiently robust to take into account the very rapid product refresh cycle (and concurrent consumption of capital), differences in blade lengths, and the potential future composite technologies such as carbon fiber and thermoplastics that could impact the blade design and resulting manufacturing processes. To investigate the long-term costs and benefits, the TEM also takes into account the cash flows over a multi-year period so that the true value of improvements can be identified and used to justify capital investment in automation and other process changes. The complimentary simulation is built in DELMIA. DELMIA allows for a visual tool to evaluate how changes in the manufacturing steps will impact process flow and timing. The integration of these two models into a full Techno-Economic Analysis (TEA) provides a comprehensive tool to identify opportunities for increasing throughput and for exploring the impact of capital investments.
Fiber Angle Optimization and Tow Path Planning on 3D Curved Surfaces Using the Multiple Mesh Approach
Variable stiffness composites are composite structures in which some of the plies are steered, the tow paths are allowed to change orientation within the ply. Finding the optimal fiber orientation, and corresponding tow paths, is possible through the lamination parameters framework and the manufacturing finite element mesh framework (MFEM). In this article the implementation of the 2D MFEM framework is extended to incorporate the optimization of 3D shell structures. Both the optimization methodology and the tow path planner for 3D surfaces are presented and discussed here.
Manufacturing and Evaluation of an Optimized Composite Panel with a Cut-Out
Three different panels assuming a representative part of aircraft lower wing are designed, manufactured and evaluated to quantify the manufacturing capability of current state-of-art AFP technology as well as the potential performance of an optimized composite panel.
Strength Prediction of Non-Conventional Laminates by Incorporating Intralaminar Shear Stresses
A quasi-isotropic laminate, a laminate of which the in-plane stiffness matrix is independent of the laminate orientation, similar to a metal, can be achieved in multiple ways. One method is to create a laminate with layers orientated at conventional angles, such as 0, 90, 45, and -45. Another method is to use layers orientated at angle different than the aforementioned; using non-conventional orientation angle of 15, -15, 30, -30, 60, and -60. For the sake of clarity, the former (a laminate with only 0, 90, 45, -45 degree plies) is referred to as a ‘conventional’ laminate, while the latter (a laminate with ply orientations other than only 0, 90, 45, -45 degree plies) is referred to ‘non-conventional’ laminate in this investigation. It is important to note that a laminate consisting of conventional angle can have a non-conventional stacking sequence (the order of which the layers are stacked), however, the term conventional and non-conventional are reserved to the angles only.
Interfacial Thermal Resistance Based Effective Thermal Properties of Nanocomposite Systems at Various Strain States: A Multiscale Computational Approach
Single Walled and Double Walled Carbon Nanotubes (SWCNTs) have excellent thermal characteristics that qualify them as ideal candidates for thermal management in electronics and nano-composites. The thermal conductivity of the nanotube system is found to be dependent on the size of the nanotube (length and diameter) and its strain state. Additionally, the effect of size of the nanoparticle also influences the thermal characteristics in the form of interfacial thermal resistance of the nanostructure. Two Molecular Dynamics based simulation methodologies namely the equilibrium MD (EMD) and non-equilibrium MD (NEMD) methods are widely used to determine the thermal conductivities of nanotube systems. In this work, the thermal conductivities of the nanotube systems are determined using LAMMPS with AIREBO potential using a Reverse Non-Equilibrium Molecular Dynamics (RNEMD) method. The effect of interfacial thermal resistance of the nanotube together with the effect of strain state is used in a continuum based formulation to determine the effective thermal conductance of the nano-composite system.
Quantitative Microscopic Investigation of Mode I Fracture Surfaces of Nanosilica-Filled Epoxies
The addition of functionalized nanosilica (NS) particles to epoxy resins is known to improve certain mechanical properties such as modulus of elasticity and fracture toughness. In the current investigation, epoxies with and without NS reinforcement were investigated. Four NS concentrations were evaluated: 0, 15, 25 and a maximum wt% NS dependent on which of the two curing agents was used. The tensile modulus of elasticity and quasi-static Mode I fracture toughness were measured and the Mode I fracture surfaces were examined using a field emission scanning electron microscope for general imaging and a scanning laser confocal microscope for quantitative information on surface morphology. Fracture toughness, as measured by critical strain energy release rate (GIc), and fracture surface area increased monotonically with increased NS content in the epoxy cured with diethyltoluenediamine (DETDA). However, for the material cured at a higher temperature with 4-4’ diamino diphenyl sulfone (DDS), GIc and surface area reach their respective peaks at NS concentrations less than the maximum value. The primary morphological toughing mechanisms observed were particle pullout and crack deflection. The DDS cured system had higher surface area than DETDA system for any non-zero NS content, but less GIc. Analysis of the experimental results led to the conclusion that GIc of the DETDA was mostly explainable in the context of NS particle pullout, as both fracture surface area and GIc varied in rough proportion to NS content. In the DDS system, however, such proportional behavior was not observed and it is believed that competing mechanisms influence GIc at NS concentrations above 15 wt%.
The Impact Resistance of Thermoplastic Fiber-Metal Laminates Based on Glass And Basalt Fibers
In an attempt to reduce the elevated processing temperature and pressure required for a prolonged period for consolidation of components made from prepreg in conventional fiber metal laminates (FMLs), thermoplastic matrix FMLs, known as TFMLs, have recently been introduced. The present work is an experimental study on the low velocity impact response of a TFML based on thin layers of an aluminum alloy and a polypropylene (PP) matrix reinforced with basalt fibers. The response of the TFML based on basalt fibers under low velocity impact loading is evaluated, where the effect of varying the stacking configuration of the constituent materials is studied, with the results being compared to those offered by glass fiber/PP reinforced FMLs, basalt/epoxy reinforced FMLs and neat aluminum. The results show that basalt TFMLs behaved better than aluminum plates and glass TFMLs, especially for the energy level causing FC (first crack) due to a greater deformation ability of basalt fiber metal laminates.
Micromechanical Progressive Failure Analyses of Composite Materials Using Continuum Decohesive Finite Element
An efficient framework for microscopic progressive failure analysis of fiberreinforced composite is established using the continuum decohesive finite element (CDFE) method. CDFE is a novel finite element technique connecting continuum and cohesive crack modeling seamlessly. In CDFE, the transition from a continuum to non-continuum (cracked solid) is modeled by physically introducing pairs of crack dummy nodes. A static condensation algorithm is applied to solve for and preserve the crack separation information on the crack dummy nodes to facilitate the implementation of CDFE into general-purpose finite element solvers. In this paper, CDFE is applied to micromechanical progressive failure analyses on a representative volume element (RVE) of composite materials with randomly packed fibers. No crack path is predefined and the maximum principal stress criterion (MPSC) is used to determine the crack orientation and initiation. According to CDFE results, multiple microscopic cracks initiate at different locations and coalesce into a macroscopic transverse crack. By comparison with existing FE results and high-fidelity generalized method of cells (HFGMC) results, the CDFE framework proves to be accurate and efficient.
Modeling and Simulation of Carbon Composite Blast Behavior
The design of new generation aircraft is driven by the vastly increased fuel cost and the resultant imperative for greater fuel efficiency. Carbon fiber composites have been used in aircraft structures to lower weight due to their superior stiffness and strength-to-weight properties. However, carbon composite material behavior under dynamic ballistic and blast loading conditions is relatively unknown. For aviation safety consideration, a computational constitutive model has been used to characterize the progressive failure behavior of carbon laminated composite plates subjected to ballistic and blast loading conditions. Using a meso-mechanics approach, a laminated composite is represented by a selected number of representative unidirectional layers with proper layup configurations. The damage progression in a unidirectional layer is assumed to be governed by a set of strain-rate dependent layer progressive failure criteria using the continuum damage mechanics approach. The composite failure model has been successfully implemented within LS-DYNA as a user-defined material subroutine. In this study, a series of experimental, close-in shock-hole blast tests on carbon composite panels, were simulated using the LS-DYNA-ALE method integrated with the ARL progressive failure composite model, which include strain rate effects on damage and fracture. The computational constitutive model has been validated to characterize the progressive failure behavior in carbon laminates subjected to close-in blast loading conditions with reasonable accuracy. The availability of this modeling tool will greatly facilitate the development of carbon composite structures with enhanced ballistic and blast survivability1.
Effect of Transverse Compression on the Residual Tensile Strength of UHMWPE Yarns
Ballistic impact induces multi-axial loading on fiber-based armor systems in the form of axial tension, axial compression, transverse compression and transverse shear. Transverse compression during ballistic impact leads to permanent deformation and fibrillation resulting in degradation of material strength. This work investigates the effect of quasi-static transverse compression on the residual tensile strength of Ultrahigh molecular weight polyethylene (UHMWPE) yarns. Specifically, Dyneema® SK- 76 yarns are tested under quasi-static transverse compression and then under quasi-static tension. The results indicate that yarns are compressed non-uniformly under transverse compression and on an average there is only about 4% decrease in the strength of yarns compressed at average nominal strains of 77%.
Micromechanical Prediction of a Composite Failure Under Longitudinal Compression
A micromechanical approach is presented to predict strength of a composite under a longitudinal compression only in terms of the original properties of its constituent fiber and matrix materials. An initial fiber misalignment and further rotation due to matrix shear strain are taken into account in evaluating the internal stresses of the fiber and matrix through micromechanics Bridging Model. The thus obtained homogenized matrix stresses are converted into true values by means of the corresponding stress concentration factors (SCFs) of the matrix before a failure assessment against its original strength parameters is made. An efficient Mohr’s type criterion is established to determine a matrix failure and the failure orientation angle, which in turn has an influence on the fiber rotation. A fiber compressive failure is detected using the maximum normal stress failure criterion. Whichever of these two kinds of failures occurs first corresponds to the composite compressive failure. Our theory has been applied to predict the longitudinal compressive strengths of the UD (unidirectional) composites used in the worldwide failure exercises (WWFEs). The predictions correlate reasonably well with the available test data.
Simulation on Kink-Band Formation Based On X-Ray Computed Tomography Modeling
Kink-band failure is a representative failure mode of a unidirectional carbon fiber reinforced plastics (CFRP) under axial compressive loading. Unidirectional CFRP has initial fiber waviness, and it may trigger kink-band failure. However, initiation of kink-band is not clarified yet. In this paper, three-dimensional finite element model with an actual fiber waviness was constructed by means of X-ray computed tomography (XCT) imaging. Simulation on compression was carried out using the three-dimensional model. A small unidirectional CFRP cylinder was fabricated and scanned the internal fiber structure using an XCT system. The three-dimensional model of the unidirectional CFRP was developed from the XCT images by tracking each fiber positions along the longitudinal direction. Numerical simulation on compression was performed using the constructed three-dimensional finite element model. With increase of compressive loading, matrix deformation was increased locally at some volumes inside the unidirectional CFRP due to fiber random waviness. Matrix started to yield at the volumes, which was considered as an initiation of kink-band formation. The applied compressive load started to decrease after the matrix yielded showing snap-through behavior in the load-displacement relation. The yielded volume of the matrix expanded through the cross-section with rotation of carbon fibers, and kinkband was gradually formed. The numerical simulation revealed initiation and formation of kink-band in a unidirectional CFRP by the random fiber waviness model.
A Machine Learning Technique to Predict Biaxial Failure Envelope of Unidirectional Composite Lamina
A machine learning technique was used to predict static, failure envelopes of unidirectional composite laminas under combined normal (longitudinal or transverse) and shear loading at different biaxial ratios. An artificial neural network was chosen for this purpose due to their superior computational efficiency and ability to handle nonlinear relationships between inputs and outputs. Training and test data for the neural network were taken from the experimental composite failure data for glass- and carbonfiber reinforced epoxies provided by the world-wide failure exercise (WWFE) program. A quadratic, stress interactive Tsai-Wu failure theory was calibrated based on the reported strength values, as well as optimized from the experimental failure data points. The prediction made by the neural network was compared against the Tsai-Wu failure criterion predictions and it was observed that the trained neural network provides a better representation of the experimental data.
Modeling the Microbond Test of Different Sizes of Droplets to Quantify the Failure Properties of Fiber-Matrix Interface
Carbon fiber reinforced plastic composite exhibits very complex interactive failure mechanisms. To conduct the progressive damage analysis, FEA software which could simulate large scale detailed models including interfaces is required. We have developed the large-scale parallel FEM program which analysis the model using cohesive elements. It is also required to measure properties of interface accurately in order to simulate the strength of interface. The microbond test is one of techniques for evaluating failure of the fiber–matrix interface. Cohesive properties include several parameters that define the stiffness and damage. Therefore, in this study, we have modeled different sizes of droplets to quantify material properties of the fiber-matrix interface. We have explored the failure properties of the fiber–matrix interface by comparing the experiment results and the three-dimensional analysis results using the software that we have developed.
Carbon Unidirectional Composite Flexure Strength Dependence on Laminate Thickness
A study to develop fundamental failure prediction methods for high strain composites (HSCs). As their name implies, HSCs are designed to operate at strains significantly larger than traditional composites, particularly under bending. Several different phenomena at the micro-scale level contribute to this remarkable flexibility, which are not addressed by standard protocols such as those defined in CHM-17. This investigation presents a first step towards the development of new set of standard tests and failure criteria able to characterize the specific mechanics of HSCs. In particular, we present testing on the large curvature bending of HSC laminates performed using the column bend test (CBT) method. The samples are made from unidirectional intermediate modulus carbon fibers set in a toughened aerospace epoxy matrix, with the thickness varied systematically, in order to explore its effect. Samples were tested until failure and then imaged using SEM and micro-CT x-ray methods in an attempt to understand the failure mode. These results were then compared to linear multiscale material model, where the modeling methods are focused on relative fiber stress/strain relationship. Further investigation will build upon these results by incorporating fiber and matrix material nonlinearity as well as composite layup shear stabilization methods.
Numerical and Experimental Assessment of a Modified Transverse Cut Tension (TCT) Specimen for In-Situ Loaded X-Ray Computed Tomography of Mode II Dominated Composite Damage Progression
The principal means by which delamination may progress in appropriately designed composite aerospace structures is via Mode II fracture. This can arise near bolted and bonded joints, following in-service damage such as impact or following the inclusion of integrated electronics into the composite structure and manufacture-induced defects. A new specimen type based on the Transverse Cut Tension (TCT) test is proposed to allow the use of in-situ X-ray micro-computed tomography for observation of mode II fracture processes. The tomography seeks to non-destructively observe the progression of three dimensional crack front features that cannot be seen in typical aerospace composite tests. The paper comprises an account of the design using analytic and finite element methods and preliminary experimental verification. The results showed that the measured toughness of the miniaturized specimen compared well with published results for this material in the literature. In addition, a small period of stable fracture growth was found that would permit in-situ X-ray micro-tomography of progressing Mode II cracks. The work provides the means to observe three-dimensional Mode II crack fronts, potentially in the presence of friction effects, which will contribute to establishing a firmer basis for improved physics-based simulation development.
Determination of Full Elastic Constants of Carbon Fiber in Carbon Fiber Reinforced Plastic Composites
The full elastic constants of a carbon fiber in a unidirectional carbon fiber reinforced plastic composite were investigated by using a resonant ultrasound spectroscopy technique. The equivalent elastic properties of the composite were determined by comparing resonance frequency obtained from experiment and analysis. The subspace method which is the kind of the iterative method for calculating eigenvalues and eigenvectors of the real symmetric matrix such as the stiffness and mass matrix, was used to estimate the resonance frequency of the composite. The error function was calculated using the natural frequency obtained by the experiment and finite element analysis. It was found that the estimated elastic constants of the carbon fiber in the composite based on both Eshelby-Mori-Tanaka theory were reasonably consistent with the previously-reported values. The effectiveness of the method used here was confirmed by comparing the experimental results and those obtained by numerical analysis for the carbon fiber in the composite. The technique could be applicable to any other continuous fibers including those with much smaller diameter, as well as randomly oriented discontinuous fibers.
Multiscale Progressive Damage Analyses for Fiber Reinforced Composites Subjected to Biaxial Loading
A multiscale integrated Finite-Element (FE) and High Fidelity Generalized Method of Cells (HFGMC) micromechanics modeling analysis is proposed to generate failure envelopes for both unidirectional and multidirectional carbon/epoxy composite laminates subjected to biaxial loading conditions. The proposed modeling approach is implemented as a user material subroutine (UMAT) within displacement-based layered FE models, whereby each layer (lamina) is represented by a hexagonal repeating unit-cell (RUC) recognizing the fiber and matrix phases of a lamina at the microstructure. The fiber is considered to behave as an elastic transversely isotropic material. The nonlinear elastic behavior along with the J2-deformation plasticity are used to account for matrix nonlinearity. Failure of a lamina is determined by using both strain-based and stress-based failure theories. The lamina stress and stiffness contributions are eliminated from the FE model in case of a failure. The in-situ mechanical properties and damage parameters of the RUC are determined by calibration using available axial, transverse, and shear experimental responses. The efficiency of the proposed multiscale analysis is demonstrated by a comparison with measured failure envelopes available in the literature.
Coupled Thermo-mechanical Micromechanics Modeling of the Influence of Thermally Grown Oxide Layer in an Environmental Barrier Coating System
Environmental Barrier Coatings (EBCs) have emerged as a promising means of protecting silicon based ceramic matrix composite (CMC) components for high temperature applications (e.g., aircraft engines). EBCs are often used to protect an underlying material (substrate) such as silicon carbide from extreme thermal/chemical environments. In a typical CMC/EBC system, an EBC may or may not be adhered to an underlying substrate with a bond coat (e.g., silicon). Irrespective, systems that utilize EBCs are susceptible to a number of failure modes including oxidation/delamination, recession, chemical attack and dissolution, thermomechanical degradation, erosion, and foreign object damage. Current work at NASA Glenn Research Center is aimed at addressing these failure modes in EBC systems and developing robust analysis tools to aid in the design process. The Higher-Order Theory for Functionally Graded Materials (HOTFGM), a precursor to the High- Fidelity Generalized Method of Cells micromechanics approach, was developed to investigate the coupled thermo-mechanical behavior of functionally graded composites and will be used herein to assess the development and growth of a lowstiffness thermally grown oxide (TGO) layer in EBC/CMC systems without a silicon bond coat. To accomplish this a sensitivity study is conducted to examine the influence of uniformly and nonuniformly grown oxide layer on the associated driving forces leading to mechanical failure (spallation) of EBC layer when subjected to isothermal loading.
Meso-Scale Strain Measurements in Fiber Reinforced Composites
Effective bulk properties of fiber-reinforced composites can be determined from individual constituent properties through spatial homogenization. Homogenization, in this regard, is conducted over a specifically selected volume of the material that is sufficiently small to capture complex local deformation response, while large enough to encompass all individual components, i.e. fibers and matrix. The physical dimension of such homogenization volumes is a key parameter in multiscale studies. Experimental measurement of the length scale at which the transition from micro to macroscale response occurs is challenging. In the present study, we propose a systematic approach to estimate the physical dimensions of a micro-to-macro transition length scale in terms of the number of fibers in the transverse plane of a cross-ply laminate subjected to remote tensile load. In-house fabricated cross-ply composite samples are loaded in tension in a miniature tensile frame inside a scanning electron microscope, while images are acquired from a small area of interest located on the transverse ply. Digital Image Correlation (DIC) is utilized to obtain full-field strain distribution within the area of interest at various global stress/strain intervals. Spatial averaging of strains at mesoscale is used to determine the micro-to-macro transition scale.
VCCT with Progressive Nodal Release for Simulating Mixed-Mode Delamination: Formulation, Algorithmic Improvements and Implications
To simulate the propagation of cracks using the Virtual Crack Closure Technique (VCCT), intermediate crack positions between existing node pairs can be modeled by progressively releasing the nodes using fracture mechanics principles. This approach has been implemented in commercial software and can be applied to simulate delamination growth. Recent research has provided insight into the synchronized mixed mode nodal release, extending and formalizing the methodology for delamination growth. This paper documents the development of the Jacobians associated with delamination propagation under quasi-static mixed-mode conditions, leading to improvements in convergence of the implicit nonlinear solvers used.
Benchmarking Mixed Mode Matrix Failure in Progressive Damage and Failure Analysis Methods
The verification and validation of progressive-damage-analysis finite element methods are difficult but critical tasks to undertake during their development. Verification exercises assess whether a predictive analysis tool produces results that are consistent with the fundamental concepts and assumptions of the tool under evaluation. Ideally, closed-form analytical solutions can be derived for which method verification results can be compared. Problems selected for computational tool verification are often simple and isolate individual features of the tool. In the case of progressive damage finite element methods, verifications should be performed to evaluate the ability of the model to predict the initiation of damage and its growth through the finite element mesh under a variety of conditions.
Buckling Performance Optimization of Steered Composite Panels while Accounting for Manufacturing Constraints
Automated Fiber Placement (AFP) technology has revolutionized manufacturing of composite structures and is quickly replacing traditional hand-layup manufacturing. Among others, the notable advantage of AFP is that the designer is no more restricted to the use of straight ber paths, enabling optimizing for desired structural performance. The scope of this work is to identify the manufacturing parameters and associated constraints imposed by these parameters on the manufacturability of steered ber structural panels, referred to as the manufacturing signature (MS), using currently available AFP machines. This is demonstrated by an optimization problem to maximize the buckling load of a panel with steered bers. Further, deviations of as- manufactured panels from the ideal pristine designs of these optimal solutions are discussed and an investigation on their e ects on performance of the manufactured panel is presented.
Optimization of Fiber Arrangement Around Circular Hole Considering Curve Shaping by AFP
In the fiber reinforced composite laminates, breakage due to stress concentration around the open-hole is a big problem. It cannot be said that conventional linear lamination can fully exploit the characteristics of anisotropic material, but there is a possibility that this stress concentration can be reduced by curved lamination by automated fiber placement (APF). In this study, we optimized the curved prepreg tape arrangement by genetic algorithm (GA) so that the strength of the open-hole-plate would be improved, on the premise that the prepreg tape could be laminated in a curved shape by AFP. As the result of optimization, the strength could be improved with respect to linear lamination. The arrangement was such that the bottom tape concentrates on top of the circular hole edge. In addition, as the maximum allowable curvature was increased, the strength was improved. This result could be used to improve the strength around open-holes such as windows of aircraft.
Optimum Design of Lay-Up Configuration and Ply Drop-Off Placement for Tapered Composite Laminate
The present study optimizes lay-up configurations and drop-off placements to maximize the failure strength of tapered laminates. A lookup table of stress components for calculation of Christensen’s criteria is prepared in advance for all possible combinations of 3-ply laminates which is the minimum structure of ply dropoff. Stress components at three corners of resin pockets modeled triangular are first obtained from the finite element analysis (FEA). Then, the optimization is conducted for the laminate which tapers from 16- to 8-ply by using a simple genetic algorithm method. Objective functions are evaluated by referring only stress indexes listed in the lookup table and the FEA are not performed during the optimization. Both thick and thin sections are limited to quasi-isotropic laminates with 45° increment angle, but a stacking sequence of plies is variable and optimized. Placement of ply drop-off are also design variables. It was revealed that 0° and 90° plies are dropped near the thick and thin sections for the optimum result. Experimental results validated that the optimum structure has higher failure strength than the reference one.
Modeling of Polymer/Carbon Nanotube Nanocomposite to Estimate Structural Damping in a Rotorcraft Blade
The development of next-generation rotorcraft requires advancement in certain key areas such as cruise speed, payload, range, along with cost considerations. Coaxial and tiltrotors are two promising candidates for future of high speed vertical flight. Traditional lag dampers are unable to produce adequate levels of damping for the rigid fiber composite blades of coaxial rotorcraft. A promising approach for achieving lightweight damping in composite blades is to add carbon nanotubes (CNTs) to the matrix material. The current investigation aims to predict the damping behavior of a rotating beam that represents a simplified fiber reinforced polymer/ CNT nanocomposite blade. The developed model takes in account different physical variables such as the critical shear stress ( ) for slip onset of the CNTs and CNT aspect ratio (l/d), volume fraction ( ), and orientation. The study first looks at a case where the nano-inclusions are aligned with the direction of loading for the rotor blade. The model predicts maximum damping for low in combination with relatively low l/d. The damping trend increases with increase in of the nanoinclusions. A damping ratio of 7.3% and 8% was observed with optimally configured parameters for aligned and randomly oriented CNTs respectively. Overall, the results suggest significant damping augmentation from the CNTs.
Improving the Interlaminar Strength of Carbon Fiber Reinforced Polymer Composite Laminates Using Cellulose Nanocrystals
This study focuses on enhancing interlaminar strength of hybrid (micro/nano) carbon fiber reinforced polymer (CFRP) composite laminates by coating the carbon fabric (CF) layers with cellulose nanocrystals (CNC). Although carbon fibers offer one of the best strength and stiffness to weight ratio among reinforcing fibers, excellent in-plane properties of CFRP composite laminates are often damped by their inferior out-of-plane properties that may result in early delamination and catastrophic failure. In order to improve CFRP interlaminar shear strength, we coat the CF layers with CNC by immersing them in a CNC aqueous suspension bath prior to impregnation via vacuum assisted resin transfer molding (VARTM). Flexural and short beam tests were done for composites with different concentration of CNC and their results were compared.
Experimental Evaluation of Carbon Nanotubes for High-Stiffness Damping Augmentation in Carbon/Epoxy Composites
Carbon nanotubes (CNTs) are promising materials for increasing the damping of fiber reinforced composites on account of a hypothesized stick-slip dissipation mechanism. The current investigation aims to increase the damping of a carbon/epoxy composites by adding different types and amounts of CNTs to the interlayer regions. [90] , [0] and [0/±45] carbon/epoxy laminates were manufactured using various types and concentrations of CNTs and surfactant. Dynamic behavior was characterized in terms of the storage and loss moduli and loss factor under tensile cyclic loading. For [0] and [0/±45] laminates, the maximum increases in loss factor and loss modulus of roughly 400-600% were obtained with 10 volume percent CNT yarns aligned in the 0-deg. loading direction. The storage modulus of these laminates was not appreciably affected by the CNT yarns. Aligned CNT yarns provided higher damping than equivalent amounts of unaligned CNT buckypapers. In a series of tests on 0-deg. laminates with and without aligned CNT yarns, damping increased with strain amplitude but not with mean strain, while damping in the baseline laminate was nearly invariant with strain. This series of tests also detected no lower threshold of strain for the onset of slip by CNT and a 10x reduction in the rate of increase of damping with strain amplitude when the amplitude exceeded 190 Î¼ï¥.
Multiscale Modeling of Crack Formation in Composite Laminates with Manufacturing Defects
As a part of a hybrid approach to composite materials damage and failure analysis combining synergistic damage mechanics and peridynamics, the work presented here deals with multiscale modeling of the formation of cracks. The failure events in a ply of a composite laminate are analyzed from the earliest stages induced by a brittle cavitation phenomenon to the emergence of a crack of a continuous plane. Manufacturing induced fiber distribution disorder is simulated by statistical methods. The loading condition considered is quasi-static transverse tension. The earliest failure event of brittle cavitation in an epoxy matrix causing fiber/matrix debonding is studied by molecular dynamics simulation to understand the effect of stress triaxiality. The energy release rates of debond cracks are examined in the presence of other fibers surrounding a debonded fiber to study the effect of interfiber spacing on the debond growth and its kinking out of the interface and eventually causing linkup with the neighboring debonds. The growth of a potential transverse crack in an environment of distributed fibers is studied by the J-integral method.
Identification of the Dynamic Behavior of Composites Using the Virtual Fields Method
Polymer matrix composites containing continuous carbon fibers are becoming more prevalent in the construction of aerospace structures due to their high strength to weight ratios when compared to metallic counterparts. In that regard, understanding damage detection and loss of stiffness of composite panels under dynamic loading environments is critical. These materials are particularly challenging to investigate due to their hierarchical structure and potentially ratedependent and nonlinear behavior, stemming from the constituent components. Thus, full-field experimental measurement techniques are particularly advantageous as they allow for a wealth of data to be acquired, including heterogeneous kinematic fields. To that end, this study presents a unique analytical and experimental framework used to examine the dynamic damage evolution of composite panels by combining the grid method (a full-field measurement technique) with the virtual fields method (an inverse identification technique).
Intraply Fracture in Fiber-Reinforced Composites: A Peridynamic Analysis
In fiber reinforced composites (FRCs), microcracks often initiate at the interface between fibers and the matrix, or from pores in the matrix. With an explicit microstructure representation, a peridynamic model can simulate the initiation, growth, and coalescence of intraply microcracks into macrocracks in the transverse loading of a FRC. The size of the simulated sample, in this case, is limited by the computational cost. For lamina-level simulations, we employ a homogenization strategy for the transversely loaded FRC with a peridynamic model, the Intermediately Homogenized Peridynamic (IH-PD) model for two-phase composites. We compare our results with three-point bending experiments from the literature. The results show that the IH-PD model mimics the tortuous crack path observed experimentally, without having to rely on the detailed microstructure. Moreover, the IH-PD results for the load versus crack opening displacement match very well the experimentally measured data.
A New Approach to Alleviating Mesh Size Independence in Multiscale Fatigue Life Prediction in Composites
This paper presents a new reduced order multiscale computational methodology for prediction of damage accumulation and failure in composite materials and structures. The proposed reduced order modeling approach relies on the idea of prescribing a finite number of discrete failure paths that can occur within the material microstructure, and tracking the evolution of damage within these failure paths as a function of loading. The key contribution and the novelty of the approach compared to existing and similar formulations is that the failure paths are expressed as surface morphologies, within which the failure is tracked by traction-separation laws, as opposed to unit cell sub-volumes, where the failure is tracked by continuum damage mechanics models. The homogenization and localization operations are performed by employing computational homogenization principles. This paper introduces the formulation of the proposed reduced order modeling approach in the context of a unidirectionally reinforced composite unit cell. The capabilities of the multiscale model in capturing damage propagation and fracture are demonstrated using lamina and laminate configurations including open hole specimens. This paper particularly focuses on the analysis of the approach with regard to mesh sensitivity of damage accumulation when subjected to static and cyclic loading conditions. We demonstrate that the proposed formulation achieves mesh consistency.
Effect of Automated Fiber Placement (AFP) Manufacturing Induced Imperfections on Composite Performance
Automation of composite manufacturing provides speed, agility and precision to produce large aerospace and automotive structures. While automated fiber placement (AFP) is a key technology, there are still barriers to widespread industrial adoptions. One such barrier is the understanding of processing parameters on development of unintended imperfections during manufacturing. In this paper, experimental results on influence of process induced imperfections when using AFP technology will be presented. Controlled carbon fiber composite panels with imperfections like gaps, overlaps and missing tows were manufactured using AFP machine. Panels without imperfections were also manufactured using traditional hand-layup, for baseline measurements. Microscopy was also conducted for accurate measurement of imperfection microstructure produced due to curing and consolidation. Specimens were cut from the panels for tests in tension and compression. The paper will report initial test results for variation of tension and compression stiffness and strength, due to the presence of manufacturing induced imperfections.
Effect of Stacking Sequence on Compressive Strength Reduction of Aircraft Composite Structures
Compressive strength reduction of composite structures due to notches or impact damage is an important factor for aircraft structure design. Compressive strength reduction is caused by stress concentration around notches such as skin manholes or impact damage due to tool falling during manufacturing. In particular, the compressive strength reduction due to the edge-on impact is large, so it is one of the most important issues for strength design of the aircraft. In this study, in order to evaluate the effect of stacking sequence on compressive strength after edge-on impact, edge-on impact survey test and compression after edgeon impact test using CFRP laminates with different stacking sequence were conducted. As a result, in the Soft laminate, including many ±45°plies, cracks due to edge-on impact spreading in the laminate thickness direction could be observed, and detectability of the damage was better than that of other specimens. And the residual strength after edge-on impact was improved in Hard3 laminate which retained laminate stiffness along the loading direction while suppressing the edge-on impact energy that necessary to apply the dent depth equivalent to the barely visible impact damage (BVID). These results show the possibility of improving the compressive residual strength by the optimization of stacking sequence.
Ply-Orientation Dependence of Notched Strength of Multidirectional CFRP Laminates and Prediction Using a Finite Fracture Mechanics Model
A method for predicting the fracture toughness of a given laminate of any stacking sequence on the basis of the fracture toughness values of the constituent plies is proposed. This method is combined with the finite fracture mechanics (FFM) model to predict the notched strengths of multidirectional laminates with balanced off-axis layers. Static tension tests are carried out on the specimens with a circular open hole for different values of hole-diameter to specimen-width ratios, respectively. The angleply laminates exhibit different notch sensitivities depending on their stacking sequences. The use of the ply-based fracture toughness values of laminates with the FFM model predicts slightly smaller values of notched strength compared with the experimental results, regardless of notch size. The accuracy of prediction using the FFM model can be improved by taking into account the energy release due to local delimitation.
Multiscale Analysis of CFRP Laminates Including the Effect of Fiber Waviness
This paper proposes a numerical analytical method for carbon fiber reinforced plastic (CFRP) laminates, where fiber orientation is disturbed due to manufacturing defect. In the CFRP laminates, it is not unusual to find internal microscopic defects including fiber waviness, voids, or resin pockets. Among them, fiber waviness has a strong effect to the compressive strength of the CFRP. Therefore, it is very important to simulate the effect of the fiber waviness on the stress distribution in the CFRP laminates. In this paper, the authors propose a numerical method which can include various modes of fiber waviness by simple method. In this model, fiber waviness is introduced as a combination of geometrical translation of the position of the node, and change of the material orientation. The analytical method is formulated based on homogenization method together with finite element method, and fiber waviness is considered by using perturbation method. Fiber waviness is introduced as an input to the calculation, and the calculation is always done using the idealized perfect geometry, thanks to the utilization of perturbation method. This has a distinct advantage in numerical calculations where the same mesh can be used for a series of fiber waviness. The method is implemented using general-purpose finite element code ABAQUS. The proposed method is validated by comparing the results to those of a conventional calculation. The results demonstrate that the proposed method can accurately capture the stress distribution when the amplitude of the imperfection is small.
Characterization of Polymer Matrix Composite Ply Thickness
The ply thickness variation in the layup of polymer matrix composites has not been previously characterized. In this article, we used a normal distribution to characterize the ply thickness of a carbon fiber reinforced polymer. We used thresholding of optical microscopy images to identify the interlaminar boundaries within the specimen. The distance between interlaminar boundaries created the sample set to estimate the parameters of a normal distribution. We examined the resulting distribution using quantile-to-quantile plots, which indicated that ply thickness is well characterized by a normal distribution.
Nano Mechanical Testing for In Situ X-CT Observation of CFRP
Carbon fiber reinforced polymer (CFRP) composites are of growing use in aircrafts owing to their high specific strength and stiffness. Micro-mechanism of damages and microscopic chemical properties of the CFRPs is a key to understand the mechanical properties and durability of these materials. Especially the initiation and propagation along fiber/polymer interfaces is of great importance. However, most experimental observations have been carried out for surfaces (cross sections) of CFRP specimens typically using electron microscopes, where the crack propagation along the fibers, i.e. along the thickness direction of a specimen, cannot be observed. Recent reports pioneered non-destructive and three-dimensional (3D) observation using X-ray computed tomography (X-CT), but their special resolutions are limited down to ca. few or sub μm. Here we have succeeded in non-destructive and 3D observation of cracks initiation and propagation with a resolution of less than 50 nm under an applied stress with a new x-ray transmission microscope (XRM) using synchrotron radiation. Optimization of apparatus and experimental techniques of mechanical testing is the key for in situ X-CT observation, which is especially a challenging task in the case of X-CT with a high resolution less than μm. We have developed techniques of nano-mechanical testing for in situ X-CT observation by indentation with a diamond edge to initiate and propagate cracks, or tensile testing. Using the nano-mechanical testing for in situ XCT, we have succeeded in non-destructive and 3D observation of the initiation and propagation of cracks with a resolution down to 50 nm under an applied stress.
In situ X-CT Observation of Crack Initiation and Propagation in CFRP with X-ray Microscopy
Carbon fiber reinforced polymer (CFRP) composites are of growing use in aircrafts owing to their high specific strength and stiffness. Micro-mechanism of damages and microscopic chemical properties of the CFRPs is a key to understand the mechanical properties and durability of these materials. Especially the initiation and propagation along fiber/polymer interfaces is of great importance. However, most experimental observations have been carried out for surfaces (cross sections) of CFRP specimens typically using electron microscopes, where the crack propagation along the fibers, i.e. along the thickness direction of a specimen, cannot be observed. Recent reports pioneered non-destructive and three-dimensional (3D) observation using X-ray computed tomography (X-CT), but their special resolutions are limited down to ca. few or sub μm. Here we have succeeded in non-destructive and 3D observation of cracks initiation and propagation with a resolution of less than 50 nm under an applied stress with a new x-ray transmission microscope (XRM) using synchrotron radiation. Using the phase-contrast imaging technique, we have succeeded in non-destructive and 3D observation of the initiation and propagation of cracks with a resolution down to 50 nm under an applied stress. Fibers, matrix polymer, and cracks were clearly identified in reconstructed 3D images. This first observation has successfully shown that cracks are initiated by (a) de-bonding at the fiber/polymer interfaces and (b) void formation in the polymer, and that the two factors compete with each other and determine the propagation path.
Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Structures
Aeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology based on analysis and experiments to assess and improve the strength, life, and damage tolerance of stiffened composite structures subjected to postbuckling deformations. Two regions were identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deflection and the region of maximum twisting. Both regions have been studied using a finite element model of a representative single-stringer specimen. For the region of maximum deflection, a seven-point bending configuration was used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting was studied using an edge crack torsion configuration, with two supports and two loading points. These two configurations were studied by changing the positions of the supports and the loading points. An optimization procedure was carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the fourstringer panel.
XIGA based Intralaminar and Translaminar Fracture Analysis of Unidirectional CFRP Laminate
Intralaminar and translaminar fracture behaviour of unidirectional carbon fibre reinforced composite laminate is investigated numerically and experimentally in the present work. Experimental determination of fracture properties associated with failure mode along the fibre direction is very important for material characterization and numerical study. The experimental fracture toughness testing is performed on compact tension (CT) specimens of unidirectional CFRP laminate manufactured by laying up 26 layers of prepreg (Hexply 914/34%/UD 160/AS4-12K). The Mode I intralaminar and translaminar fracture tests have been performed using a universal testing machine (UTM) with a loading rate of 0.1 mm/min. Fractured surfaces have been examined using scanning electrode microscope (SEM). A NURBS-based Extended Isogeometric Analysis (XIGA) code has been developed for fracture analysis of orthotropic laminate. The XIGA code is capable of an efficient analysis of crack problems using non-uniform rational B-spline basis functions for both the solution field and geometric description of the model. The level set approach is used to track the crack geometry. Crack faces are enriched by Heaviside enrichment function and crack tip singularity is captured by asymptotic crack tip enrichment functions. The domain-based interaction integral method is used to extract the stress intensity factor values. The numerical results for compact tension (CT) specimen shows a good agreement with experimental results. In case of experiment as well as numerical modelling, the initiation of crack propagation is estimated with accuracy.
Progressive, Large-Scale Damage Modeling in Ultra Short Fiber Tailorable Feedstock Composite Materials
This study combines unique modeling approaches to simulate the progressive failure of ultra short carbon fiber composite materials. The reinforcement phase in traditional chopped fiber composites generally consists of glass fibers 10 μm in diameter with lengths on the order of 15 mm in length. Ultra short carbon fiber composites with lengths of 3 mm show promise as a versatile inexpensive reinforcement. Furthermore, some level of microstructural design may be achieved by inducing uniform alignment in the ultra short fiber arrays. In order to investigate progressive failure of various microgeometries, finite element based micromechanical approaches are utilized to perform virtual tests on highly detailed microstructural representations. This study demonstrates viable approaches to build models large enough to capture all relevant micromechanical damage effects, especially the interactions between fibers across the entire domain. Both the Abaqus/Standard and Abaqus/Explicit finite element codes are used together to efficiently simulate progressive damage on a large scale. Careful parametric analysis elucidates the effects of various microstructural factors, including: fiber-matrix interface effects, fiber diameter, chopped fiber length, uniform fiber alignment / misalignment, occasional fiber misalignment, fiber spacing, and the constituent properties of both fiber and matrix. Furthermore, the generation of geometries and meshes is accomplished programmatically, which enables the simulated testing of a large and high-resolution parameter space. Trends in microstructural effects on material performance will be carefully analyzed to provide guidance for processing targets to achieve desired properties in tailorable feedstock.
Thermal Failure of Composites under Heat Flow
In this paper, theoretical investigation to predict the overall failure condition of particulate-reinforced composites due to thermal stress is presented assuming that the composite fails by yielding. The composite is subject to heat flux which becomes uniform at infinity. The thermal stress for a single inclusion problem is solved first and this solution is used to estimate the thermal stress for the composite with multiple inclusions by replacing the properties of the matrix by those of the composite. The maximum von Mises stress is computed at the interface of the inclusion and the matrix to derive the overall failure criterion of the composite.
Implementation of a Matrix Crack Spacing Parameter in a Continuum Damage Mechanics Finite Element Model
Continuum Damage Mechanics (CDM) based progressive damage and failure analysis (PDFA) methods have demonstrated success in a variety of finite element analysis (FEA) implementations. However, the technical maturity of CDM codes has not yet been proven for the full design space of composite materials in aerospace applications. CDM-based approaches represent the presence of damage by changing the local material stiffness definitions and without updating the original mesh or element integration schemes. Without discretely representing cracks and their paths through the mesh, damage in models with CDM-based materials is often distributed in a region of partially damaged elements ahead of stress concentrations. Having a series of discrete matrix cracks represented by a softened region may affect predictions of damage propagation and, thus, structural failure. This issue can be mitigated by restricting matrix damage development to discrete, fiber-aligned rows of elements; hence CDM-based matrix cracks can be implemented to be more representative of discrete matrix cracks. This paper evaluates the effect of restricting CDM matrix crack development to discrete, fiber-aligned rows where the spacing of these rows is controlled by a user-defined crack spacing parameter. Initially, the effect of incrementally increasing matrix crack spacing in a unidirectional center notch coupon is evaluated. Then, the lessons learned from the center notch specimen are applied to open-hole compression finite element models. Results are compared to test data, and the limitations, successes, and potential of the matrix crack spacing approach are discussed.
Discrete Damage Modelling of Clamped Tapered Beam Specimen Under Fatigue Loading
Clamped Tapered Beam (CTB) sub-element, proposed by Advanced Composite Project Team at the NASA Langley Research Center was studied under monotonic fatigue loading. The specimen was designed to study matrix crack initiation from pristine condition, delamination initiation and propagation including migration from one interface to other. Regularized eXtended Finite Element Method (Rx-FEM), which allows modeling displacement discontinuities associated with matrix cracks independent of the mesh orientation was used for analysis. Previously developed fatigue Cohesive Zone Method (CZM) was modified to include the S-N information in the Newton-Rapson equilibrium loop. The analysis was conducted in two phases, before and after the experimental data was revealed. The blind predictions were conducted at R=0.1 load ratio for 80% and 70% values of the fatigue load amplitude with respect to static strength and overestimated the rate of delamination growth. The corrected predictions took into account the R=0.2 load ratio, thermal residual stresses and were based on the modified algorithm. The failure event sequence and the respective cycle counts were consistent with experimental data.
Discrete Damage Modeling of Matrix Dominated Failure Including Random Spatial Variation of Strength
Discrete Damage Modeling (DDM) was applied to strength prediction of three types of composite tape specimens exhibiting rather brittle behavior. These were transverse tensile coupons, three-point bend 90° coupons and NASA LaRC Clamped Tapered Beam sub-element. The performed strength predictions are sensitive to the value of the transverse tensile strength Yt. Deterministic strength predictions required different values of Yt for realistic prediction of strength for the three specimen categories. Weibull scaled seeding of transverse tensile strength was introduced to address this problem. Cohesive Zone Method (CZM) in the field of random initiation strength distribution was examined and revealed that a finite seed length is required in order the reproduce brittle behavior. A 0.4mm seed length window was applied and resulted in realistic predictions of strength in all three specimens based on the Yt=64MPa measured on standard ASTM 90° coupons and Weibull modulus of a=13.
Bmanc-A Versatile Software for Failure Analysis of a Composite Structure Essentially Upon Original Constituent Properties
A three-dimensional elastoplastic micromechanics Bridging Model is implemented into a subroutine in ABAQUS, named as BMANC. All of the formulae involved are analytical, and are in closed form if a planar problem is concerned. A unique feature of this model is its consistency in the internal stress calculation. Namely, the internal stresses in the fiber and matrix of a composite calculated by the 2D (two-dimensional) and 3D (three-dimensional) bridging model formulae are exactly the same if the composite is subjected to a planar load. This feature is not sheared by any other micromechanics model such as finite element method, meaning that a full 3D approach should be employed if the other model is used in analysis. Another feature is that Bridging Model is among the most accurate micromechanics models. The most important feature is that the homogenized internal stresses of the matrix must be converted into true values before an effective property of the composite is calculated in terms of the original fiber and matrix properties. The conversion for all of the stress components has been achieved and summarized in the paper. The prediction results are validated with corresponding experiment data.
Survey of Sensitivity Analysis Methods During the Simulation of Residual Stresses in Simple Composite Structures
Process-induced residual stresses occur in composite structures composed of dissimilar materials. As these residual stresses could result in fracture, their consideration when designing composite parts is necessary. However, the experimental determination of residual stresses in prototype parts can be time and cost prohibitive. Alternatively, it is possible for computational tools to predict potential residual stresses. Therefore, a process modeling methodology was developed and implemented into Sandia National Laboratories’ SIERRA/Solid Mechanics code. This method requires the specification of many model parameters to form accurate predictions. These parameters, which are related to the mechanical and thermal behaviors of the modeled composite material, can be determined experimentally, but at a potentially prohibitive cost. Furthermore, depending upon a composite part’s specific geometric and manufacturing process details, it is possible that certain model parameters may have an insignificant effect on the simulated prediction. Therefore, to streamline the material characterization process, formal parameter sensitivity studies can be applied to determine which of the required input parameters are truly relevant to the simulated prediction. Then, only those model parameters found to be critical will require rigorous experimental characterization. Numerous sensitivity analysis methods exist in the literature, each offering specific strengths and weaknesses. Therefore, the objective of this study is to compare the performance of several accepted sensitivity analysis methods during the simulation of a bi-material composite strip’s manufacturing process. The examined sensitivity analysis methods include both simple techniques, such Monte Carlo and Latin Hypercube sampling, as well as more sophisticated approaches, such as the determination of Sobol indices via a polynomial chaos expansion or a Gaussian process. The relative computational cost and critical parameter list are assessed for each of the examined methods and conclusions are drawn regarding the ideal sensitivity analysis approach for future residual stress investigations.
Analysis of Open Hole Tensile Strength in a Prepreg Platelet Molded Composite with Stochastic Meso-Structure
The variability of open-hole tensile strength of a prepreg platelet molded composite with stochastic meso-morphology and deterministic platelet size was simulated by progressive failure analysis. Continuum damage mechanics was used to model the constitutive response of the platelet material. The analysis showed that openhole coupon fracture may occur at or away from the hole, depending on the stochastic meso-morphology details. It was also demonstrated that as the notch diameter is increased, the probability of fracture at the notch is increased.
Comparison of Fiber Microstructural Characteristics for Two Grades of Carbon Fiber Composites
One of research thrusts to push the limits of advanced fiber reinforced composites is to determine the link between manufacturing, resulting microstructure, and final structural properties. By bridging the gap between these topics, not only can we better understand how and why composites structurally work as they do, but we can potentially tailor the manufacturing processes for a desired resultant set of properties. To better illuminate the effects of manufacturing on microstructure and microstructure on properties, computational models are often employed. Using these models, we can gain insight on relationships that may otherwise remain unexplored. This research thrust is often labelled as ICME, integrated computational materials engineering.
Variability of Mechanical and Dielectric Properties in Testing Electrospun PAN Nanofiber Mat
Electrospinning technology has been found to have applications in various fields including medical, structural reinforcement, filtration, storage, energy, and others. Electrospinning has been successfully used to develop aligned micro- and nano-fiber mats for lightweight reinforcement of composite materials and structures. A horizontal apparatus has been designed and fabricated for improved micro- and nanofiber alignment that provides the possibility of developing better reinforcement materials in the form of aligned fiber mats. These nano fiber mats are stiffer, stronger and have better mechanical, electrical and thermal properties for structural and multifunctional applications. Understanding the behavior of nanofibers at its nano scale is important for realizing benefits in these fields. In this research, the force rate behavior of nanofibers; its toughness properties, dielectric properties, specimen geometries, measurement challenges and characterizing failure modes at the failure regions have been demonstrated.
Nanocomposites: Manufacturing, Microstructural Characterization and Mechanical Testing
Carbon Nanotubes (CNT) offer exceptional thermal, electrical and mechanical properties. While an increase in thermal and electrical conductivity can be readily achieved by addition of CNT to a polymer base, the subsequent effect on mechanical properties must be investigated. In this study, nanocomposite samples were manufactured using injection molding process. Multiwall Carbon Nanotube (MWCNT) masterbatch with 15 wt.% MWCNT concentrations were diluted with PA 6/6 pellets to create five different CNT concentration ranging from 3 wt.% in 3 wt.% increments. The neat polymer sample was also manufactured as a control specimen. Mechanical properties such as Young’s modulus, Tensile strength and elongations were determined to see the effect of CNT content on overall properties. Scanning Electron Microscopy (SEM) images were used to evaluate the uniform distribution of CNT in the polymer phase. The results showed that the stiffness increased as the CNT content increased, however, the increase in strength reached a threshold value around 6 wt.% beyond which the strength decreased. It was observed that the elongation decreased significantly by addition of CNT into the polymer. The elongation dropped from an average of 190% for the neat sample to 5% for 15 wt.% CNT content sample. Such decrease in elongation might render the polymer unsuitable for the application it has been designed for. The findings of this study show that improving thermal and electrical properties of polymers does not come without a sacrifice on mechanical properties.
Fabrication of Cellulose Nanofiber/Glass Fiber-reinforced Composites and Their Bending Behavior Evaluation
It is required a joint technic with excellent mechanical properties to fabricate glass fiber reinforced plastic (GFRP) components. However, it is well-known that the mechanical jointing often causes the fracture of fiber reinforced plastics. It seems that the adhesive bonding is suitable to joint GFRPs, and it is necessary to develop an adhesive with outstanding mechanical properties. Recently, cellulose nanofibers (CNFs) have been obtained from plants by mechanical and chemical methods and it has been reported that CNFs have remarkable mechanical properties. In this study, we inserted epoxy resin with CNFs between GFRP and investigated its bending behavior, to estimate the capability of epoxy resin with CNFs as an adhesive for GFRPs. The flexural behavior and modulus of GFRP was similar regardless of the inserted part of epoxy resin with CNFs layer in GFRP. The flexural strength of GFRP was drastically increased (up to 600 MPa) by the insert of epoxy resin with CNFs of 5 or 10 wt.%, although that of GFRP was decreased by the insert of epoxy resin with CNFs of 25 wt.%. Therefore, it seems that the epoxy resin with CNFs is available as an adhesive for GFRP. However, it is required the observation of CNFs dispersion in epoxy resin to understand the strengthening mechanism of epoxy resin with CNFs.
Atomistically-informed Continuum Modeling of Damage Mechanisms in Radially-Grown CNT Nanocomposites
A micromechanical modeling framework that integrates an interphase damage model is used to study damage evolution, and failure in radially-grown carbon nanotube (CNT) reinforced nanocomposites. The atomistically-informed damage model is developed using the continuum damage mechanics approach with damage evolution equations derived using molecular dynamics simulations. The developed damage model is integrated with a high-fidelity micromechanical analysis and captures the underlying physical behavior that could be attributed to the enhancement of the out-ofplane properties at the higher length scales. To illustrate the effects of nanoarchitecture, the response of the radially-grown CNT nanocomposite is compared with that of the traditional carbon fiber reinforced polymer (CFRP) composite.
Multi-functional Nano-porous Ceramics1958
Ceramics are stiff, hard, lightweight, and thermally stable (> 1000 oC), but are rarely used in structural components due to their brittleness. If toughened, ceramics can be an effective, light-weight alternative to existing metal components that are currently used for high-temperature applications and/or for protection coatings against heat and radiation. Recent investigations on toughening of ceramics has been mostly focused on introducing particles, fibers, and whiskers to arrest and/or deflect crack initiation and propagation. Another common toughening method is compositing with ductile phases, such as metals and polymers, but thermal stability of the resulting composites is lower. In this work, a novel toughening method is attempted by introducing nano-porosity to monolithic ceramics without degrading thermal stability. Traditionally, pores are considered as defects, but when pores are very small (<~100 nm), the nano-pores are observed to deform locally in a non-propagating manner. Thus, fracture toughness can be potentially increased by such quasi-plastic deformations uniquely triggered by locally weak nano-porosity. In this project, we study to understand the fundamental toughening mechanisms of ceramics that arise from the introduction of nano-pores, and to study a scalable manufacturing method of these novel tough and strong ceramics. If achieved, tougher ceramics will be in high demand as improved lightweight alternatives to metal alloys in structural applications that require mechanical strength, stability in thermal and corrosive environment, such as engines, gas turbines, thermal protections, nuclear and solar energy and biomechanics.
Material State Monitoring using Embedded Sensors for Validating Models for Detecting Process-Induced Damages in Polymer Composites
Composite material cure recipes are based on manufacturer-recommended cure cycle inputs, which are developed as a result of various stochastic cure kinetics models during the development of resin formulation. The time, temperature, pressure, and vacuum integrity are inputs for autoclave cure, but none of them are direct measurements of the material state and have no direct correlation to viscoelastic characteristics or mechanical performance of the cured material. The proposed process development system includes autoclave controls that are modified so that the set points are controlled based on externally determined material state of the laminate in the autoclave, which is monitored remotely by a rheological measurements and a material state estimate based on the process and cure state models. With the proposed process-control architecture, material will be cured to achieve the desired viscoelastic and mechanical properties with minimizing the variability cause by the initial material state (ex., shelf- and out-life) by monitoring the rheological response and managing the autoclave temperature to achieve the desired cure state. It also directly measures the initial viscoelastic state, which is a measure of material performance at the point of production rather than arbitrarily constraining the process solely to a time-temperature history, in order to achieve an assumed cure state. Furthermore, the methodology can also be used for interrogating the manufacturing-induced defects of polymer composites in a controlled environment (discrepancy analysis) and link them to boundary conditions. The material-state database developed through validated process models brings about the virtual environment, where the actual boundary conditions can be interrogated during the cure, to ensure that the material achieves its optimal performance. The database approach alleviates using the computationally-heavy simulations (ex., fluid dynamics) for every cure cycle.
Effect of Geometrical Imperfections on Structural Integrity of Laminated Composite Structures: Experimental Approach and Characterization
An experimental approach is developed to mimic actual geometrical imperfections in composite samples with introduced but controlled idealized through-thickness waviness defined by different levels of their severity. The samples are then tested under typical representative load conditions to correlate the level of imperfections with measured metrics of structural integrity, such as damage initiation, damage growth, damage networking, etc. Generated test results are evaluated to make qualitative and quantitative conclusions regarding the effect of such imperfections. Demonstration of the approach is performed on examples of thick laminated composite beams under representative 3- and 5-point bending load conditions. Commercially available IM7/8552 carbon-fiber composites are considered as typical materials with four levels of imperfections. It is observed that, under bending conditions with dominant interlaminar failure mode, waviness can actually improve structural integrity due to additional resistance to interlaminar crack initiation and growth. In addition, the observed trends are explained by independent modeling predictions.
Global Prediction of Discrete Local Damage Interactions Using Broadband Dielectric Spectroscopy
With the increasing demand for composites in many modern applications, it is important to predict the initiation of failure events. Due to the heterogeneous nature of these materials, a single damage mode will not lead to final failure, hence detecting the initiation and growth of a single crack (defect) will not yield accurate predictions of the strength or the material state. It is important to capture the interaction of various damage modes that tend to change the material state leading to initiation of failure. To assess the material state, one needs to determine the appropriate global physical variables that can capture the interaction of local damage events that lead to progressive degradation of materials. In the recent past, various researchers have turned towards the application of vector electric fields that can interact with various local damage events created by single or multiple loading events. Broadband Dielectric Spectroscopy (BbDS) is one such robust technique that can predict the interaction of such local events. In the current research, Regularized Extended Finite Element Method (Rx-FEM), which allows modeling displacement discontinuities
Effect of Intralaminar Failure Properties on Compressive Strength of CFRP Structure after Edge-on Impact
Strength of compression after impact is one of the most important design criteria for structures using laminated fiber-reinforced plastics. Effects of face-on impact damage on compressive strength have been widely studied over the past decades. However, knowledge about effects of edge-on impact damage is limited compared to that of face-on impact. Therefore, in this study, edge-on impact tests and compression after edge-on impact (CAEI) tests were conducted. In addition, intralaminar fracture toughness (energy) of compressive failure of fiber (0°) and matrix (90°) was obtained by single edge notched compression tests and VCCT analysis, since intralaminar failure properties seem to be essential from our previous work. The obtained toughness was used in progressive damage analysis (PDA) of CAEI failure. In the PDA, CAEI failure was simulated under several modelling assumption of impact damage, including delamination and intralaminar pre-crack. As a result, CAEI strength was predicted with relatively small error when intralaminar pre-crack was included as impact damage. Although modelling assumption of impact damage, such as information about crack length or cracking ply, has to be validated based on experimental observation as future task, utility to predict CAEI strength by modelling impact damage as pre-crack was shown.
Effects of Localized Manufacturing-Induced Defects in Wind Turbine Blades
Localized manufacturing-induced defects can compromise the overall structural behavior of wind turbine blades, including compressive strength and fatigue life. While much research has been pursued on the effects of defects, there is still a need for empirical models that can assist in understanding the relationship between defect geometries and their effects on blade durability. The objective of this current research is to develop an empirical model that can be used to estimate the impact that a local high-porosity defect will have on the fatigue performance of a typical fiberglass reinforced wind turbine blade. Flat plates are manufactured using a variety of specially designed processing conditions with the intent of directing the flow to induce defect regions in composite plates that span the range from high- to low-porosity densities. The plates are cut into slices, and the slices examined using optical microscopy, SEM and CT-scanning techniques. A set of coupons is tested in flexure to assist in the design of the size of specimens to be used for fatigue testing. Future work is to conduct a fatigue test program to develop an empirical model that relates the number of cycles to failure to the state of stress and the degree and depth of porosity.
Ply Curving Termination for Suppressing Delamination in Composite Ply Drop-Off
Composite tapered sections with ply drop-off are prone to delamination due to stress concentration at the edge of the terminated 0° plies. Currently quite gradual taper angle is used in aircraft composites to suppress delamination, which leads to weight increase. Structural modification that can suppress the stress concentration is necessary to realize further-optimized lightweight structures with steeper taper angles. This study proposes a new approach called “ply curving termination (PCT)â€ in which local fiber orientation is modified at the edge of the terminated 0° plies. The curved area has significantly low stiffness and thus the stress concentration at the ply edge is suppressed. This paper reports the numerical analysis and experiment to validate the effectiveness of this new concept.
Microscale Simulation of Composites with Various Microstructures by Using eXtended Finite Element Method (XFEM)
In recent years, freedom in design of composite microstructure has been improved because of the development of the manufacturing technology of various cross-sectional carbon fibers. Therefore, numerous candidates of composite microstructure must be considered for microscopic optimization of composite. To this end, this study develops mesh-free microscale simulation tool consisting of two kinds of computational techniques; homogenization method and extended finite element method (XFEM). For the evaluation of an effect of microstructure on the macroscopic mechanical and fracture properties, homogenization method was introduced. Additionally, the composite microstructure (i.e., fiber / matrix interface) is able to be modeled independently of the mesh by the XFEM. The proposed tool makes it possible to conduct comprehensive numerical investigation into various composite microstructures without remeshing.
Progressive Axial Crushing of Composite Laminates: A Comparison between LS-DYNA Continuum Damage Models
We are presenting a detailed comparison of well-established intra-laminar material damage models to assess predictive capabilities and the influence of additional nonphysical parameters with regards to compressive damage behavior and axial crushing of Carbon Fiber Reinforced Polymers (CFRPs) structures. Implemented in the commercial explicit Finite Element (FE) software LS-DYNA, three material models MAT058, MAT219 and MAT261 are applied to a range of characteristic tests that lead towards the axial crushing of quasi-isotropic IM7/8552 CFRP laminates. The findings of this study aid in fully understanding the efficient use of built-in intralaminar damage models for CFRP structures under compressive loading. It further leads towards robust and reliable calibration schemes to determine typically used parameters in crashworthiness simulations.
Numerical Modelling of Impact Damage in Fibre-Reinforced Plastic Composites with Smoothed Particle Hydrodynamics
This study develops a computational simulation method to predict nonlinearity, progressive failure behavior and failure strength of CFRP laminates using the SPH method. The fracture mechanism of CFRP laminate subjected to crushing may be categorized in four major failure modes, fiber tensile failure, fiber compressive kinking, matrix cracking and delamination. Hence, these failure modes are modeled in this study. In previous studies, the stiffness or stress values of damaged particles becomes zero instantaneously when the damage occurs. But, this may cause the unrealistic localization of damage. In order to overcome this problem, softening models are implemented in this simulation. The dissipated energy with fiber failure is exceedingly larger than with matrix damage. Therefore, one of the energy based damage model, called smeared crack model (SCM) was used to treat adequately the dissipated damage during fiber kinking. Additionally, in crushing phenomena, countless matrix cracks are generated and it is difficult to simulate individual cracks respectively. Hence, continuum damage mechanics (CDM) is used for modeling multiple cracks that are difficult to discretize in mesoscopic scale. For the modeling of delamination, the damage model considering the separation of interactions between particles is used to reproduce the separation of adjacent plies. In addition, it is known that CFRP shows different elasto-plastic response depending on the loading direction due to the pressure-dependence of epoxy resin. To reproduce this nonlinearity, an anisotropic pressure-dependent elasto-plastic material model is applied for conventional SPH.
Mechanisms of Energy Absorption in Hybrid Material Systems Consisting of Sheet Metal and Advanced Composites Under Bending Load
In previous experimental investigations [1], the positive characteristics of hybrid fiber reinforced polymer (FRP)-steel material systems in terms of energy absorption were analyzed in 3-point-bending tests on coupon scale. Depending on the specimen orientation and thus on the differing tensile or compressive loading of the respective components, a differing energy absorption was observed. For predominant compressive loading of the FRP phase, some material systems have shown a higher energy absorption compared to the reversed specimen orientation. To understand the observed effect and the relevant damage mechanisms, a particular test method consisting of the combination of stepwise mechanical testing, digital image correlation (DIC) as well as intermediate microscopic and computer tomographic (CT) analysis is developed in this study. The results of the DIC allows for the detection of the neutral strain plane and its shift during the progressing deflection and damage. Based on the data obtained, an estimation of the respective effects of the detected damage mechanisms on the change in the components loading situation is possible. These results are discussed with respect to the phenomenological failure models given by Puck and Schürmann (see [2–4]) and provide a basis for further study.
Crush Response of Prepreg Platelet Molding Compound Tubes
Crush tubes made of prepreg platelet molding compound (PPMC) were manufactured with a bladder molding process. These discontinuous fiber tubes were crushed at quasi-static speeds, and the failure morphology, load-displacement curves, and specific energy absorption values were recorded. PPMC tubes displayed a failure morphology similar to that of a splaying failure mode seen in continuous fiber tubes, but the splaying segments were significantly larger in this case. The load-displacement curves were similar in shape to those seen in the continuous fiber fragmentation mode. The average specific energy absorption for this material system was determined to be 42.3 kJ/kg. A 2D orientation analysis application was used to determine platelet orientation distribution on the surfaces of the PPMC sheets. No significant correlation was found between platelet orientation distribution and specific energy absorption.
High Rate Testing of Composite Fastener Joints With and Without Clamp-up
The NASA Advanced Composites Consortium (ACC) High Energy Dynamic Impact (HEDI) group has been investigating experimentally the effects of testing speed and fastener clamp-up on the behavior of single lap shear composite fastener joints. Two fastener types, a titanium pin and a Hi-Lok® fastener, were used to simulate the two conditions of clamp-up force (zero and non-zero). The tests were performed at the rates of 21×10-6 m/s, 2.54 m/s, 7.62 m/s, and 12.7 m/s. Strain gages and digital image correlation (DIC) were used to measure deformations, displacements, and strain fields. This paper elaborates results from tests conducted at 12.7 m/s for pin and Hi-Lok® fastener types. While the strains at bearing failure initiation appeared to be slightly rate sensitive for test articles employing pin joints, no trend could be associated with that for the fastener joints. The strain levels corresponding to bearing failure initiation were much higher for test articles using Hi- Lok® joints (with washers), because of the clamp-up. The strain rates measured at a remote location for the pin and the Hi-Lok® joint approached 200 s-1 and
A Nonlocal Progressive Damage Model for Composite Materials
Fiber reinforced composite materials are widely used in aerospace structures due to their high specific stiffness and strength. These materials exhibit complex deformation, damage and failure mechanisms under high strain rate loading conditions. Many different failure criteria have been proposed in literature to describe the damage initiation and evolution of fiber reinforced composite materials. Most of these damage models are based on a local framework. In a local framework, the material behavior is based on a point-wise constitutive relation which is independent of the effect of the surrounding points. These models include stiffness degradation when damage initiates in the material, which within a local framework leads to a highly mesh dependent result. This could be overcome using a nonlocal finite element approach. In the current work, a nonlocal formulation based on the work by Andrade et al [1] is adapted to describe composite material behavior. The LS-DYNA rate dependent progressive damage model, MAT162 is employed to detect the initiation and evolution of damage in orthotropic composite materials. This model is developed as a FORTRAN user material subroutine in the LS-DYNA environment. The results of MAT162 with and without the nonlocal formulation are compared at different mesh densities to validate the model. The numerical analysis exhibits the advantages of the nonlocal formulation.
Non-Local Damage Modeling for Composite Laminates: Application to Isogeometric Analysis for Impact Simulations
High-fidelity progressive damage simulations of composite materials are important for advancements in damage tolerant design. We recently proposed a novel modeling approach for damage analysis of composite laminates, in which multi-layer structures are represented as individual plies connected through zero-thickness cohesive interfaces. The model is developed in the framework of Isogeometric Analysis (IGA). By using Non-Uniform Rational B-Spline (NURBS) basis functions for representing geometries and discretizing the displacement field, IGA allows for a more direct connection between numerical simulation and CAD software. In addition, compared to traditional polynomial basis functions, NURBS functions allow for better representation of geometries and higher order inter-element continuity properties. The computational efficiency of the proposed modeling approach stems from the adoption of Kirchhoff–Love shell elements for the modeling of individual lamina. Intralaminar damage is introduced in the framework of continuum damage mechanics, in which a strain-softening damage model drives the degradation of material elastic properties. However, the use of local strain measures, in combination with strainsoftening degradation models, may lead to damage localization problems. These cause the governing equations to become ill-posed and their approximate solution to be highly mesh-sensitive. Our work aims to re-establish the objectivity with respect to the adopted discretization.
Stochastic Process Modeling of a Prepreg Platelet Molded Composite Bracket
Prepreg platelet molding compounds (PPMCs), formed by slitting and cutting prepreg tape, are used to produce complex parts with moderate structural performance while maintaining good processing characteristics associated with compression molding. The fiber orientation state in the final part is driven by the processing (i.e. the initial orientation state of the preform and the flow). Then, the fiber orientation distribution within the structure determines its mechanical performance. However, there can be significant part to part variability for these systems which is accommodated through reduced design allowables. A recently developed molding simulation methodology predicts the final fiber orientation state given the initial conditions. Yet, the variability of the initial orientation state caused by preform deposition can contribute significantly to the variability in part microstructure, particularly for low flow processes. The method for anisotropic flow and fiber orientation analysis was employed to analyze the molding of a bracket geometry. Initial orientation states were stochastically generated. The predicted orientation state was transferred to a structured mesh to perform a structural analysis and obtain the part stiffness. The predicted stiffness was used as a metric to assess the ability to predict variability caused by stochastic initial conditions. The resulting predicted stiffness and variability were consistent with mechanical testing.
Experimental and Numerical Characterization of the Intra-Laminar Fracturing Behavior in Discontinuous Fiber Composite Structures
In this paper, we investigate the intra-laminar size effect of discontinuous fiber composites (DFCs) with three different unidirectional prepreg platelet sizes (75×12, 50×8, and 25×4 mm). Experimentally, we test five different sizes of single edge notched specimens, geometrically scaled (1:2/3:1/3:1/6:1/20), with the constant thickness. We observe notch insensitivity meaning that the crack initiate away from the notch, when the structure sizes are small (from the ratio 1/20 to 1/6). However, the crack always initiate for the ratio of 2/3 and 1. Bazants size effect law is used to analyze such unconventional fracturing behaviors. The experimental results are fitted using the linear regression analysis follow by the size effect law. The transition behavior of the DFCs from the strength based criteria to the energy based criteria is clearly observed. Also, as the platelet size increases, the fracture behaviors shift away from the energy based criteria, which implies a decrease in brittleness. To obtain the intra-laminar fracture energy, Gf , we have developed a finite element model based on the stochastic laminate analogy. The platelet size of 75×12 mm shows 96.8% increase in the fracture energy compared to the platelet size of 25×4 mm while behaves less brittle way. In conclusion, this study examines the effect of the platelet sizes of the DFCs in the presence of the notch. In this process, capturing the quasi-brittleness of the material using the nonlinear fracture mechanics is essential and we accomplish this using the simple size effect law. This work expands on an earlier SAMPE conference proceeding [1], and thus, there is a significant overlap in texts and figures between this and the SAMPE conference proceedings.
Fabrication to Performance: A Comprehensive Multiscale Stochastic Predictive Model for Composites
We propose a comprehensive framework for uncertainty management within the manufacturing process of non-crimp fiber composites (NCF), including the forming, resin injection, curing, and distortion. The challenge of meeting performance requirements while having incomplete knowledge about the fundamental physical processes is addressed with the objective of proposing manufacturing guidelines that are agnostic to these uncertainties. We accomplish this by making the functional dependence of uncertainties in the performance metrics and uncertainties in the various parameters and models explicit. We tackle the issues associated with dimensionality, which has hampered similar efforts in the past, through a basis adaptation procedure that permits the development of functional dependencies, for realistic systems, without any loss of accuracy. These representations are uniquely suited for design optimization as they provide explicit, yet highly accurate, stochastic reduced order models (SROM) that can be analytically differentiated and integrated. We compute several Quantities of Interest (QoI) as functions of random variables and processes of material properties and process conditions. During the forming stage, we consider the mechanical properties of the fibers and the local fiber directions to be random. The deformation of the fabric was computed via a reduced model consisting of an effective shell with embedded upscaling algorithms at the integration points. Forming induces stochastic fluctuations in the relative shearing angles of the fabric, which are mapped, through a stochastic model, into spatial fluctuations of the permeability field. The simulations were carried out using the PAM-COMPOSITES™
Uncertainty Quantification of Simulated Residual Stresses in Multi-Material Composite Structures
Multi-material composite structures develop residual stresses during the curing process due to dissimilar material properties, which eventually may lead to failure in the form of fracture, delamination, or disbonding. Experimentally determining residual stresses can be both time and cost prohibitive, whereas accurate simulated predictions of residual stresses can be cheaper and provide equivalent information during the design process. Residual stresses can be simulated through several different approaches of varying complexity. The method employed in this study assumes the majority of residual stresses are developed due the mismatch of coefficients of thermal expansion and polymer shrinkage, which is indirectly accounted for by calibrating the simulation with an experimentally determined stress free temperature. This method has shown success in predicting the residual stress states across different material combinations and structures in previous studies.
Dispersion and Properties of Graphene Oxide and Reduced Graphene Oxide in Nanocomposites
Reducing graphene oxide (GO) is currently seen as one of the most cost effective and scalable methods to produce graphene sheets. This method takes exfoliated graphite in the form of graphene oxide (GO) and reduces it to reduced graphene oxide (rGO). This reduction process recovers the mechanical, thermal, and electrical properties of GO,1 making it more appealing for many applications including fillers in polymers. However, the reduction of oxygen functional groups tends to lead to lower dispersion quality and activity of rGO in polymers. This remains an issue as researchers search to produce graphene based nanocomposites for different applications. This study characterizes the thermal and mechanical properties of graphene oxide and reduced graphene oxide epoxy nanocomposites to determine the overall performance in relation to dispersion quality and nanoparticle loading. For this purpose, epoxy nanocomposites of GO (C:O ratio 1:1) and rGO (C:O ratio 5:1) with various loadings (0.5, 1.0, and 2.0 wt.%) and dispersion qualities (3 different combinations of shear mixing and horn sonication) were fabricated and characterized. Transmission optical microscopy (TOM) and scanning electron microscopy (SEM) were used to qualitatively asses the level of dispersion for each dispersion technique. Flash diffusivity analysis and differential scanning calorimetry (DSC) were employed to measure the thermal diffusivity and specific heat capacity, respectively, for each sample, from which the thermal conductivity was calculated. The thermal conductivity was then correlated to the level of dispersion and filler (GO or rGO) for the composites. Nanoindentation was utilized to assess the mechanical properties of the nanocomposites with respect to dispersion, loading, and filler type.
Cycloaliphatic Epoxy -Silica Nanocomposite Provided from Perhydropolysilazane
Epoxy-silica nanocomposites were synthesized with an asymmetric epoxy monomer, CEL3000, which have a glycidyl group and an alicyclic epoxy group, and a combination of amine and thiol compounds, as an initiator and a hardner, respectively, in presence of perhydropolysilazane, PHPS. Highly transparent epoxy-silica nanocomposites with clear microphase separation of epoxy and silica domains were synthesized at various PHPS content. The competition of curing of epoxy groups were considered by FT-IR investigation. PHPS disturbed the reaction of thiol because of the interaction between PHPS and 2° amino group of the initiator. Transparency and water vapor barrier property were quantitatively measured for the composites. It was found that the interaction between PHPS and 2° amino group of the initiator improved the transparency of epoxy resin. Water vapor barrier property of CEL3000 resin was improved by not only by the increasing of path length of water vapor but also the reduction of hydroxy group in the resin owing to the capping with SiH group of PHPS.
Manufacturing Process of CNT/BMI Composites and CF/CNT Hybrid Composites with Continuously-spun CNT Prepregs Synthesized by FCCVCD
The Floating Catalyst Chemical Vapor Deposition (FCCVD) method has attracted extensive research interests from both academia and industry due to its high potential for carbon nanotube (CNT) mass production. As aerospace aims become increasingly demanding, there is an ever growing need for mass producing lightweight structural composites. By infiltrating bismaleimide (BMI) resin and curing under pressure, CNT/BMI nanocomposites with high CNT loading are exhibiting excellent mechanical properties. By further integrating CNT into carbon fiber (CF), the mechanical and electrical improvements in axial, transverse and through-thickness direction demonstrate potential for multifunctional applications. In this work, a FCCVD reactor designed and assembled at the High-Performance Materials Institute was used for CNT synthesis. The CNT aerogel formed was continuously spun onto a rotating mandrel while continuously spraying a BMI/acetone solution, forming an ultra-thin prepreg material ranging from 1-5 μm in thickness. The quality of the CNTs collected was characterized using Raman Spectroscopy, Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), and Transmission Electron Microscopy (TEM). To evaluate the ultra-thin FCCVD-CNT prepregs, six layers were stacked in the same alignment direction, and cured using a vacuum-assisted bagging system and hot press. Electrical conductivity tests were conducted in both the aligned and the transverse directions. Furthermore, the ultra-thin CNT prepregs were integrated into the aerospace-grade unidirectional IM7/CYCOM® 5250-4 prepregs following an alternating order. The assembly was finally vacuum-bagged to expel air and cured with a hot press. The microstructure and interface of CF/CNT hybrid composites and CNT/BMI composites were observed under SEM. This paper reports on a fabrication process of CNT prepregs, which could be potentially used to scale-up the CNT/BMI composites and CF/CNT interply hybrid composites manufacturing.
Multiscale Modeling of Bonded T-Joints Using Atomistically Informed Method of Cells
In this research, a multiscale modeling framework is developed and applied to the analysis of adhesively bonded composite joints under mechanical loading. The primary goal is to obtain an improved understanding of damage initiation and failure at the relevant length scales and predicting the consequent effects at the structural scale. The methodology utilizes damage information at the atomic level, addressed using molecular dynamics (MD), and couples it with a method of cells based micromechanics model for the nonlinear and damage analysis of carbon fiber reinforced polymer (CFRP) composite. This damage analysis technique is then used to predict the multiscale nonlinear effects in hot-spot zones, such as the adhesive/adherend interface, in adhesively bonded T-joints which will assist in the development of methods for prevention or delay of the most common forms of failure in such built-up components.
A Stabilized Finite Element Formulation Remedying Traction Oscillations in Cohesive Interface Elements
The standard finite element implementation of intrinsic cohesive zone models (CZMs) based on the penalty method exhibits a distinct lack of numerical stability and/or convergence for stiff cohesive laws. This lack of stability is typically observed in the form of spurious oscillations in the normal and tangential tractions recovered at the cohesive interface. In this paper, we will present a robust, stabilized finite element formulation for CZMs that remedies traction oscillations, thus ensuring stability and convergence for any value of initial cohesive stiffness. A key advantage of the proposed formulation is that it generalizes the Nitsche’s method for modeling cohesive fracture with a large initial cohesive stiffness, thus enabling the implementation of intrinsic and extrinsic CZMs in a unified and variationally consistent manner. We present several numerical examples to demonstrate the stability, convergence and accuracy of the proposed formulation in two-dimensions. First, we will verify the accuracy using simple patch tests considering uniaxial tension, compression and shear loadings. Second, we will demonstrate the lack of spurious traction oscillations at cohesive interfaces of rectangular beams loaded under shear and three-point bending. To demonstrate the stability issues related with the spurious traction oscillation, we consider both isotropic as well as anisotropic CZMs, wherein the normal and tangential cohesive stiffness values are different. Our numerical results for high stiffness cases clearly show that the proposed formulation yields a smooth oscillation-free traction profile and ensures stability, whereas the standard formulation suffers from instability and/or convergence issues.
A Comparative Study on Pin Bending Effect Under Bearing Static and Fatigue Failure
Primary aerospace composite structures are often joined by bolted joints due to its capability in transferring high loads and ease in disassembling and assembling for inspections and repairs. Joint strength in composites can be significantly reduced with operational use beyond originally intended design life due to the accumulation of internal damage which necessitates more frequent inspections. Internal damage (delamination, matrix cracks) can continue to propagate without being seen through visual inspections, thus nondestructive testing is required. However, this may be costly, particularly for a full-scale aircraft. Thus, a comprehensive understanding of joint damage growth behavior is necessary to guide what damage modes to inspect for as well as the frequency of inspection. In this work, an experimental investigation was performed on single-countersunkbolted- joint specimens and semi-circular through-hole and countersunk hole specimens under static load to study the effect of pin bending on bearing damage (i.e. shear cracks). The static tests results were compared to cyclic loaded specimens from previous published work to develop a greater understanding of the relationship between hole elongation and damage growth in a double lap shear test configuration [1]. The static test results indicate pin bending did not contribute to the bearing failure. Shear crack formation at the shank was found to be caused by the compression-induced instability of fibers (fiber kinking) near the free surface of the laminate.
Matrix Crack Formation and Growth in the Presence of Nonuniform Fiber Distribution and Matrix Voids
The current work is a continuation of the work presented at the last ASC meeting [1]. In the previous work, the initiation of brittle cavitation in the epoxy matrix reinforced by unidirectional fibers and subjected to transverse tension was studied. Using the dilatation energy density criterion for brittle cavitation, the initiation of fiber/matrix debonding was predicted for realizations with varying degree of fiber mobility. The objective of the current work is to analyze the driving force for crack propagation from the debonds arising from these points of cavitation. Loading of the model is continued until two, four or six adjacent debond cracks were found to form. The debonded fibers are replaced by a crack normal to the applied tension load. For each of these transverse crack cases, the strain energy release rate is calculated via the J-integral. The objective of these calculation is to find the real crack driving force and compare it to the assumed driving force based on homogenization of the composite. The crack driving force (J-integral) is monitored at increasing degree of fiber mobility.
Progressive Damage and Failure Prediction of Interlaminar Tensile Specimen with Initial Fabrication Induced Defects
An automatic solution process to evaluate the effects of fabrication induced defects in composite components is developed. The framework of this approach includes defects extraction from the X-ray computed tomography (CT) scan data, defects mapping into a finite element model, characterization of intra- and inter- ply damage progression and their interaction, and evaluation of the ultimate strength in the presence of fabrication and load induced damage. Initial CT scans are performed along with photomicrographs to determine the location and geometric configuration of voids and ply waviness in an interlaminar tensile (ILT) test specimen. Voids induced initial delamination zones are inserted at ply interfaces. The effects of voids are characterized using a micromechanics model to capture the degradation of initial properties at the ply-level. Two key solution modules are developed and integrated with our Composite Bolted and Bonded Analysis Toolkit for Abaqus (CB2ATA) toolkit to assign initial intra-ply properties in the perturbed local material coordinates resulting from the ply waviness and insert initial delaminations at corresponding ply interfaces. In order to explore the effects of these defects, two analyses are performed for the ILT specimen with and without the observed defects. The predicted load deflection curves and progression of failure patterns are compared with the experimental measurements.
Effect of Defects on the Mechanical Properties of Virtually Cured Composite Structures
This paper addresses the effects of defects occurring during the manufacturing process on mechanical performance of Polymer Matrix Composites (PMCs). Damage accumulation during cure and subsequent in-service performance are studied by using randomly packed Repeating Unit Cell (RUC) representative of unidirectional fiber ply material or a textile fiber tow, at the microstructural level. Computations are carried out using the Finite Element (FE) method within the commercial code ABAQUS in which constitutive models are implemented via user-written subroutines. The transition from a continuum to damage and failure is determined by using the Bažant-Oh crack band model. Results are presented for RUCs that are first subjected to curing and subsequently to mechanical loading in the transverse direction. The analysis shows the differences in both the cured RUC transverse strength, S , and stiffness, E , when cure induced damage and imperfections such as presence of voids, fiber contact, imperfect fiber/matrix bonding and presence of matrix pockets are considered.
State Variable Methods of Assessment, Prognosis, and Control of Composite and Bonded Structures
It is generally recognized that the determination of material state is the baseline foundation for the assessment and prognosis of current and future performance of structural materials in which distributed damage leads to progressive property degradation. Many classes of composite materials, both naturally occurring and engineering designs, are governed by such behavior. However, finding appropriate measureable physical variables to make such assessments, especially for asmanufactured and subsequent real-time assessment and prognosis conditions is challenging. Measurement and analysis of multiple single defect initiation, interaction, and collective effects is the most rigorous current methodology, and is the foundation for certification and quality assessment in the aerospace industry. But it can be difficult (or impossible) to find all of the defects in as-manufactured composites or (especially) to assess their growth and interaction during real-time service, and to conduct the proper analysis of their collective effect on strength and life as a function of real-time load and environmental history of a given structure. Global state variables are well suited to this challenge, but finding suitable measurables and methods is challenging. The current paper will examine several aspects of this problem for structural composite materials, bonded joints, and additive manufactured composite elements with complex shapes. Recent experience and results of development efforts in our group to exploit electrical and dielectric methods will be described. Applications to materials aging, degradation during highly nonlinear deformation, bonded joints, and “materials in the loopâ€ structural control concepts and examples will be presented [1].
Enhancing Damage Tolerance of Composite T-joint Using Fiber-Reinforcement-Based Crack Arrester
One of the difficulties in composite structural application is joining components. T-joint is one of the important elements in aircraft structures that transfers load between vertical and horizontal panels. In T-joint, there is a problem that cracks occur in the deltoid and readily propagate along the interface between the flange and the skin. Therefore, a crack arrest feature is necessary to improve the damage tolerance of composite T-joints. This current study utilized a novel crack arrester which introduces continuous fibers in the flange/skin interface and suppresses the crack propagation using massive fiber bridging. At first, the feasibility of this arrester and the effect of the stacking sequence were evaluated using double cantilever beam (DCB) tests. The apparent fracture toughness increased more than six-fold in the specimens with arresters. This arrester concept was then applied to T-joint and its effectiveness was evaluated through tensile tests. The specimens with arresters showed totally different failure progress and much higher energy absorption compared to the specimens without arresters. It was confirmed that damage tolerance of T-joints is remarkably improved by introducing the proposed arrester and its performance significantly depends on the stacking sequence of the arrester.
Experimental and Computational Investigations of Process-Induced Stress Effects on the Interlaminar Fracture Toughness of Hybrid Composites
Hybrid composites allow designers to develop efficient structures, which strategically exploit a material’s strengths while mitigating possible weaknesses. However, elevated temperature curing processes and exposure to thermally-extreme service environments lead to the development of residual stresses. These stresses form at the hybrid composite’s bi-material interfaces, significantly impacting the stress state at the crack tip of any pre-existing flaw within the structure and affecting the probability that small defects will grow into large-scale delaminations. Therefore, in this study, a carbon fiber reinforced composite (CFRP) is co-cured with a glass fiber reinforced composite (GFRP), and the mixed-mode fracture toughness is measured across a wide temperature range (-54°C to +71°C). Upon completion of the testing, the measured results and observations are used to develop high-fidelity finite element models simulating both the formation of residual stresses throughout the composite manufacturing process, as well as the mixed-mode testing of the hybrid composite. The stress fields predicted through simulation assist in understanding the trends observed during the completed experiments. Furthermore, the modeled predictions indicate that failure to account for residual stress effects during the analysis of composite structures could lead to non-conservative structural designs and premature failure.
Closed-Form Mixed-Mode Strain Energy Release Rate Expressions for Unidirectional Laminate Configurations
A closed-form fracture methodology based on first order shear deformable plate theory (FSDT) was developed in Reference [1]. By using a sub-laminate model and adopting transverse shear-deformable laminate theory, general expressions for total strain energy-release rate (SERR) and its individual components were derived. Making use of the first order shear deformable plate theory, closed-form expressions for the calculation of the individual modes of the strain energy release rate for traditional unidirectional laminate test specimens used to obtain fracture toughness properties (Double Cantilever Beam, End Notch Flexure and Mixed-Mode Bending) are presented.
The Importance of Energy Release Rates in Failure of Composites
Kevin O’Brien’s work on delamination has had a huge impact on the field. His seminal paper [1] highlighted the importance of the energy release rate in both static and fatigue delamination of composites. The current paper describes areas where this work has had a major influence, going beyond delamination to also include in-plane and notched failure.
Progress in Failure: Toward Reliable Failure Predictions in Composites
Achieving reliable means for failure prediction in composites is a standing challenge. To this end, an integrated approach for the diagnosis and prognosis of composites is underscored. It encompasses three key elements. The first is nondestructive inspection enabling 3D measurement of defect size, location and geometry coupled with an automated transition capability to finite element models. The second is accurate and cost effective 3D material property measurements with a minimum number of tests and methods. Finally, achieving structural strength and fatigue life prognosis results from combining the prior elements into comprehensive methods that would ultimately allow for capturing the failure mechanisms associated with multiple damage modes and their interaction. Future research directions emphasize the development of composites processing simulation tools to accelerate the attainment of quality standards and associated dependable allowables.
Quantification of Error Associated with Using Misaligned Meshes in Continuum Damage Mechanics Material Models for Matrix Crack Growth Predictions in Composites 2385
The ability of a material model to capture in-plane matrix mode I and mode II crack growth is an essential component for modeling ply level damage evolution in composite structures. Previous studies using a continuum damage mechanics (CDM) approach have shown success in satisfying benchmark solutions for mode I and II crack growth. However, success was shown using a fiber-aligned meshing strategy, which encourages matrix cracks to propagate in a single band of elements, along the fiber direction. Generating a fiber-aligned mesh becomes a highly involved process for laminates including off-axis (non 0° or 90°) plies. The objective of this study is to quantify the effect of non-fiber aligned mesh discretization on predictions of inplane matrix crack propagation. The approach taken incrementally varies the mesh orientation angle relative to the fiber orientation; more specifically, misaligned meshes are used to quantify the effect of element angle orientation relative to the initial crack orientation on the energy released during matrix crack propagation simulations using a CDM method. CDM solutions obtained with the misaligned meshes are evaluated against known benchmarks for mode I and II matrix crack growth. The CDM solutions reveal a near-polynomial trend of increased predicted failure stress with increased mesh misalignment angle; hence implying a potential relationship between element orientation angle and apparent fracture toughness.
An Engineering Approach to Analyze Damage Initiation Modes in Tapered Composite Structures
In this work an engineering approach is demonstrated for analyzing damage initiation modes in tapered composite structures. The analysis methodology includes simulation of the non-linear static response of tapered composite structures under static tension loads to predict the location of interfacial delamination initiation. Furthermore, the developed methodology provides a strength-based criterion to assess whether damage initiation will occur in the inter-laminar delamination or intra-laminar matrix cracking mode. Based on the results of the analysis, a tapered composite structure is fabricated and tested under displacement-controlled quasi-static tension loading. The damage initiation location captured experimentally is compared with the analysis towards achieving preliminary qualitative validation. The linear stiffness of the tapered composite structure is predicted within 15% of the experimental average thereby achieving preliminary quantitative validation.
The Influence of Variability and Defects on the Structural Performance of Discontinuous Composites
Composite materials often feature defects, particularly for composites which feature a complex microstructure, such as aligned discontinuous composites. This study uses an accurate but efficient virtual testing framework, which was used to predict the influence of defects on the structural performance of both hybrid and nonhybrid aligned discontinuous composites. Fibre vacancy defects were found to cause the strongest reduction in material properties, while hybridisation was found to be an effective means to reduce the influence of defects on the structural response.
Stochastic Finite Element Analysis of Composites
Composite materials can have a large variation in material properties, especially in the transverse direction. This can cause difficulty when designing with these materials and may lead to overdesigning the material. Using finite element software, a model was created to simulate failure of composite materials in the transverse direction. Using micrograph images, a simulation of microstructures was generated based on fiber volume fraction and special positioning of fibers. Using a multi-level approach, the material properties were calculated in the micromechanical model. The comparison of the obtained elastic properties with rule of mixture revealed that the transverse properties are poorly predicted by rule of mixture. A mesomechanical model was then developed based on the material properties obtained from the micromechanics model. Using a progressive failure approach, each element could independently be disabled when the element fails, causing a simulated propagation of failed material. The results showed that failure follows a Weibull distribution consistent with experimental observation. The developed stochastic model will allow for infinite possibilities of failure replicating experimental findings.
Meso-Scale Computational Simulation of Mechanical Response of Carbon Nanotube Yarns
In this paper, a numerical model is proposed to predict the mechanical response of Carbon Nanotube (CNT) yarns under uniaxial tensile loading. The model implements a Finite Element (FE) modeling technique to formulate the idealized structure of randomly formed configurations of CNT yarns. In the model, the bundles of the CNT yarn are simulated as bar elements randomly connected to the ends of each other. A set of transverse and diagonal members are introduced to capture the coupling effects of the frictional forces between the adjacent bundles. The FE model is loaded in a step-by-step monotonically-increasing pattern. At each step, the internal forces of the elements are checked against a failure criterion and the elements with internal forces higher than the set strength value are removed. The model is validated by comparing its results with the results of an experimental study conducted elsewhere. A very good correlation is achieved between the FE model results and the benchmark test results.
Conforming Element Mesh for Realistic Textile Composite Micro-Geometry
In recent years, several computer tools, e.g., DFMA, TexGen and WiseTex have been developed to derive realistic yarn-level micro-geometries for textile composites. However, due to numerical errors, the generated micro-geometries by these computer design tools have unavoidably exhibited artificial surface interferences or narrow gaps between yarns. It is therefore problematic to directly input the micro-geometry into a commercial FEM code to generate a conforming element mesh. In this paper, a procedure is developed to generate a conforming FE mesh that matches actual yarn-toyarn and yarn-to-matrix surface inside a textile composite with a complex microgeometry. It improves the accuracy of micro-mechanics analysis. The procedure divides into five steps. Initially, the unit cell domain is discretized into a uniform cuboid finite element mesh and the yarn surface is discretized into triangular plane elements. The second step consists of calculating the intersecting points between yarn surface triangle elements and mesh gridlines in the z-direction. The third step is the removal of numerical error driven artificial surface interferences or narrow gaps between yarns. If the distance between two intersection points from two adjacent yarns is smaller than a specified tolerance, the two adjacent intersecting points are merged to the mid-point. A material type, defined by yarn number, interface or matrix, is assigned to each node. In the fourth step, the initial uniform cuboid finite element mesh is modified so as to match yarn boundaries to the finite element mesh. In the final step, material types/yarn numbers are assigned to each element based on nodal material types. If an element is composed of nodes of two different material types, it is split into two or more elements. As such, a conforming FEM mesh, which matches the element boundary to the yarn-to-yarn or yarn-to-matrix interface, can be generated.
Compressive Strength Prediction of 3D Woven Textile Composites: Single RVE Multiscale Analysis and Imperfection Sensitivity Study
Results for the compressive strength of hybrid 3D woven textile composites (H3DWTCs) that consist of carbon, glass and kevlar fiber tows and a polymer matrix material, obtained through a combination of experiments and a multi-scale analysis of a single representative volume element (RVE), are presented. The RVE model considers micro scale based modeling, where tows are considered homogenized entities of fibers and matrix, and the matrix outside of fiber tow as an isotropic material. The homogenized tow is further analyzed via a 2scale model. The progressive failure modes of the fiber tow and matrix that are observed in experiments are seen to be captured by the models presented in this paper.
Applicability of Two-Step Homogenization in High-Crimp Woven Composites
In the present work, we analyze the applicability of two-step homogenization applied to 3D woven composites with high crimp reinforcement. The available micromechanical homogenization approaches (Hashin, Chamis, Hashin-Shtrikman bounds etc.) were developed and validated for unidirectional composites. These formulas have also been used by the community to homogenize tows in 2D and 3D woven composites including reinforcement architectures with high crimp ratios. However, a rigorous study of their applicability to high-crimp geometries is yet to be performed. We utilize Finite Element Analysis (FEA) to calculate the overall engineering constants (Young’s moduli and shear moduli) of tows having various crimp (ð¶ð‘…) and wavelength-to-fiber diameter (ðœ†/ð‘‘) ratios. For this analysis, periodic sinusoidal unit cells following shapes of individual fibers are used. Fiber volume fraction is set to 70% and is the same in all cases. Transversely isotropic carbon fiber and isotropic epoxy matrix are used. The results are compared with overall responses of tows modeled using homogenized tow properties obtained from micromechanics and FEA as well as explicitly modeled tows containing multiple parallel fibers. The results of our analysis show dependence of the overall elastic properties on both crimp ratio and the normalized wavelength. Separation of fiber/tow scales is achieved at ðœ†/ð‘‘ = 50.
Optimization of Carbon Fiber Surfaces for Reinforcement in Advanced Polymer Composites
This paper summarizes recent efforts within our research group to optimize the interfacial adhesion of carbon fiber reinforced polymer composites (CFRPs). This effort has been approached by several avenues including surface modification of carbon fibers, and the use of molecular dynamics to determine key interfacial interactions determining optimal adhesion. Typically, surface manipulation of carbon fibers is carried out using reductive electrochemical techniques, employing irreversible single electron reduction of aryldiazonium salts. Though recent efforts have shown oxidative surface grafting of carbon fibers is possible using the Kolbe decarboxylation reaction. Both approaches create a fiber which possesses a covalently bound surface modification, able to present a myriad of chemistries to the supporting resin. Determination of interfacial shear strength (IFSS), using single filament fragmentation in epoxy resin, has shown IFSS gains of over 150%, relative to pristine unsized fiber. Interrogation of the fiber-matrix interface using molecular dynamics simulation has shown that a large degree of the IFSS gains are derived from the molecular ‘drag’ effect of the surface bound molecules through the polymer phase. Further benefits of this approach can also be realized by combining the surface manipulation techniques with novel sizing agents, able to plasticize the localized resin around the carbon fiber, giving a gradient interphase. When used in concert, the synergistic effects of surface modification and interphase manipulation has realized IFSS gains >250% relative to control fibers.
Multifunctional MENs Doped Adhesives for Bond Quality Evaluation
Adhesive bonding for composite structures offers multiple advantages over traditional fasteners such as reducing the weight, creating a more uniformly distributed stress state in the joint, and elimination of stress concentration factors due to joining. However, the strength of adhesive bonds can be reduced due to environmental exposure, contamination, mechanical damage and fatigue and assurances of long-term durability and bond strength are not available. Before adhesive bonding of composites can be used on primary structures, a method for guaranteeing the bonds strength must be developed. Due to magneto-electric principles, magneto-electric nanoparticles (MENs) can be used to detect minute changes of electric fields at the molecular level through detectable changes of the nanoparticles’ magnetization. As a result, when integrated into epoxy based adhesives, MENs are capable of detecting chemical or mechanical induced material imperfections at the molecular level. Current efforts are focused on developing a field tool that can be used to obtain magnetic signatures from doped adhesives similar to those obtained via laboratory scale equipment (vibrating sample magnetometer). To achieve similar sensitivities, FIU is investigating the use of a B-H looper system. In this approach, the MENs material is probed with a specifically designed setup that includes small electric coils wrapped around the sample. The coils are arranged into a noisecancellation configuration to measure the magnetic susceptibility of the sample under various conditions with a lock-in amplifier. With the goal to identify signature response characteristics of specific environmental and mechanical effects, various epoxy based adhesive samples were doped with 30 nm diameter MENs. Differences in magnetic signatures were observed between environmentally aged samples and baseline samples, demonstrating the viability of the B-H looper system as a bond inspection tool.
Hybrid Structured Phenylethynyl Silsesquioxane Resin Composites
Thermosetting polyimides terminated with phenylethynly phthalic imides are the current state-of-the-art high temperature resin for use in structural composite applications. However, due to the presence of imide group these resins often suffer high moisture uptake leading to property degradation during use. In addition, the need to remove condensation near the crosslinking reaction temperature as well as high glass transition temperature of unreacted oligomers, the processing window for this class of thermosetting is very narrow. Hence, the need to develop compounds with the same terminating group with ease of processing is of significant interest. In this work, double-decker shaped silsesquioxane (DDSQ) terminated with multiple phenylethynyl groups was developed and curing process investigated. It was anticipated that DDSQ as the backbone can provide the needed monodispersed characteristics in its molecular weight, while phenylethynyl groups form different isomers (region- and stereo-) about the SiO core of DDSQ. This approach provides ease of processing while eliminate crystallinity. In addition, the inorganic nature of these compounds also exhibited a significant reduction in the moisture uptake which can greatly enhance in-service performance of composites. Synthesis and purification of needed chlorosilanes and the subsequence separation of these functionalized DDSQs by liquid chromatography were performed without the need to use fractional crystallization as the first preparation step are presented. This approach greatly reduces the complexity and enables continuous process.
Full-Field Strain Patterns of Sandwich Beams of Different Length under Three-Point Bending
Sandwich material has high stiffness and strength with little resulting weight penalty and it has a high-energy absorption capability. Different theories have been proposed to study a sandwich beam’s deformation under load, but experimental verifications are relatively lacking. In particular theoretical and numerical predications are often limited to the global deformation of the beam. Little attention is paid to the stress/strain fields of the core. In this paper, we present some results of the core deformation of three sandwich beams under three-point bending by using the Digital Image Correlation (DIC) method.
Low-Velocity Impact Damage of Woven Carbon Sandwich
The rise in demand and interest in arctic exploration has brought new challenges with regard to the mechanical behavior of lightweight sandwich composites used in offshore structures. Sandwich composites are typically preferred for ship structures due to their lightweight, enhanced performance and affordability. However, these materials typically experience drastic changes and degradation in their macro-andmicrostructures when exposed to seawater and cold temperatures in-service. Therefore, it is critical to have a detailed comprehension of the mechanical behavior and failure mechanisms of sandwich composites in low temperature conditions. Within the scope of the current study, low-velocity impact behavior of woven carbon/vinyl ester sandwich composites in low temperatures is investigated. Impact responses such as the contact force, displacement and absorbed energy, at three impact energies of 7.5, 15 and 30 J are determined for 4 different temperatures of 25 °C, 0 °C, -25°C and -50 °C. The change in impact force, the degree of damage and the failure mechanisms with varying impact energies and in-situ ambient temperatures are studied in this paper.
Debonding of Sandwich Panels and Solid Laminates Exhibiting Fiber Bridging
Experiments have shown remarkable similarities between the debonding of carbon fiber reinforced polymer facesheets from the aluminum honeycomb sandwich core on the one hand and the delamination of monolithic laminates on the other hand under the influence of severe fiber bridging. A mixed-mode cohesive zone formulation which was shown to describe the mode I debonding of sandwich panel exhibiting severe fiber bridging has been extended to also address mixed-mode behavior. The model has been calibrated with pure mode I and pure mode II experiments. The validation has been performed with mixed mode experiments without additional mixed mode parameters. The results obtained with the mixed-mode formulation are presented herein.
Modeling Nonlinear and Tme-Dependent Behaviors of Polymeric Sandwich Composites at Various Environmental Conditions
Polymeric sandwich composites are appealing for lightweight structures that require high strength and stiffness such as parts of aircraft, marine vessels, civil infrastructures and wind turbine blades. During service, polymeric sandwich composites undergo complex loading histories, such as combined static and cyclic loading, in addition to a constant exposure to hostile environments, i.e., temperature and humidity changes. In addition, one of the characteristics of polymers is their prominent viscoelastic response when subjected to mechanical loading and the viscoelastic response of polymers becomes more pronounced at elevated temperatures and high humidity. Coupled mechanical loading and hostile environments cause the constituents of the sandwich structures to experience different time-dependent behavior and degradation, leading to complex failure mechanisms. The aim of this study is to be able to predict the performance of sandwich composite subjected to mechanical loading histories and various environmental conditions, by incorporating knowledge of the behavior in each constituent (skin and core). We present experiments and numerical analyses in order investigate the overall mechanical response of a sandwich composite and its constituents, immersed in fluid at 50oC. The observed behavior of sandwich composite is strongly influenced by the response of the constituents at different environmental conditions.
In-Plane Thermal Characterization of Fiberglass/Phenolic Honeycomb Core through an Experimental-Numerical Approach
In the present study, a series of in-plane quasi-static tensile and compressive tests are carried out on a commercial Fiberglass/Phenolic hexagonal cell honeycomb core at elevated temperatures to investigate the high temperature mechanical responses of the bulk core under large deformations. Dynamic Mechanical Analysis (DMA) testing is also conducted on the core cell walls to determine the temperature dependence of the mechanical properties of the constituents of the core. A 3D non-linear finite element model (FEM) with large displacements of the repetitive unit cell is employed in which the cell walls, node bond adhesive layers and adhesive fillets at the intersections of the cell walls are modeled based on the measured geometry of a commercial Fiberglass/Phenolic honeycomb core. The homogenized stress-strain curves obtained from test data for uniaxial tension and compression loadings along the ribbon and transverse directions at different temperatures are compared with finite element predictions. It is shown that, by increasing the temperature, the honeycomb core responses to applied displacements become softer in both the ribbon and transverse directions, which are attributed to the degradation of phenolic resin used in the cell wall matrix and node bond adhesive. The predictions of the finite element model are in good agreement with the test data.
Residual Tensile Strength of Adhesively Bonded Double Lap Joints Aafter Transverse Impact
The aim of this study was to understand the effects of transverse impact on the residual tensile strength of double lap joints. The experimental results were compared to baseline cases with no impact damage. The adherend and adhesive used were the Hexcel Hexply® IM7/8552 and Hexcel Redux® 319 respectively. During impact, delamination occurred within the 0° ply at the inner adherend surface ply opposite to which the impact was applied. A significant decrease in failure load was also found from the following residual tensile strength tests for the impact case compared to the pristine case. While the failure mechanisms were similar, the reduction of residual strength appears to be mainly due to the presence of the sharp crack as a result of the impact, the decreased effective overlap length of the specimen and the lack of intact adhesive fillets after the impact.
Experimental Approach to Investigate Facesheet Delamination of Honeycomb Sandwich Panels Under Ground-Air-Ground (GAG) Pressurization
Sandwich structures can exhibit several failure modes, depending on specific configuration of the structure as well as details of the applied load. One failure mode is delamination of the face sheet(s) from the core when subjected to Ground-Air-Ground (GAG) pressure cycles. The pressure differential develops with each ascent/descent cycle, causing mechanical stress and potentially face sheet delamination. The likelihood of delamination is increased if any defect(s) exists in the bondline between face sheet and core. In this study, the GAG cycle was recreated in a controlled laboratory environment. Three panels consisting of Nomex honeycomb core and 3ply, 4ply and 8ply face sheets were investigated. It was seen that the bulge created due to pressure differential in the 3ply facesheet was higher compared to 8ply facesheet. The out-ofplane displacement measured by Digital Image Correlation (DIC) for 3ply, 4ply and 8ply face sheets ranged between 0.42-2.46 mm. Moreover, fatigue analysis was performed on the three face sheets upto 50th GAG cycles which showed that there was slight increase in delaminations for the three face sheets after higher number of cycles. This test method proved to be an efficient way to predict delamination growth in sandwich panels when subjected to GAG pressurization cycles
Application of Laminated Composite Grids as a Reinforcing Element for Automotive Components
This paper intends to present the application of laminated grid structures as a new class of stiffeners for reinforcing body and chassis of transportation vehicles. A laminated grid plate is constituted from several grid plies with different orientations. Therefore, the grid layers with various fibers, patterns, and orientations can be used, resulting in laminates with enhanced stiffness and coupling effects. In this study, a hypothetical trunk floor is assumed as a sandwich panel with two skins and a composite laminated grid core, which is clamped along all edges. Three different grid structures are considered as the core to strengthen the trunk floor subjected to arbitrary lateral loads. Moreover, the first natural frequency of the plates are achieved. The Ritz method is employed to obtain the maximum deflection and free vibration frequencies of the trunk’s floor panel. The results indicate that employing the laminated grids considerably enhances the response of the panel in comparison with conventional grids.
Development of a One-Step Analysis for Preforming of Tri-axial Fiber Reinforced Prepregs
Carbon fiber reinforced polymer composites are drawing great attention in the automotive industry due to their lightweight, high stiffness/strength properties. Fiber reinforced prepregs in various fabric architectures are preformed to a designed part shape before a final compression molding of the parts. Most of the current numerical simulation techniques are developed for woven composites, which are fabricated with bi-axial fiber orientations. There is no modeling technique for braided and NCF (Non- Crimped Fabric) prepregs with tri-axial fiber directions. In this paper, the authors’ previous work on bi-axial woven prepregs is extended to model the tri-axial braided prepregs based on a one-step analysis approach. The algorithm developed for this analysis treats the matrix and fibers as different materials. Any material model in the commercial FEA software can define the matrix, while the fiber is modeled as an elastic material. The material deformation on the final formed part is obtained by using the minimum energy method. This feature has been successfully implemented in LS-DYNA and can be activated by the modified keyword: *DEFINE_FIBERS. No tooling information is needed for the model setup. The initial prepreg shape and size can be obtained based on the final part geometry. The prediction accuracy and computational efficiency of the developed one-step analysis is demonstrated through modeling the preforming of a double dome part. Good agreement is obtained in the initial prepreg size prediction when compared to experimental data.
Basalt Fiber Based Sheet Molding Compound and Composites for Automotives
The motivation of present research is to evaluate the potential of Basalt fiber based Sheet Molding Compound (SMC) as a novel and cost effective material form for the use of composites in automotive applications for net-shape production and light weighting. The present paper reports results from our ongoing research on the feasibility trials for introducing Basalt fibers in sheet molding compounds through material manufacturing trials, panel fabrication, testing and comparison to current SMC materials. The technical approach included: Multi-Scale Characterization of Basalt fibers and Interface Shear Strength, Evaluation of Tow Properties, Manufacturing of SMC using Basalt fibers, Fabrication of test panels from SMC containing Basalt fibers, Characterization of the physical and mechanical properties of the Basalt fiber SMC test panels, and finally the Comparison of the Basalt containing SMC with standard SMC containing glass fiber as a reinforcement. The sheet molding compounds were formulated by Hexion using Mafic Basalt Fiber with optimized sizing and suitable resin systems and the composite panels were provided to the authors for its detailed characterization at the University of Tennessee, Knoxville. Multi-scale characterization work started with evaluating single basalt fibers using a very unique nano-tensile testing system. Through CSM (Continuous Stiffness Measurement), dynamic properties are determined throughout deformation process of single basalt fibers, and highly precise tensile modulus as well as its variation with axial strain till failure was obtained. Suitable number of single fibers were evaluated to obtain a two parameter (shape and strength) Weibull statistics for a gage length of 25 mm.
Micro-Mechanics Based Modeling of Joule Heating Induced Damage Propagation in Carbon Composite Laminates
Recent experimental studies on electrical conduction behavior of carbon fiber reinforced polymers (CFRP) laminates under various current densities have revealed that even when a composite is subjected to a relatively small current, for example five Ampere current applied to a 2.5cm by 2.5 cm specimen, it results in irreversible change in electric resistance of the CFRP laminates in the through-thickness direction, indicating internal structural damage. The understanding of damage initiation and propagation in CFRP under electric current application is of great significance in design and development of applications that utilize the electric properties of CFRP. In this study, a model is formulated with special attention to address the stochastic nature of composite microstructure and material properties. A localized Joule theory is developed to demonstrate the current concentration effects at multiple levels. A micro-structure based resistor network framework from previous studies has been modified to integrate thermal degradation and failure analysis. The model includes: (i) statistical description of composite microstructure including fiber arrangement and distance between fiber contacts; (ii) description of resin rich layer; and (iii) thermal and dielectric breakdown of the material system. By iteratively assessing damage states based on degradation analysis and updating material properties based on damage severity, the damage propagation of laminated carbon composite materials is established. The model is capable of predicting the electric resistivity in multiple directions, and damage states of composite laminates with arbitrary stacking sequence under variable current applications. The research is intended to raise awareness of internal structural damages induced by localized Joule heating. In addition, the quantitative relation between composite microstructure, internal damage state, and electric resistance established from the numerical model can be utilized for designing damage monitoring systems for CFRP laminates.
Anisotropic Soft Composite Based Hyperelastic Model
Tissues and organs are soft composite systems made of collagen fibers embedded within an extracellular matrix. These soft composite systems exhibit directional and regional material anisotropy due to varying fiber distributions and orientations. To date, anisotropic material properties of soft composite systems have been characterized using simplified isotropic and transversely isotropic hyperelastic material models in various computational and experimental studies. These simplifications may not only lead to inaccurate results from analyses, but also inhibit the study of the effect of important tissue modifications due to disease. In this work, a novel composite based hyperelastic model was developed which can characterize tissue anisotropy. This model considers both the soft fibers and matrix to be hyperelastic materials, which interact through shear deformation. The effect of the individual contribution of the fibers and matrix were introduced into the numerical framework for a single soft composite layer and the changes due to variations in the fiber orientation were incorporated into the strain energy functions. Also, strain energy formulations were developed for multiple soft composite layers with varying fiber orientations and contributions, which can describe the biomechanical behavior of organs consisting of multiple tissue layers. Stress versus strain relationships were derived from the strain energy equations for a uniaxial mechanical test condition. To validate the model parameters, experimental models of soft composites tested under uniaxial loads in literature were characterized using the novel anisotropic hyperelastic model. An average R2 correlation index of 0.983 was estimated for the developed model. To date, such a robust anisotropic hyperelastic model has not been developed to the best of our knowledge, which would be indispensable for experimental characterization of tissues and for improving the fidelity of computational models of organ systems in future.
On The Use of Multifunctional Z-Pins for Sensing Internal Damage in Composite Laminates Based on Electrical Resistance Measurements
Experimental studies have demonstrated that multifunctional z-pins can significantly improve structural damage resistance while also providing a sensitive response to damage. While TTRs have been commonplace in the composites world for decades, multifunctional TTRs with self-sensing capabilities are a relatively unexplored area. This paper focuses on investigating the feasibility of leveraging the inherent electrical conductivity of carbon-fiber and metallic z-pin reinforcements for sensing internal damage in composite laminates. The study utilizes finite element models in ABAQUS to analyze the electrical resistance of the z-pins during progressive failure of pins subjected to combined pullout and shear failure. The electrical finite element models were utilized to understand the interaction between impact-induced mechanical damage and electrical responses for these multifunctional reinforcements. The finite element simulations were conducted in two phases. Initially, the damage was evaluated using continuum shell elements with damage evolution. Next, the damaged elements from the mechanical analysis were incorporated into electrical models of the z-pins, and the influence of mechanical damage on electrical response was studied. The results of this study indicated that carbon fiber z-pins provide a stronger response to damage compared to titanium zpins. The stronger damage response of the carbon fiber was attributed to the inherent electrical anisotropy of the carbon fiber composite. The results from this paper will be used to inform the development of multifunctional through-thickness reinforced composite laminates that demonstrate improved damage tolerance and have smartsensing capabilities.
Computational Study of Major Loop Hysteresis in Active Fiber Composites
The non-linear and time-dependent response of Active Fiber Composites (AFC) under an application of a high electric field is studied in this paper. We consider an electro-mechanical constitutive model for polarization switching of Lead Zirconate Titanate (PZT) fibers. Small deformation gradients and large electric field beyond the coercive limit of PZT is considered. The model is implemented in Finite Element (FE) method. FE analyses for AFC under cyclic electric field are performed. A parametric study of the polarization model is conducted in order to understand the effect of material parameters on the hysteretic response.
Multi-Objective Optimization for Coupled Mechanics-Dynamics Analyses of Composite Structures
Fiber reinforced composites are increasingly used in advanced applications due to advantageous qualities including high strength-to-weight ratio. The ability to tailor composite structures to meet specific performance criteria is particularly desirable. In practice designs must often balance multiple objectives with conflicting behavior. Objectives of this work were to optimize lamina orientations of a three-ply carbon fiber reinforced composite structure for the coupled solid mechanics and dynamics considerations of minimizing max principal stress while maximizing fundamental frequency. Two approaches were investigated: Pareto set optimization (PSO), and multi-objective genetic algorithm (MOGA). In PSO, a single objective function is constructed as a weighted sum of multiple objective terms. Multiple weighting sets are evaluated to determine a Pareto set of solutions. MOGA mimics evolutionary principles, where the best design points populate subsequent generations. Instead of weight factors, MOGA uses a domination count that ranks population members. Results showed both methods converged to solutions along the same Pareto front. The PSO method calculated fewer function evaluations, but provided many fewer final data points. At a certain threshold, MOGA provides more solutions with fewer calculations. The PSO method requires more user intervention which may introduce bias, but can largely be run in parallel. In contrast, MOGA generation are evaluated in series. The Pareto front showed the trend of increasing frequency with increasing stress. At the low stress and frequency extreme, the stacking sequence tended toward (45°/90°/45°) with max principal stress located in the inner ply in the hoop direction. At high stress and frequency, the stacking sequences (90°/*/90°) indicated that the middle ply orientation was less significant. A mesh convergence study and dynamic validation experiments gave confidence to the computational model. Future work will include an uncertainty quantification about selected solutions. The final selected solution will be fabricated and experimental validation testing will be conducted.
Reliability-Based Approach for Sandwich Composite Structural Applications
Composite sandwich structures have many advantages in engineering design, due to their light weight and resistance to corrosion and in-service environmental conditions. Recent interest in such composite structures has increased in structural applications. Liability concerns prevent most structural engineers from considering this material, specifically, when the basis of the technical design data and long-term durability behavior are still uncertain. For this purpose, several sandwich composite samples (Balsa and PVC foam) were molded and subjected to severe weathering and environmental conditions. Experimental tests to predict the core shear strength, compression and moduli of the sandwich structures were performed on 30 samples. The Load and Resistance Factor Design (LRFD) method was employed and the resistance factors of the core shear and compression strengths were developed. This work advances the current state of the art by assisting engineers to quantify the reliability of their design and increase the utilization of composites as a reliable material for structures. The results of this research will ultimately lead to a probabilistic analysis model as well as act as a reference to guide those applying this technology.
Micromechanical Finite Element Modeling of Micro Punch Shear Experiments on Unidirectional Composites
Punch shear penetration of unidirectional composite shows micromechanical damage mechanisms such as (i) mode II dominated fiber facture, (ii) fiber-matrix debonding, and (iii) large deformation and cracking of matrix. Prediction of non-linear progressive punch shear damage behavior is of interest in determining the properties and parameters of continuum damage model such as, MAT162 in LS-Dyna®. A 2D micromechanical finite element model of unidirectional composite is developed by defining (i) crack planes orthogonal to fiber axis at an interval of 5 micron, and (ii) fiber-matrix debonding planes between the fiber and matrix. Zero thickness cohesive elements are used to model fiber fracture and fiber-matrix debonding. Energy release rate and cohesive parameters for glass is obtained from the literature. The stochastic distribution of fiber tensile strength is taken from experimental data of S glass at sub-millimeter length scale. Tangential tractions are estimated using maximum strain energy theory. Validated cohesive laws for fiber-matrix debonding is obtained from computational simulations of micro-droplet experiments. Elatic-plastic properties of matrix resin is obtained from experimental measurements.
Interesting Properties of 3D Warp Interlock Fabrics as Fibrous Reinforcement for Composite Material
3D fabrics tend to be more used as fibrous reinforcement into composite material due to their near-net-shape ability as well as their specific mechanical properties. Among these 3D structures, 3D warp interlock fabrics have been investigated in order to highlight their physical and mechanical properties. Several product parameters are needed to clearly define such a woven structure. In order to highlight the some product parameters influence, two different 3D warp interlock architectures with two different yarn’s raw material have been produced on the same weaving loom and tested. Significant mechanical behaviors have been observed in the weft direction compared to the warp direction, which tend to be stiffer due to less crimped yarns inside the woven structure. Additionally, it has been found that the shrinkage values of binding and stuffer warp yarns play an important mechanical behavior of 3D warp interlock fabrics during quasi-static tensile tests.
Pseudo-ductile Composites with Micro-wrapped Hybrid Tow
A novel dry fibre architecture has been developed in order to improve the ductility of high-performance composites. Micro-wrapping of high-modulus fibres with high strain (to failure fibres) resulted in a unique structure that is capable of delivering ductility without significant stress-drops. This paper presents a detailed experimental investigation of fibre architectures including side-by-side, micro-wrapped with single and double helix wrapping. Influence of matrix ductility on the composite laminate ductility has also been investigated.
Measurement of Intrinsic Residual Stresses in 3D Woven Composites Using Measurement of the Displacement Fields from Hole Drilling by Electronic Speckle Pattern Interferometry and Digital Image Correlation
Intrinsic residual stresses in woven composites result from the coefficient of thermal expansion mismatch between the fibers and the matrix. Extrinsic residual stresses result from large scale thermal gradients during curing and cooling. Intrinsic residual stresses in 3D woven composites are sometimes severe enough to cause micro-cracking in the matrix. They are also expected to impact the fatigue resistance and the impact resistance. To the best of our knowledge, there have been no spatially resolved measurements of the intrinsic residual stress field as a function of position in the repeating weave pattern. We used digital image correlation (DIC) and electronic speckle pattern interferometry (ESPI) to measure the surface displacement field resulting from drilling a 1 mm diameter hole at four selected locations in two different 3D woven composite architectures that represent low and high through-the-thickness constraint. The two methods are used because the displacements sometimes on the lower end of the resolution for the DIC method and the displacement gradients are sometimes too steep to resolve the fringes for the ESPI method. Finite element models constructed with realistic fiber geometry using Dynamic Fabric Mechanic Analyzer software were utilized to estimate the residual stress field from cooling from the curing temperature. Holes were manually inserted by deactivating the elements in the hole region and the resultant displacement fields were compared to the measurements. In general, the measured displacement fields were lower in magnitude than the model predictions. In some cases, the sign of the predicted displacement field is opposite to the observed field which could be attributed to differences between the actual hole location and the hole in the model.
Investigation of Mode I Crack Growth of VARTM Carbon Composites Using Optical Fibers
The objective of this study is to examine the development of Mode I delamination using strain distributions obtained from optical fibers embedded within composite laminates. Optical fibers were interlaced into dry carbon preforms near the midplane of [0/90/90/0] and [0/90/90/0] laminate configurations prior to infusion. Two different cure cycles, a manufacturer-recommended one-step cure cycle and a selected two step cure cycle, were used in the fabrication of the laminates. Double cantilever beam tests were performed to estimate the mode I crack growth. The internal strain was obtained from the embedded optical fibers and crack length was visually measured on the side of the test articles. In addition, a numerical method was developed to determine the crack growth based on the strain distribution associated with the crack front. Good agreement is obtained between the crack lengths obtained from optical fiber and surface measurements.
1D-Patterned Nanocomposites Structured Using Oscillating Magnetic Fields
Polymer nanocomposites are sought-after materials due to their low density and multi-functionality, but their bulk application of polymer nanocomposites (PNCs) is currently limited because scalable methods to manufacture PNCs with organized nanofiller structures, which is critical to enable advance properties, are missing. In this work, non-contact assembly of nanofillers within polymer matrices is investigated as a scalable, energy-efficient method to tailor nanofiller organization within PNCs using external, oscillating magnetic fields (~100s of G for ~10s of min). Last year, experimental and theoretical studies were presented about one-dimensional (1D) magnetic assembly behaviors of iron oxide nanofillers in DI water and also in a aerograde thermoset polymer. This year, experimental work was continued about 1D alignment of iron oxide nanofillers using nanofiller surface treatment for improved dispersion and suspension in a thermoset. In addition, electrical and thermal conductivities of these PNCs are being measured and will be correlated with their various nanofiller structures. The effect of controlled nanofiller interfaces, enabled uniquely with magnetic assembly, on the PNCs’ transport properties will be evaluated. If the nanofiller structuring capability is obtained, PNCs can provide tailorable multifunctional properties that are essential for aerospace, automotive engineering, medical devices, consumer products and sporting equipment applications. Near-future work of this project include 2D and 3D organization of nanofillers using a newly installed tri-axis Helmholtz coil set-up to provide elastomer PNCs for anisotropic transport, actuation, and sensing applications. This work is supported on the BAA (#N000141612172) and DURIP (#N000141712023) funds from the ONR Sea- Based Aviation Airframe Structures and Materials Program.
A Cohesive Zone Modeling Study on the Fracturing Behavior of Thermoset Polymer Nanocomposites
This work proposes an investigation on the fracturing behavior of polymer nanocomposites. Towards this end, the study leverages the analysis of a large bulk of fracture tests from the literature with the goal of critically investigating the effects of the nonlinear Fracture Process Zone (FPZ). It is shown that for most of the fracture tests, the effects of the nonlinear FPZ are not negligible, leading to significant deviations from Linear Elastic Fracture Mechanics (LEFM). By means of Size Effect Law (SEL) on the assumption of a linear cohesive crack law, the fracture tests in the literature were re-analyzed. As the data indicate, this aspect needs to be taken into serious consideration since the use of LEFM to estimate mode I fracture energy, which is common practice in the literature, can lead to an error as high as 157% depending on the specimen size and nanofiller content. This was further confirmed by matching the data by means of a cohesive zone modeling featuring a linear cohesive law. Taking advantage of size effect tests on thermoset-based graphene nanocomposites, it was also found that these materials are better described by a bilinear cohesive law. It was shown that, while the use of a linear cohesive law provides a good approximation, a bilinear cohesive law provides a superior description of the fracturing behavior for different sizes.
Effects of Density and Cell Rise Ratio on 3D Failure Strengths of Rigid PVC Foam in Different Loading Modes and Loading Directions
A study was conducted to investigate the important effects of foam density and cell rise ratio on failure strength of rigid closed-cell PVC foams. Experiments were performed on test samples made from an as-received Divinycell H100 PVC foam panel which had relatively large density variations in both planar and through thickness directions. With the inherent foam density variation that existed in the foam panel, tension, compression and shear specimens with different foam densities were prepared, which eliminated the possible effect of variation in processing conditions on foam strength. Recently developed accurate 3D foam test methods by the authors were employed in the study for the foam strength determination. Foam failure strengths in both out-of-plane and in-plane directions were determined. The effects of foam density and foam cell rise ratio on the foam in-plane and out-of-plane strength properties are found to be significantly different. To augment the experimental study a recently developed predictive models for foam compression, tension and shear strengths were used to further investigate the effects of foam density (r) and foam cell rise ratio (b) on the measured foam failure strengths. Good agreements are found between the experimentally determined and theoretically predicted foam strengths for all loading modes in different loading directions. The importance of the combined effects of foam density and cell rise ratio on foam failure strengths is clearly demonstrated. The results of the present study on foam strength and our earlier results on foam stiffness show that for in-plane and outof- plane foam shear strengths, foam density r is the governing parameter. For out-ofplane and foam compression and tensile strengths, (=br) is the key parameter that must be used to provide accurate description of closed-cell PVC foam strength and stiffness.
A Methodology for the Analysis of the Initiation of Inter-Fiber Failure and Local Delamination in Wind Turbine Blade Shell Sandwich Structures
Sandwich structures are extensively used in wind turbine blade shell structures design to reduce total blade weight while providing buckling resistance. In this paper, a novel methodology to analyze the initiation of inter-fiber failure (IFF) and IFF induced local delamination in sandwich face-sheet laminate under full blade fatigue test is proposed. The detrimental influence of in-plane shear stress on IFF initiation under multiaxial fatigue loading was accounted for using a strength-based knockdown factor approach. The initiation of the local delamination was analyzed by a Fracture Mechanics model, where energy release rate (ERR) was studied as the delamination driving force. It is found that the critical ERR for local delamination depends on face-sheet laminate layup as well as resultant forces. The proposed methodology provides a very simple but useful method for evaluating the potential associated damage of blade structure during fatigue testing and it can also be extended to analyze the static loading scenario.
Efficient Manufacturing Method of CFRP Corrugation by Using Electro-Activated Deposition Resin Molding
CFRP corrugation structures are used for various purposes, and application to morphing wing has recently been drawing attention. However, an effective manufacturing method for the CFRP corrugation has not established. Vacuum-assisted resin transfer molding (VARTM) method can decrease manufacturing cost, but it cannot be applied for corrugation since vacuum bagging is impossible. Therefore, the conventional method to laminate prepreg and consolidate by using autoclave has been applied. In this study, by using electro-activated deposition resin molding, it was confirmed that CFRP corrugation structures could be easily manufactured. Neither autoclave nor vacuum bagging was necessary. The corrugation and corrugated core sandwiches with different pitches were fabricated and mechanical properties were investigated.
Finite Element Analysis of Moisture Diffusion into Sandwich Composite Using Thermal-Mass Diffusion Analogy
A finite element model was developed in ABAQUS to predict the moisture ingression into sandwich composites due to moisture diffusion. The finite element model was based on thermal-mass diffusion analogy which had been used to develop finite element models for predicting moisture diffusion into small-scale electronics. On the other hand, several sandwich composite specimens were exposed to elevated humidity levels, and the resulting increase in core humidity over time was measured. Predictions based on the ABAQUS analysis were then compared to the experimental measurements and also with predictions of a finite difference scheme developed in-house previously. The trends of predictions by the ABAQUS analysis were found to be in accordance with the experimental results and finite difference predictions, with scope for improved prediction with more accurate material properties.
Coupled Diffusion/Large-Deformation Behavior of Epoxy Matrix Resin in Corrosive Environments
Since thermoset resins that contain ester groups (including vinylester, polyester, and anhydride-cured epoxy) are susceptible to hydrolytic degradation, amine curing agents with epoxy are a widely used alternative because they are much more stable against water or alkaline solutions and also allow for curing at lower temperature compared to anhydride. However, amine groups react with inorganic acids to form a salt which can have detrimental effects on the epoxy resin depending on the composition and curing conditions. Experimental data pertaining to diffusion kinetics of sulfuric acid in aminecured epoxy resin is lacking in the literature, but the available studies have shown that it forms a step-like concentration profile, rather than a Fickian-type gradient, and its mass-change tends to be proportional to the square-root of time, similar to Fickian diffusion. Although usually no cracking or mass-loss is observed, strength and stiffness decrease over time with almost no warning, making it a very dangerous durability problem. The current study examines the unique high-swelling phenomenon of bisphenol-F epoxy (DGEBF) cured by polyamine in hot/wet and low-pH environments. Most studies of epoxy or vinylester matrix resins report swelling between 0.5-3%, within small-strain approximation for coupled diffusion-deformation models. However, considerable swelling of DGEBF/polyamine is observed, reaching a total volume increase over 30% for the most aggressive condition. This creates considerable internal stresses that in turn severely affect the mechanical properties of the resin in the shortterm time scale (i.e., pre-saturation). The step-like concentration profile of sulfuric acid creates a moving swelling front where rubbery and glassy phases are separated by a small transition region; compressive stress is distributed through the swollen layer while the glassy core experiences tensile stress as it constrains some swelling, and the transition layer has some associated shear stress. This affects long-term strength and overall material performance.
Erosion of Uni-Directional Carbon-Fiber Reinforced Polymer Composite—A Micromechanical Approach
Fiber reinforced polymer composites are used in wind turbine blades and helicopter rotor blades because of higher strength to weight ratio and better fatigue resistance. However, these blades while operating in dusty environments are subjected to particulate impacts and can cause leading edge erosion affecting the performance of blades. Therefore, it is necessary to understand the mechanism of erosion in fiber reinforced polymer composites. In this study, a 3D finite element model of unidirectional carbon fiber-epoxy composite is subjected to impact by a spherical erodent particle made of steel with a velocity of 45m/s. The results indicate the influence of fiber orientation and particle impingement angle on the erosion resistance of the composite. The analysis results also capture for the first time the underlying reasons for the semi-ductile response of polymer composites to erosion.
Improvements in the Structural Analysis of a Composite Material T-joint Structure
In this work, a T-joint structure made of woven fabric carbon/epoxy composite has been numerically investigated, using the commercial software LS-DYNA, by referring to previous experimental tests performed by the authors. The analyzed component can be considered as a simplified version of the real T-joint which connects the B-pillar and the longitudinal rocker in car body structure. Its mechanical behavior under transverse bending has been firstly evaluated through a series of experimental tests and then simulated with transient nonlinear finite element analysis with explicit time integration. The goal is to properly account nonlinear effects and improve the accuracy of the finite element prediction. In particular, the attention has been posed on the nonlinearities due to the manufacturing process and design of the experimental setup. Parameters related to the material properties and to the compliance of constraints are carefully considered to understand their influence on the load-carrying capacity and the failure mode of the structure. By properly setting those parameters in LS-DYNA environment, a very good agreement between experimental and numerical simulation results is obtained.
Crashworthiness Analysis of Short Fiber Reinforced Composite Bumper Beam Using Multiscale Modeling and FE Simulation
In this work the crashworthiness response of a vehicle bumper beam made of short glass fiber reinforced composite was mainly investigated using multiscale material model and explicit time integration numerical technique. The matrix of the short glass fiber reinforced composite adopted for the bumper beam was a polyamide, i.e. a thermoplastic resin. Materials characterization was executed according to ASTM test procedures to verify the suitability of the material model and obtain the required characteristic data for FE model. For comparison purpose, reference material, steel, and CFRP were considered for FE simulations. FEM results showed that, based on equal bending stiffness criterion, the 30% glass polyamide based bumper beam exhibited the highest deceleration and least crashworthiness performance compared to the steel and CFRP bumper beams. On the other hand, result shows that CFRP composite beam has comparative performance with better energy absorption, weight saving, and gives reduced peak deceleration.
Effectively Reduced Damages with Increased Through-thickness Electrical Conductivity of CFRPs Against Artificial Lighting Strike
Most common method to make Carbon Fabric Reinforced Plastics (CFRPs) electrical conductive is to add conductive fillers, but how much through planeelectrical conductivity is needed to be safe against lightning strikes has not been investigated yet. In this work, three Carbon Fabric Reinforced Plastics (CFRPs) panels with 0.8, 0.56 and 0.17 S/cm Through-thickness electrical conductivity respectively are prepared using a conductive polymer-based conductive resin. Thermal degradation and de-doping of the conductive polymer were capitalized to design the composite panels with various electrical conductivities. Prepared CFRP panels were tested against simulated lightning current of 40 kA. The effect of the Through-thickness electrical properties on the effectiveness against lightning strike is characterized using various techniques. The most severe damage after lightning strike usually occur due to the rapid increase in the temperature due to Joule’s effect. This suggest that the damage behavior of the CFRP panels are highly dependent on the orthotropic electrical conductive nature of the specimen. By increasing the through-thickness electrical conductivity, orthotropic behavior of CFRPs are tried to mitigate. A thermography camera was used to observe the temperature profile on the surface of the samples at the time of impulse current arc attachment to the samples. Least rise in temperature was observed with high through-thickness electrical conductivity samples. Ultrasonic non-destructive testing images confirmed the least damage to sample with electrical conductivity of 0.8 S/cm.
In-situ Damage Precursor Detection in Fiber Reinforced Composites Using Mechanochemical Materials
Optical responses of mechanophore (stress-responsive materials) in fiber reinforced polymer composites under mechanical loads were characterized. A new experimental system was developed to capture in situ mechanophore activation by recording ultraviolet (UV) excited fluorescence during uniaxial load tests. Anthracene- based mechanophore, dimeric 9-anthracene carboxylic acid (Di-AC) was synthesized and incorporated into an epoxy-based thermoset successfully. This Di-AC embedded epoxy mixture was applied to glass fiber fabric to fabricate mechanophore embedded glass fiber reinforced polymer (GFRP) composites through hand-layup process. Quasistatic and cyclic loads were performed to investigate the effect of different types of loads on mechanophore activation. The results indicated that mechanophore activation occurred at the beginning of the test during the quasistatic loading test and continued linearly before yield. Microcracks were formed in the matrix prior to yield, and UV intensity of the mechanophore exhibited nonlinear response. During fatigue tests, the intensity of fluorescence increased after a certain number of cycles. Microcracks were initiated around the middle stage of the fatigue test, the intensity also showed a nonlinear response. The potential of anthracene-based mechanophore for early damage detection in GFRP under complex loading was observed.
Additive Processing of Sacrificial Polymers to Enable Pressure Sensing in Structural Composites
Additive manufacturing of polymers and their composites is an area of increasing academic and industrial interest due to the ability to agilely create complex prototype structures. While efforts continue to attempt to directly print more structural composite element, an alternative approach is to use additive printing with traditional composite processing methods to enable structural multifunctional composite concepts. Within this paper we demonstrate the application of fused deposition printing of sacrificial polymer material to enable embedded microvascular channels within structural composites to enable pressure sensing of morphing structures and for installing pressure tappings for studying hypersonic fluid-structure interactions. Overall, this processing methodology enables many type of microvascular composites whose channels can be computer designed, additively printed, integrated with traditional laminate processing, and effectively evacuated to enable a variety of multifunctional concepts.
MoS2 Dispersed Epoxy Composite: Influence of Solvent Quality and Surface Chemistry to Local Chemical Network Formation and its Influence on Nanoscale Toughening Mechanism
Despite the tremendous interest, an understanding of the onset of crack formation and propagation in nanocomposites is still very incomplete. This is due to a lack of a) characterization techniques to capture the mechanics at sub-100 nm level, and b) accurate models for the estimation of failure-modes. Current models based on linear fracture mechanics fail to capture sub-nanometer crack-tip formation or describe the nanoscale fracture toughness mechanism. Recently, molecular dynamics simulations have helped to illuminate the influence of nanoscale crack length and local void formation on a fracture. However, the accuracy of these simulations relies on refinement of the model parameters from specially designed experiments. With this motivation, the present experiments are designed to illustrate the toughening mechanism of MoS2 reinforced epoxy composites. A systematic processing technique was developed to ensure proper dispersion of the nanocomposites of high wt% of MoS2 in a DGEBA epoxy polymer matrix. The detailed characterization using in-situ FT-IR, DSC, DMA, AFM-IR enabled insights into the local polymer network and interphase formation based on the solvent quality and surface functionality of MoS2. High-resolution TEM, SEM, and AFM, as well as nano-X-ray computed tomography, lead to an understanding of the 3D dispersion of these nanosheets. Experiments associated with nanoscale visualization of fracture in the composites are conducted using in-situ tensile testing stages placed within SEM. These experiments may provide an in-depth understanding of the influence of flaw size, and platelet size, thickness, and dispersion of nanoparticles on fracture toughness and will be indispensable in guiding the refinement of multi-scale models.
A Stochastic Structural Finite Element Model for Trabecular Bone and other Structural Foams
Background: Although trabecular bone is a highly porous heterogeneous composite, most studies use homogenized continuum finite element (FE) approaches to model trabecular bone. Such models neglect the porous nature of the tissue. When microstructural models are desired, the use of continuum elements may require costly CT/MRI imaging and detailed meshing. The purpose of this study is to demonstrate an approach that simulates trabecular bone with less dependency on medical images while capturing the effect of porosity. Methods: A stochastic structural FE model was created representing the trabecular micro-architecture as beam elements. Beam orientation, length and connectivity were stochastically determined by random placement of nodes and meshing the resulting Voronoi diagram. Boundary conditions were applied on the structure to attain normalized axial and shear strain. Also, apparent mechanical properties, apparent densities and anisotropy ratios were calculated from the model output. Results: The number of generated seeds within the model and cross sectional area of the random beams were observed as parameters that affect model outcome. The apparent mechanical properties and apparent densities were found to be sensitive to number of beams; beams mean length and beams cross sectional area. Clinical Relevance: The proposed finite element technique provides a stochastically accurate structural representation of trabecular tissue and its reaction to applied loads. It incorporates several advantages of high fidelity methods but at lower cost and requiring only clinical imaging. Therefore, this approach may be useful for patient specific musculo-skeletal biomechanical modeling.
Defects Characterization, Damage Mapping, and Property Evaluation of Composites
The presence of voids in the composite laminates can reduce the mechanical properties in composites and these voids can become precursors in the activation of other failure modes such as delamination. Knowing the volume fraction of voids is not enough since their criticality is highly reliant on their location, size, and geometric configuration. While the use of X-ray computed tomography (CT) combined with an advanced image processing and segmentation procedure can extract geometric configurations of voids, the introduction of these voids into a finite element model has posed a great challenge due to the presence of distinct length scales in the description of these voids and evaluation of their effects on the performance of composite components. In order to map an arbitrary distribution of voids into a 3D finite element model, two modeling approaches are developed based on an immersed method for an implicit representation of voids and a computational geometry method to explicitly determine the intersection of a discretized void with its surrounding elements. Based on the intersection of an arbitrarily oriented void with elements and a ply interface, the resulting volume fraction of voids within each element and the surface area at the ply interface is determined and used to compute the initial stiffness degradation and initial delamination area for the subsequent progressive failure analysis. Modeling efficiency and accuracy of both methods are demonstrated using a void with an analytical geometry and a discretized geometry generated from the X-ray CT data.
A Visco-hyperelastic Constitutive Model for Fiber-Reinforced Rubber Composites
The focus of this study is to develop a fabric architecture-based constitutive model to predict the time-dependent mechanical behavior of a knitted-fiber-reinforced rubber. The rubber composite has a sandwich structure consisting of a fabric layer embedded into two rubber layers. Here, the rubber is a visco-hyperelastic material, while the fibers tend to be linear elastic with little extensibility. The rubber composite shows a nonlinear, anisotropic constitutive response under finite strawin and stress relaxation under constant deformation, which is modeled through a novel strain energy function. The anisotropic behavior comes from the conservation of the total length of the fiber cords. The strain energy function comprises two parts to define both the static behavior and the time-dependent response. This approach enables the determination of the constitutive behavior of the rubber composite based on the constituent material properties and geometry, such as the viscoelastic properties of the rubber and the fiber architecture. The proposed strain-energy-based approach is validated through a full Finite Element (FE) model on basis of a Representative Unit Cell (RUC), in which the textile reinforcement and rubber layers are modeled explicitly.
Process and Health Monitoring of FRP by Rayleigh-Scattering Based Distribution Optical Fiber Sensors
Distribution optical fiber sensors are attractive because of capability of distribution sensing of strain for large-scale FRP structures. Recently, Rayleigh-scattering based distribution optical fiber sensors, which has high spatial and strain resolution, have been developed. Therefore, it is expected that local strain distribution produced by such as local damages or unevenness of temperature distribution can be monitored by the Rayleigh-scattering based sensors. In the present study, feasibility studies of process and health monitoring of FRP by the Rayleigh-scattering based sensors were conducted. In the monitoring of VaRTM process of textile GFRP, flow front of resin impregnation and process-induced strain were monitored. From the monitoring results of impregnation process, it was found that the flow-front of the resin could be observed as the maximum compressive strain in the strain distribution. From the strain curves after start of heating process (40 minutes), the optical fiber sensor could measure thermal strain by heating from 40 to 100 minutes, curing shrinkage from 100 to 150 minutes and thermal shrinkage by cooling from 150 to 170 minutes successfully. Therefore, it can be concluded that the Rayleigh-scattering based sensors could monitor impregnation status of resin and distribution of process-induced strain during VaRTM process. In the health monitoring, damage identification of cross-ply GFRP laminates with delamination by the attached distribution optical fiber sensors was carried out. Three types of specimens, no delamination, delamination between the central layers and delamination between the outer layers were prepared. The distribution sensor was attached on the bottom surface. From the results, it appeared that the strain varied largely on the delaminated section and the measured distribution is similar to the analyzed one. Therefore, it was found that the Rayleigh-scattering based sensors could identify delamination from surface strain distribution.
Repeatability of Non-autonomous Self-Healing with Thermoplastic Healing Agent in Fiber Reinforced Thermoset Composite
A repetitive in-situ healing system for fiber reinforced polymer composite is being developed by incorporating the thermoplastic polycaprolactone (CAPA) healing agent (healent) and shape memory polymer in a thermoset epoxy (SC-780) system. Doublecantilever beam (DCB) specimens with embedded healant were repeatedly fractured and healed by heating above the melting temperature of CAPA (58-60°C). In-situ macro fiber composite (MFC) actuated heating/healing of the fractured specimens was performed by applying a voltage to MFC in the range of 200 to 250 V with 4 kHz excitation frequency for 2 hours to generate 80°C at MFC locations. Heating of the DCB specimens, followed by 24 hours cooling at room temperature yielded as much as 81 % recovery of the virgin interlaminar fracture toughness for different healing cycles, which is close to the maximum healing efficiency (86 %) of conventional oven heating for 2 hours at 80°C. The effect of variation of heating time and other heating parameters are also reported. A parametric study of healing efficiency as a function of heating time showed the effective healing with only twenty minutes of MFC actuated heating was obtained. Scanning electron microscope and optical microscope were also used to qualitatively analyze the fracture surfaces to understand the mechanisms responsible for repetitive self-healing.
3D Printed Continuous Fibre Composites: Exploiting Design Flexibility to Achieve Application Specific Properties
In the current study, the response of continuous fibre composite structures with variable fibre topology was investigated. These structural configurations are designed to be manufactured via continuous fibre three-dimensional (3D) printing. In order to take advantage of the design flexibility afforded by 3D printing a periodic sinusoidal fibre printing pattern was introduced. It would be difficult to manufacture this type of sample using conventional manufacturing methods, however, these structures can be easily produced using commercially available continuous fibre 3D printers. In order to understand the influence of variable fibre topology the response of these structures under both axial and shear loading was investigated. For axially loaded samples a moderate fibre deviation or ‘waviness’ resulted in an increased failure displacement with a slight reduction in ultimate strength. For similar samples subjected to pure shear loading it was found that the fibre misalignment introduced both transverse and axial stresses. There is experimental evidence that moderate transverse compressive stresses require a larger shear stress to initiate failure, compared to the pure shear failure stress. However, the transverse tensile stresses that were introduced would effectively weaken the structure negating any benefit that would be obtained from the moderate transverse compressive stresses. Future work will focus on eliminating the transverse tensile stresses whilst introducing moderate transverse compressive stresses. If this can be achieved, the effective laminate shear strength, can be increased with no change to the mechanical performance of the constituent materials.
Temporal Enhanced Ultrasound as a Novel NDT Technique for Characterization of Defects in Composites
Nondestructive Testing (NDT) methods are commonly employed to monitor manufacturing and in-service defects of composites. These defects include porosity, foreign object debris, impact-induced delamination and heat-induced resin degradation. Conventional ultrasonic methods such as pulse-echo analysis based on loss of signal amplitude are widely used for NDT. However, application of conventional ultrasonic methods is challenging for cases where the loss of signal is negligible, e.g., micro-porosities, micro-cracks, heat-damage or under-cured parts. Temporal-enhanced ultrasound (TeUS) is a novel non-invasive imaging technique that captures information from a temporal sequence of backscattered US radio frequency data obtained from a fixed location. Recent studies have demonstrated numerous effective applications of this technology for in vivo and ex vivo characterization of biological materials. In this paper, we demonstrate the feasibility of using TeUS as a novel, inexpensive NDT approach for composites inspection, and present the results of experiments designed to investigate its classification capability for composite defects.
Failure Prediction Using Viscoelastic/plastic Constitutive Equation Considering Entropy Damage for Polyimide
In recent years, it has been attempted to clarify the damage evolution in solid materials from irreversible thermodynamics. In this study, tensile tests were carried out polyimide and entropy generation amounts were calculated until failure based on irreversible thermodynamics. The parameters of viscoelastic/plastic model were determined by fitting with the result of creep and recovery tests. We predicted fracture of polyimide in the tensile test by using viscoelastic/plastic model which considered entropy generation amounts as criterion of damage.
Tensile and Compressive Failure Behaviors of Triaxially Braided Composite
The failure mechanisms of triaxially braided composite under in-plane tensile and compression loads are investigated in this study. The testing specimens are cut from a typical 8-layer 0°/±60° triaxially braided composite panel which is fabricated with Toray T700 carbon fiber and a toughened epoxy resin 3266. The quasi-static tension and compression loads that parallel or vertical to the 0° fibers are applied by using a Hydraulic testing machine, respectively. The Digital Image Correlation (DIC) technology is implemented to measure the displacement and strain fields of gauge section for both the tensile and compression tests. As a result, the failure mechanisms under different load conditions are elaborated contrastively.
Multi-Scale Evaluation for Effect of Reinforcements on Viscoelasticity of Shape-Memory Polymer Composites
The present study investigated the effect of reinforcement on the viscoelasticity of shape-memory polymer composites (SMPCs) from two different scale views utilizing two different techniques. From a nano-scale view, we conducted dynamic viscoelastic simulations using molecular mechanics simulation models proposed by our research group [1]. These nano-scale simulations could reproduce a clear glass transition from glassy state to rubbery state in the frequency domain; therefore, they are suitable for studying the filler effect including interface properties. From a macro-scale view, we conducted experiment dynamic mechanical analysis using woven fabric based SMPCs. Through comparison of multi-scale analyses, we investigated the importance of nano-scale analysis for the composites’ design of viscoelasticity.
Effective Diameter of Added Glass Fiber into Matrix of Carbon-fiber Reinforced Thermo-Plastics for Improving Mechanical Properties
The aim of this study is to clarify the effective diameter of added glass fiber into the matrix of chopped carbon fiber tape reinforced thermo-plastics (CTT) for improving mechanical properties. Three types of submicron glass fibers of which the diameter were comparatively different were added into PP (polypropylene) with MAPP (Maleic-Anhydride-modified polypropylene) to modify the matrix using twinscrew extruder. The extruded modified resin was formed to the sheet shape with 0.1 mm in thickness in a T- die. The diameter of submicron glass fibers were relatively changed to 0.18, 0.28 and 0.69 μm of three types. The weight content of MAPP and that of submicron glass fibers were 5 wt% and 3 wt%, respectively. The static bending strengths obtained by three point bending tests were improved due to modification of matrix with submicron glass fiber. The particles of matrix modified with submicron glass fiber were remained on the carbon fiber protruded from the fracture surface after bending fracture. This result suggested that adhesive property between matrix and carbon fiber was improved by the matrix modification with submicron glass fiber. To check the adhesive property, micro-droplet test was performed. Interfacial shear strength obtained by micro-droplet test was significantly improved when the diameter of added submicron glass fiber was 0.28μm. These results indicated that adding submicron glass fiber into matrix was effective for improving mechanical properties and the glass fiber whose diameter is 0.28μm as additive for matrix of CTT was most effective for improving mechanical properties in a range from 0.18 to 0.69μm in diameter of glass fiber.
Experimental Investigation Into the Failure of CFRP T-joints Under Ice Impact and Quasi-static Loadings
The dynamic failure behaviors of composite T-joints were investigated through conducting the simulated hail ice (SHI) impact experiment. The SHIs were made in laboratory environment and fired by using a gas gun. Crack onset and propagation in the deltoid of CFRP T-joints were captured by using a high-speed camera. The quasistatic tensile experiment was also carried out and the digital image correlation (DIC) technique was adopted to capture the failure process of CFRP T-joints. The different failure mechanism of CFRP T-joints under tensile and impact loadings were comparatively discussed in this study and the deltoid zone was found to play a key role in the damage process under impact loading.
Experimental and Analytical Studies on the Solvent Volatilization Behavior of Carbon Fiber/Phenylethynyl Terminated Polyimide Prepreg During Molding
Phenylethynyl terminated polyimide “TriA-Xâ€ developed in Japan Aerospace Exploration Agency (JAXA) exhibits excellent mechanical properties with high glass transition temperature (> 350°C), therefore it is suitable as matrix resin of high temperature composites. Carbon fiber (CF)/TriA-X prepreg includes about 10 wt.% of NMP (N-methyl-2- pyrrolidone) as solvent. The solvent must be volatilized to make high quality laminates, however experimental and numerical researches regarding the solvent evaporation behavior have not been reported. This study examined the solvent volatilization behavior experimentally. As a result, the anisotropic solvent diffusion behavior was observed. The experimental result suggested the breather cloth around the laminated prepreg affects the solvent volatilization behavior. By considering the solvent diffusion in the laminated prepreg and breather cloth based on Fick’s law, the solvent volatilization behavior was successfully simulated. The numerical results showed good agreement with the experimental results. It was confirmed that the simulation methodology is effective to determine the adequate temperature profile for solvent volatilization process.
Long-Term Durability of Unidirectional CFRP Subjected to Tensile Loading
In the present study, the tensile strength and its long-term durability is predicted by Simultaneous Fiber Failure (SFF) model which is presented by the authors (Koyanagi et al. Journal of Composite Materials 2009). The main aim of this research is to evaluate a durability at the elevated temperature condition beyond glass transition temperature (Tg) of constituent polymer material. Two types of specimens are examined in this study. The fiber strengths, interface strengths, and matrix strength are measured in order to predict the tensile strengths of two composite materials. For evaluation of long-term durability, time-temperature superposition principle is adopted considering previous results (Koyanagi et al. Mechanics of Time-Dependent Materials 2012). DMA tests are conducted for resin to evaluate time acceleration by elevated temperature (Arrhenius-type activation energy) and Tg. The predicted durability of two composites show rather good agreements with experimental results, qualitatively. Especially for quantitative comparison, the static strengths and long-term durability of two composites agree well with experimental results. This paper suggests that the durability of unidirectional CFRP can be estimated even beyond Tg. The detailed procedure of this estimation is described as follows.
Temperature-dependent Effective Electrical Conductivity of Carbon Nanotube-epoxy Nanocomposites: A Semi-analytical Model
The semi-analytical model for predicting electrical conductivity of carbon nanotubes based composites developed by Deng and Zheng [15] was extended to high temperatures. To be able to apply such model to temperatures up to 800 C, correlations coefficients were examined under the thermogravimetric analysis perspective. Four different heating rates (10 Co/min, 20 Co/min, 40 Co/min, and 50 Co/min) and four CNT/epoxy compositions (control sample, 0.5 wt. %, 1.0 wt. %, and 2.0 wt. %) were investigated. As carbon nanotubes have the tendency to agglomerate a morphological study was performed using Atomic Force Microscopy. The investigation revealed no major agglomerations and a good spatial dispersion. The TGA data were treated considering the reacted fraction as function of the absolute temperature. Two different coefficients were proposed based on reacted fraction, activation energy, and the rate of reaction. The semi-analytical expressions are physically related to the thermal degradation phenomenon, including the carbon nanotubes de-percolation effect. The model is continuous on temperature and reacted fraction, but it is discrete on CNT concentration. The model seems to capture the physical phenomenon of thermal decomposition all three main phases with accuracy
A Coupled Thermo-chemo-mechanical Model for High Temperature Oxidations in Polymers and Polymer Composites
High temperature oxidation in polymers is a strongly coupled, highly nonlinear and complex process which limits the use of polymeric materials in several aerospace and defense application as it causes significant performance degradation. The coupled diffusion-reaction process that occurs during thermo-oxidation together with the macromolecular network behavior and finite viscoelasticity of a polymer require a time, temperature and oxidation dependent constitutive law to predict the mechanical response of the material. In this work, a thermos-chemo-mechanically coupled constitutive model was formulated for polymer oxidation. The model was studied for regular and excess oxygen case. The material properties were studied from the preliminary work. The modulus of the oxidized material increases compared to the virgin polymer. It was also noted that in presence of external strain (deformation) the situation aggravates and accelerates the oxidation process.
Elevated-Temperature Thermal and Mechanical Behavior of Carbon Fiber/Graphite/PTFE/PEEK Composite
Elevated-temperature thermal and mechanical properties of carbon fiber (CF)/graphite flakes (Gr)/PTFE filled PEEK composite are investigated by a combined experimental and theoretical study. In the experimental phase, the CF/Gr/PTFE/PEEK composite was fabricated by compression-molding, followed by a sintering process. Optical and electronic microscopy was performed to examine microstructure of the composite and X-ray diffraction (XRD) was conducted to provide morphological information. Dynamic-mechanical analysis (DMA) was performed to evaluate composite thermomechanical properties. Differential scanning calorimetry (DSC) and thermal-mechanical analyzer (TMA) were employed to determine heat capacity, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and thermomechanical preperties of the composite at elevated temperature. To augment the experimental study, micromechanics modeling and analysis of the composite thermal and mechanical behavior are also conducted to better understand the fundamental nature of composite microstructure and environmental interaction. Good agreement was observed between the experimental data and theoretical predictions. The detailed effects of elevated temperature on CF/Gr/PTFE/PEEK composite thermoelastic (storage) and viscoelastic (loss) moduli, thermal expansion and heat capacity are determined.
Optimal Design of Composite Shells with Multiple Cutouts Based on POD and Machine Learning Methods
Due to the high specific stiffness and strength, composite shells have been widely used in fuel tanks of launch vehicles. The buckling analysis of composite shells with cutouts based on the finite element (FE) method is too time-consuming. From the point-of-view of model size reduction, a novel Proper Orthogonal Decomposition (POD)-based buckling method is proposed in this paper, which can significantly increase the computational efficiency of buckling analysis. In order to improve the efficiency and effectiveness of prediction and optimization of composite shells with multiple cutouts, the POD method is integrated into an optimization framework that uses Gaussian process (GP) machine learning method. First, the training set used for the machine learning training is generated efficiently by means of the POD method. Then, the obtained set is trained and tested based on the Gaussian process method. The inputs are ply angles of the composite shell and the output is the buckling load of the composite shell containing cutouts. In order to maximize the buckling load of the composite shell against cutouts, the Genetic Algorithm is combined with the trained Gaussian process method to search for the optimal ply angles. Finally, an illustrative example is carried out to demonstrate the effectiveness of the proposed prediction and optimization framework.
An Examination on the Applicability of Compressive Buckling Allowable Design for Composite Panels and Analysis for Strength Calculations
The purpose of this study is to examine the applicability of buckling allowable design for composite stiffened panels and to establish a simple method that can predict the strength of composite stiffened panels under compression loading. Typical compression loading tests were carried out on the various fiber-direction stiffened panels. As increasing the compression load on these panels, unstable phenomena accompanied with some different deflection mode shapes were observed before their final breakdown. However, the out-of-plane displacement at the location of the anti-node point monotonously increased during the tests. Utilizing this type of increasing displacement, we propose a simple strip analysis to predict the critical load for post-buckling panels. An energy adsorption method has been applied to calculate the de-bonding load between stringers and skin panels. The results of the calculation showed good agreements with those of the tests. As shown in the test results here, the ratio (de-bonding / buckling load) is large, suggesting the possibility of further weight reduction. We got good agreement between the test results and the calculation for shear buckling allowable design by similar method. We believe that we can provide aircraft fuselage design tool.
An Investigation of Inner Flange Buckling in Furlable Composite Booms
The triangular rollable and collapsible (TRAC) boom is an attractive architecture for deployable spacecraft structures due to its minimal flattened height-to-deployed stiffness ratio. A challenge for TRAC booms however is the development of a buckling mode that occurs (on the inner flange) when furling the boom around a hub for stowage. In this research, the buckling mode was found to be sensitive to boom flange length and the composite flexural stiffnesses dictated by the laminate materials, fiber orientations and ply stacking sequence. Finite element studies were performed to investigate the influence of flange arc length and composite layup on critical stresses and strains prompted by the buckled wave. Longer flange lengths resulted in higher strains but could be offset through modifications to the laminate architecture allowing for larger booms to be packaged without increasing the minimum stowage (hub) diameter. The analysis model was validated through experimental furling tests and successful correlation between the simulation strains and experimental strain gages.
Finite Element Based Buckling Cross-Sectional Optimization for Composite Arrows
In archery, dynamic buckling during the launch phase compromises the target accuracy of arrows. For both dynamic and quasi-static arrow buckling, the critical load depends upon the area moment of inertia of the cross-section which should be increased at constant arrow weight, by redistributing the material as far away from the principal point of the cross-section as possible, and while keeping the material thick enough to prevent local buckling. In this paper we present an effort to optimize the cross-sectional shape of a composite arrow shaft, using a finite element based, quasi static buckling analysis keeping the length and area of the cross-section constant. The composite column considered is assumed pinned at both ends and is assumed made with fibers oriented along the length of the column. Four cross-sectional shapes, tubular circular, tubular equilateral triangular, star shaped and star with beads are analyzed in this study. The composite column is modeled in ABAQUS, and the buckling load is determined by using the “Linear Perturbation, Buckleâ€ analysis. The transition from global to local buckling characterized by a decrease in bucking load and change in the buckled shape of the column is determined for each cross-sectional shape. The point of transition marks the maximum load that can be sustained for that cross-sectional shape. The maximum load for all the cross-sections is determined and compared. The tubular circular cross-section composite column is found to provide the highest buckling load followed by the star with bead cross-section, star shaped cross-section and tubular equilateral triangular cross-section composite column in the respective order. Thus, of the shapes considered, the tubular circular cross-section is the optimum shape for the cross-section of the arrow shaft.
Buckling Stability of Additively Manufactured Isogrid
This work investigated the buckling stability of isogrid panels manufactured using fused deposition modelling (FDM). In particular, it verified the use of existing closed form analytical solutions and finite element analyses for predicting both global and local buckling loads and modes for these structures. FDM-produced isogrid samples were subjected to uniaxial quasi-static compression with boundary conditions approximating simple supports. Buckling values and mode shapes were obtained from Digital Image Correlation (DIC). The values obtained experimentally were compared to buckling loads calculated using finite element analysis and closed form solutions for orthotropic materials. Good agreement was obtained in comparing finite element analysis to experimental results for both global and local modes, while closed-form solutions compared well for the global modes for which the solutions were intended.
Failure Mode Transition in Transverse Tensile of UD-CFRP Under Various Temperatures and Strain rates
In the present study, strain-rate dependence and temperature dependence of failure mode are numerically simulated by finite element analyses. In the analyses, interface failure and matrix failure are expressed by cohesive zone modeling and continuum damage mechanics, respectively. It is assumed that the damage initiates dependently of strain rate and temperature, and cohesive zone modeling is assumed to be temperature- and time-independent. In the continuum damage mechanics, Christensen’s failure criterion of multi-axial stress states for each strain rate are applied into the resin properties. Interfacial strength which is obtained by microbond test is introduced into cohesive zone modeling. When temperature is high and/or strain rate is low, matrix crack occurs very often and the failure mode is matrix-failuredominant mode. On the other hand, when temperature is low and/or strain rate is high, interface crack significant, i.e. failure mode becomes interface-crack-dominant mode.
Micromechanics Model for Wavy CNT Nanocomposites with Weakened Interface
This study presents an efficient approach for deriving the modified Eshelby tensor for wavy CNT nanocomposites with weakened interface to be used in the Mori-Tanaka model. Waviness and interfacial damage between matrix and CNT fillers are two main features that influence elastic stiffnesses of CNT nanocomposites. However, both have yet been taken into account at the same time for micromechanics model. Using the proposed model, we investigated influences of the waviness, damage level, volume fraction, interfacial damage and orientation on the effective stiffness of both 3D randomly oriented and aligned CNT nanocomposites. Increases of the volume fraction showed positive reinforcing effect while the waviness and interfacial damage decreases the effective stiffness. The proposed model is expected to provide realistic effective elastic stiffness of CNT nanocomposites.
Rapid Generation of Representative Volume Elements with Non-uniformly Dispersed Reinforcements for High Volume Fraction Composites
With the increased usage of lightweight composite materials for various applications across different industrial sectors, so-called computational experiments are becoming more widely employed to tailor and optimize their properties and minimize experimental testing during design. A representative volume element (RVE) depicting the material’s microstructure is often used to evaluate effective macroscopic properties or assess local stress concentrations for damage initiation prediction. Past research efforts have generated RVEs for composites with multiple realistic nonuniformly dispersed reinforcement geometries to improve mechanical property prediction accuracy. The challenge is that existing algorithms often require a great deal of time to converge to a suitable microstructural representation when many nonuniformly dispersed reinforcements are considered, or they may not converge at all for composites with high reinforcement volume fractions. In this study a new efficient method for rapidly generating realistic RVEs for various types of composite materials containing reinforcement volume fractions up to 80% was developed based on event driven molecular dynamics (EDMD) theory. A thorough spatial statistical assessment, including use of radial distribution functions and 3D autocorrelation with density recovery profiles, was performed on the generated RVE geometries to gauge the degree of non-uniformity of the reinforcement dispersion.
Damping Properties of Polymer Lattice Materials
Microlattices, or microtrusses, are periodic open-cell structures formed by a network of interconnecting struts (Figure 1). With deliberate material arrangement – used to affect efficient manipulation of the load paths – these engineered architectures can significantly enhance the bulk material properties per unit weight. Cellular materials can also possess multifunctionality with potential to provide a suitable combination of specific strength and stiffness, vibration suppression, and acoustic absorption that could benefit a number of commercial undersea applications; for example, pressure vessels and fairings fabricated from sandwich structures with microlattice cores can potentially exploit energy absorption capabilities of these cellular solids to reduce the vehicle noise signature. The present work seeks to characterize the viscoelastic properties of several cellular architectures to better understand the frequency-dependent damping of these lattice materials. A dynamic mechanical analyzer (DMA) is used to collect experimental data for comparison with predicted behavior derived from finite element analysis (FEA). The recovery and capacity for energy absorption of each cell topology are assessed, and the details of the analytical techniques and experimental program are provided.
A Continuum Damage Model for Fatigue and Its Integration Scheme
The objective of this paper is to develop a continuum damage model for fatigue prediction. A viscodamage model, which can rigorously handle damage anisotropy, distinct tensile and compressive damage behavior, and damage deactivation, is developed to produce stress-dependent fatigue damage evolution. An affine formulation governing damage evolution, and a closed-form formulation of the constitutive relations is derived based on the viscodamage model. An adaptive stepsize control and cycle jump time integration scheme is proposed and implemented to improve the present model’s efficiency in cyclic loading conditions. Through uniaxial cyclic loading simulations, the present model and time integration scheme is found to be capable of reliably and efficiently producing cyclic damage evolution. This model can be further calibrated to facilitate both uniaxial and multiaxial fatigue analysis in composite materials.
Designing Composite Leaf Spring with a Validated Finite Element Method
The leaf springs are one of suspension elements that play an important role in driving comfort and safety. These are commonly used in light and heavy commercial trucks, vans and some passenger cars. Generally, leaf springs are made of steels or composites. In this work, parabolic steel leaf springs and composite leaf springs are compared in terms of their stiffness and weight. For this purpose, a commercially available steel leaf spring was selected and a FEA model of this steel leaf spring was created. Steel leaf spring was modeled with a commercial FE software MSC Patran. Boundary and load conditions were implemented according to the test conditions. MSC Nastran software was used as solver. Static tests were performed to validate this model. During the test reaction, forces were measured with load cells on hydraulic piston, and stiffness value of steel leaf spring was obtained according to these results. In order to obtain similar stiffness characteristics of steel leaf spring, an alternative composite leaf spring 3D design is conducted and a second FEA model is generated. According to reaction force results taken from Nastran software, stiffness values of composite leaf spring were calculated. By iterating number of plies of composite leaf spring, a design that has the same stiffness value as steel leaf spring was obtained.
A Novel and Sustainable Approach to Recycle Prepreg Trim Waste Via Sheet Molding Compound (SMC) Technique
In this study, carbon fiber (CF) prepreg trim waste was converted to lightweight, high performance and low-cost CF sheet molding compound (SMC) composite. The prepreg trim waste, which is available in the form of spools of tape, was cut into chips of various width and length to prepare the SMC composite using a standard SMC line. The effect of aging was studied by using “tackyâ€ tape, which was properly stored, and “dryâ€ tape, which was aged at ambient or elevated temperatures. The composites were made by combining two or three SMC sheets in a mold and compression molding. The temperature and pressure molding profiles that resulted in the highest degree of cure of the resin in the prepreg and the smaller void content respectively were determined. In addition, the charge size and mass were varied to optimize the flow of the charge to completely fill the mold. The tensile modulus and strength, and impact strength were determined according to the corresponding ASTM standards, as a function of the conditions used to prepare the composites and the location of the testing coupons within the plates. The strong inherent planar anisotropy dominate the behavior of these composites. The samples retained their integrity upon tensile failure, which was not catastrophic. Aged prepreg showed comparable properties to fresh ones. The CF-epoxy SMC composite shows 3 to 4 times higher tensile modulus, 40% higher tensile strength,
Pull-out Strength of Fiberglass/Epoxy Composite Rebar Manufactured Using a Three-Dimensional Braiding Process
This research demonstrated the ability to produce high quality fiberglass-epoxy rebar using a continuous three-dimensional braiding process. The tensile pull-out strength of the composite rebar embedded in concrete was measured and compared to steel rebar. Fiberglass/epoxy is less likely to corrode than steel rebar, ensuring more sustainable concrete structures in infrastructure applications. The 0.5â€ diameter composite rebar was produced by pulling hundreds of strands of unidirectional fiberglass/epoxy towpreg to form the core of a cylindrical rod. The rods were consolidated by overwrapping strands of aramid fiber under tension in a spiral fashion, and cured using the manufacturer’s (TCR Composites) recommended 250°F cure cycle. Configurations manufactured and tested included tow variations of aramid overwrap (dry and prepreg) with and without sand coating embedded in concrete cubes (8â€x8â€x8â€). The results are compared to baseline values obtained by testing traditional steel rebar and commercially-available fiberglass composite rebar. The tensile pull-out (bond) strength was quantified using a test fixture in accordance with ACI 440.3R-12. Measurements were collected at both the free and loaded ends of the rebar, with freeend results yielding a more accurate indication of rebar bond stress. The bond strength of the in-house manufactured fiberglass/epoxy rebar was comparable to steel rebar and commercial fiberglass composite rebar. The sand coating increased the surface roughness, improving the bond strength of the composite rebar by margins of 43-58% percent. Fiberglass/epoxy composite rebar consolidated with prepreg aramid tow and subsequently coated with sand resulted in a bond strength slightly higher, but comparable to steel. This study demonstrates the ability to consistently produce high quality fiberglass/epoxy composite rebar using an innovative, low cost, high production rate, three-dimensional braiding process.
Damage and Delamination Modeling of Multifunctional Composite Structural Batteries
Structural battery composites are capable of storing and delivering electrical energy and bear structural loads. It is important to analyze the stiffness and strength of these composites, particularly in bending loading configurations. Damage of structural batteries is of particular importance in structural batteries due to the energy storage component. Two structural battery material systems were considered. The first battery cell uses copper and aluminum foils as electrodes and carbon paper as current collectors. The second material system incorporates carbon fiber fabric coated with nickel and iron through electrodeposition as the multifunctional electrodes. These stiff carbon fiber fabrics provide excellent mechanical properties. For sufficient electrical energy capacity storage for use as the chassis of a CubeSat, it was calculated that seven layers per panel would be required. Multi-layer panels consisting of seven battery layers as a core with external structural reinforcement were tested. Physical and virtual three point bend tests were completed on these material systems. Prototype Cu-Al panels were fabricated and tested in a 3 point bend test configuration until ultimate failure. The damage initiation and propagation was analyzed through qualitative and quantitative analyses. Damage morphology took the form of layer delamination and material cracking within the battery layers. Computational finite element simulations matching the mechanical testing geometry were completed to further understand the mechanism involved in the damage of these composites. Simulated Cu-Al panels showed a larger stiffness, however the progression of damage was well represented in the models. The Ni-Fe carbon fiber battery was superior in stiffness and strength. Overall, these composites show promise that these can be utilized as structural materials that store electrical energy.
Nonlinear Aeroelastic Analysis of Composite Morphing Wing with Corrugated Structures
A new nonlinear aeroelastic framework enabling to study the nonlinear aeroelasticity of corrugated/composite morphing wing with orthotropic materials has been developed in this paper. A shell finite element allowing to model flat wings has been adapted to model composite and corrugated panels, which enables to investigate influences of composite configurations. A corotational approach is implemented to consider the effect of geometrical nonlinearity resulted from large deformation corresponding to wing morphing. A cost effective unsteady vortex-lattice method is coupled with the structural model to consider the aerodynamics under the large deformations. A homogenization method of corrugated panel is also implemented to model corrugated panels as equivalent orthotropic plates. Verifications of individual structural, aerodynamic, and corrugated panel models are performed. The nonlinear aeroelastic analysis framework at steady-state condition is also verified by comparing with the linear aeroelastic solution and the nonlinear aeroelastic solution with the finite beam element formulation. This work demonstrates the feasibility of the methodology to numerically simulate such morphing wings.
Alignment of Nickel Coated Carbon Fibers by Magnetic Field during Cure of Polymer Composites
Carbon fibers are widely used as reinforcement in polymer-matrix composites due to their excellent mechanical properties combined with their lightweight. Coating of short carbon fibers with nickel not only preserves the desired specific mechanical properties of both the fiber and the resulting composite but also improves their electrical conductivity and magnetic properties. The short fibers may be randomly dispersed in composites to exhibit isotropic behavior. However, controlling the orientation of the fiber reinforcement can improve the performance of the composite such as electrical conductivity, modulus, and strength in the loading direction. In this study, the alignment of nickel coated carbon (NiC) fibers by a magnetic field during cure of an epoxy resin is explored. For this purpose, the effects of several parameters such as field strength (10-50 mT) and fibers length (0.1 and 0.25 mm) on the degree of fiber alignment within composites are investigated using scanning confocal microscopy and image analysis techniques. The 0.1 mm long fibers started to align along the direction of magnetic field at around 10 mT. The degree of alignment was enhanced at about 30 mT which enabled the NiC fibers to interact easily with each other and build NiC fiber networks. Furthermore, when a higher magnetic field of 50 mT is applied, in addition to the rotation and network formation, pronounced migration of fibers towards the magnetic poles was observed. In addition, under the same magnetic field of 30 mT, longer fibers (0.25 mm) reached a higher degree of alignment.
Development of Robust Electrically Insulated Carbon Nanotube Yarns for Sensing in Conductive Composites
Carbon nanotubes have novel mechanical, thermal and electrical properties that alongside their tailorable aspect ratio can be integrated and widely distributed in composite structures to detect damage and monitor strain in real time. With the growing demand for conductive and functional composites, there is an increasing need to incorporate CNT yarns in them due to their inherent multifunctional properties. However, CNT fibers are conductive and need to be coated. Insulation of CNT yarns in conductive reinforcements like carbon fiber-reinforced composites can be challenging due to their highly porous structure, small size and non-uniform construction. This work implements a facile approach for coating of CNT yarns through a simple dip-coating of a measured length of CNT fiber in an adhesive. The adhesive serves to electrically insulate the yarn against short-circuiting in conductive medium and to preserve it against deterioration in harsh environment. It was observed that the molecular size, viscosity and adherence properties of the coating material plays a key role in achieving a uniform and durable coating. These factors alongside weight and the coefficient of thermal expansion of the coating materials was considered in selecting low density polyethylene as the coating agents for this study. Among the polymers tested, Dow Corning 732 provided the best coating for a highly piezoresistive CNT yarn while coating with Permatex-80050 increased the strain-tofailure of the CNT yarn.
Impact Damage Behavior of Basalt Fibers Composite Laminates: Comparison Between Vinyl Ester and Nylon 6 Based Systems
Polyamide 6 (PA6) and vinyl ester (VE) basalt fabric composite laminates were prepared and analyzed in terms of flexural and low velocity impact properties in order to compare the results and evaluate the influence related to the matrix type. The BS/PA6 exhibits superior flexural performances and a higher rigidity in all tests. It is also interesting to note a particular behavior of the vinyl ester samples that can be related to the damage mechanism. After the impact tests, the specimens were non destructively investigated by a ultrasound technique to evaluate the delamination extent. These data, complemented by morphological inspections and indentation depth measurements, have provided useful information about the involved damage mechanisms.
Progressive Damage Analysis of a Bioresorbable Composite Subject to Three-Point Bending
The focus of this paper is to investigate the deformation response of a new class of bioresorbable composites used for bone fixation devices under flexural loading. The Digital Image Correlation (DIC) technique are utilized to map the deformation history and identify the damage locations. Since the damage propagation path needs to be calculated by the simulation, a fracture energy based Smeared Crack Approach (SCA) is revisited and implemented in the Finite Element Analysis (FEA). In the new SCA model, 2D crack morphology incorporating two crack components are discussed. In addition, the stress degradation rule after the damage initiation is revised to guarantee the energy conservation. Compared with the analytical solution, the new SCA model is validated by the benchmark examples of the Double-Cantilever Beam (DCB) and the End Notched Flexure (ENF) simulations. The results demonstrate the predictive capability of the SCA to capture both the mode-I and mode-II failure modes. The new SCA model is further employed to predict the failure response of the bioresorbable composite under three-point bending, which shows a good correlation with the experiment. Based on the FE simulation results, a linear regression model is developed to elucidate the effect of critical stress and fracture energy on the resulting flexural response. The numerical methods in this paper can be used as an efficient tool in designing the bioresorbable composite for controlled mechanical performance.
Enhancing the Interface in Glass Fiber/Epoxy Composites with Nanocellulose
The automotive industry is undergoing a paradigm shift toward the production of “greenâ€ vehicles, and its future is dependent on the development of materials that can reduce fuel consumption and be manufactured sustainably. These materials must not only exhibit sufficient strength and durability to meet rigorous safety standards, but they must also be inexpensive to produce so as to reduce cost to the end user. Due to their high strength to weight ratio, short glass fiber polymer matrix composites are the material of preference when it comes to light-weighting. However, their potential is tied to research efforts to improve that ratio while maintaining low costs, which has thus far proven difficult. The key to enhancing their properties is the fiber/matrix interface, which affects the mechanism of load transfer and thus the composite’s overall mechanical properties. A promising method for improving the efficiency of interfacial load transfer involves the introduction of nanoparticles to the fiber surface through coating of the fibers. One example of such nanoparticles is cellulose nanocrystals (CNC), which have high mechanical properties, are renewable, and have the potential for high volume production at low cost. However, tailoring nanomaterials to optimize performance and scaling the coating process still present significant challenges. In this study, we have coated glass fibers with various aqueous dispersions and emulsions of CNC and embedded them in epoxy. Resulting changes to interfacial shear strength were investigated through single fiber fragmentation testing, which revealed that fiber coatings incorporating methyl(triphenyl) phosphoniumfunctionalized nanocellulose provide a statistically significant improvement over commercially-available fiber. The fiber-polymer interactions dictating this improvement were characterized through polarized optical microscopy of tensile coupons, which allowed for the analysis of fracture mode.
Advanced Manufacturing of Mycological Bio-Based Composites
The development of bio-based materials continues to be driven by the need to reduce the environmental impact on our planet. The development of materials made from fungi, known as mycological composites, is becoming an emergent field in the range of bio-based materials being developed. Motivated by commercial and consumer demand for less ecologically damaging options, mycological composites continue to grow as a sustainable materials option, offering reduced environmental impact due to being made from waste by-products from industrial processes such as sawdust. Naturally occurring fungi species can digest a range of carbon-containing wastes, and can convert them into bio-based composites materials formed of the fungi and the original feedstock. Current applications require mycological composites to be shaped using traditional subtractive mechanical methods, which can cause chipping and surface damage due to the frangible nature of mycological materials. Nontraditional manufacturing techniques were considered, including water jet machining, ultrasonic machining, electrochemical and electrical discharge machining, and laser cutting, to help inform developers and researchers of suitable alternative manufacturing methods. Laser cutting was determined to be the most suitable method to test, offering precision cutting without mechanical abrasion, and optional pyrography. Testing showed bulk cutting can be achieved for thin slices of material, and small feature cutting testing showed laser cutting is suitable for creating small features. The average laser cutting kerf was found to be comparable to band and table saw kerfs, with had advantages of a variable outline cutting paths and producing no cutting waste. Laser pyrography testing showed laser cutting can also be used to burn vector images into mycological materials, with potential to combine with cutting into a single process step. Overall laser cutting was found to provide a suitable non-mechanical cutting process that could create features that could not be cut using traditional subtractive or mechanical processes.
Creating Flexible Structures Out of MDF Plates
Medium densify fiberboard (MDF) is a composite comprising of wood fibers and epoxy resin, with a typical density range between 600-800 kg/m3. It is currently used for secondary structures in buildings such as architectural walls and facades. MDFs are typically available in flat panel forms which can be cut into various shapes for architectural design purposes. One cutting method is known as kerfing, in which a series of cuts are made on a wood panel to create flexible structures mainly through bending and twisting deformations. In this study, we present a micromechanics model of cut patterns in order to understand the overall deformations of the kerf panel. Three different cut densities, i.e., one, two, and three cuts per quarter unit-cell, of a square spiral pattern are studied. The effect of different cut densities on the uniaxial stretching of the unit-cell is examined. An experimental test is also done on a unit-cell under uniaxial stretching. The responses from the experiment and model are compared.
Damage Detection of Textile Composite Structures Using the Piezoelectric Impedance Method
Structural health monitoring of aerospace structures is becoming vital as lightweight composite materials are increasingly used in aero-structures. Composites have various types of failure modes, which make it even more important to identify damage compared to isotropic materials. Therefore, on-site health monitoring is one of the most preferred methodologies for detecting internal crack or damage at its initial stage to enhance operating safety and reduce repair cost. Recently, piezoelectric transducers are on high demand for on-site health monitoring because of its high bandwidth sensitivity. They simultaneously perform as actuators and sensors and embedded on the structure. The electrical impedance or admittance changes according to modification in mass, stiffness or damping properties of the host body. In this study, plain-weave textile composites are utilized to demonstrate capability of piezoelectric transducer. The elastic properties of composite are determined using the Concentric Cylinder Assemblage micromechanics model. Finite element modeling is done in ANSYS APDL to observe electrical admittance variation around natural frequency. Delamination and intra-ply damage cases are studied. Delamination is separation of adjacent plies and damage is reduction of stiffness in the whole structure. Modal frequency of the structure decreased with delamination and damage level increment within the body. Pre-preg manufacturing method is carried out to fabricate composite panel from which samples are cut for testing according to ASTM standards. Piezoelectric test provided voltage drop across the resistor that denoted the coupling effect of the patch with the host structure. Electrical admittance plot expressed response to body deformation actuated by the piezoelectric transducer. Results from finite element modeling and experimentation assert that piezoelectric impedance technique is efficient in detecting damage or delamination within the composite structure.
Influence of Local Bending of Fresnel-Based Optical Fiber Sensors on Measuring Degree of Cure of FRP280
FRP are very light and strong materials and applied to advanced FRP products such as airplanes and automobiles. A recent key issue of manufacturing FRP is cost reduction and in situ monitoring using smart sensing can be used to achieve it. Among such smart sensors, the Fresnel-based optical fiber reflectometer can be used for measuring internal degree-of-cure of thermosetting FRP during cure process and it is known that it has good accuracy and embeddability in FRP. However, the reflection power is affected by the local bending when the sensor is embedded in the complexshaped FRP products and then the quantitative evaluation of effect of the optical bending loss on the measurement accuracy is necessary. In the present paper, the effect of optical loss by local bending on the measurement accuracy of Fresnel-based optical fiber sensors was investigated. Two types of optical fiber sensors, one is a standard type and another is a highly-flexible type, were used for the measurements. First, optical loss by bending with radius of 2,3,4,5 and 6mm was measured for the both types of sensors. From this experimental result, it was found that the optical loss was a linear function of bending length and influenced strongly by the bending radius. In addition, it appeared that the optical loss by the embedment could be estimated. Second, degree-of-cure measurements of epoxy resin were conducted by the optical fiber sensors with local bending. From this experimental result, the degree-of-cure was hardly affected by the optical bending loss. Next, the degree-of-cure measurements of curved FRP plates molded by VaRTM were conducted. From the experimental results, it was found that the local bending hardly affected the measurement accuracy. Additionally, it appeared that the movement of the optical fiber due to resin impregnation influenced the accuracy but the error is correctable.
Analysis of Damaged Laminated Composite Plate Under Dynamic and Aeroelastic Environment
The effect of microscopic damages such as matrix micro-crack, fiber-matrix debonding and fiber breakage on subsonic flutter of rectangular composite plate is investigated. Flutter analysis is carried out in frequency as well as time domain and results are compared. Damage is modelled using an anisotropic damage formulation. It is observed that presence of microscopic damages in composite plates deteriorates the flutter characteristics. No generic conclusion on pattern of reduction in flutter characteristics of composite plates can be drawn, moreover it depends on the location of damage and layup configuration of the laminate. By proper arrangement of lamina with small microscopic damage, within the laminate, flutter characteristics can be optimized.
Analytical Prediction of Tensile Strength Prediction for Two-Dimensional Triaxially Braided Composite
In this paper, a new analytical model is developed to predict the strength of two-dimensional triaxial braided composite (2DTBC). According to the concept of subcell model, a representative unit cell of braided composite is divided into four subcells, each of which is approximated as a stack of unidirectional composite plies. In order to investigate the interaction of braided angle and effective properties, it is assumed that the unit cell models of different braid angles have the same fiber volume fraction and the same thickness. Then, explicit equations can be obtained to describe the geometric parameters and to quantify the specific dimensions of subcell components based on the manufacture provided properties and microscopic image analysis of realistic specimens. Micro-mechanical models are applied to estimate the elastic and strength properties of unidirectional plies. Classical Laminate theory and Parallel and Series Bridge models are implemented to connect the strain and stress responses of subcell components and effective responses of the unit cell. Hoffman and Hashin failure criteria are used to examine the onset of failure for each subcell component, which can then predict the progressive failure process and global stress-strain response of the unit cell. The developed analytical model will then be utilized to predict the tensile strength (axial tension and transverse tension) of a +60°/0°/-60° 2DTBC and validated against experiments. Finally, the validated model will be used to study strength properties for 2DTBC of different braided angles. The results can provide insights for the design and optimization of composite structures with similar braided architecture.