Proceedings of the American Society for Composites â€” Thirty-second Technical Conference
Behavior of Post-tensioned Steel-Concrete Composite Beams Subjected to Hogging Moments
This study presents a detailed finite element (FE) analysis of the nonlinear flexural response of continuous steel-concrete composite beams strengthened with externally post-tensioned tendons. The initial post-tensioning (PT) force is introduced as an initial strain in the truss element that used to simulate the external tendons. The accuracy of the FE model is validated using existing experimental works. The effects of tendon eccentricity, longitudinal steel rebars ratio, and initial PT force on the beam behavior are explored. Furthermore, deterministic and stochastic shrinkage effects are carried out to obtain the long-term random responses of the strengthened beams as well as plain beams. However, the ultimate capacity of the strengthened beam increases only by 8%, the cracked moment redoubles and an affirmative behavior over the un-strengthened beams is obtained. Also, using prestressed concrete flange deters the propagation of cracks and improves the composite action between the steel beam and the concrete flange. As the longitudinal steel rebars ratio increases, the contribution of the externally posttensioned tendons to enhance the beam capacity is diminished.
Micromechanical Modeling of Composites for Shear and Transverse Properties
An accurate micromechanical material model for predicting a compositeâ€™s elastic properties is desired in order to determine the potential structural and economic benefits of an advanced resin system for the construction of wind turbine blades. A MATLAB script and GUI are implemented for the determination of composite laminate properties based upon the properties of the individual matrix and reinforcement, as well as the fiber volume fraction. An advanced model based on periodic boundary conditions is compared with classical rule of mixture models and a composite cylinder model. The predicted properties of available systems are verified using a database of experimental results. This material model can be exported and used in finite element analysis to determine the structural effects and economic potential of a high performance resin (or reinforcement) with improved properties.
Manufacturing a 9-Meter Thermoplastic Composite Wind Turbine Blade
Currently, wind turbine blades are manufactured from a combination of glass and/or carbon fiber composite materials with a thermoset resin such as epoxy, which requires energy-intensive and expensive heating processes to cure. Newly developed in-situ polymerizing thermoplastic resin systems for composite wind turbine blades polymerize at room temperature, eliminating the heating process and significantly reducing the blade manufacturing cycle time and embodied energy, which in turn reduces costs. Thermoplastic materials can also be thermally welded, eliminating the need for adhesive bonds between blade components and increasing the overall strength and reliability of the blades. As well, thermoplastic materials enable end-oflife blade recycling by reheating and decomposing the materials, which is a limitation of existing blade technology. This paper presents a manufacturing demonstration for a 9-m-long thermoplastic composite wind turbine blade. This blade was constructed in the Composites Manufacturing Education and Technology facility at the National Wind Technology Center at the National Renewable Energy Laboratory (NREL) using a vacuumassisted resin transfer molding process. Johns Manville fiberglass and an Arkema thermoplastic resin called Elium were used. Additional materials included Armacellrecycled polyethylene terephthalate foam from Creative Foam and low-cost carbonfiber pultruded spar caps (manufactured in collaboration with NREL, Oak Ridge National Laboratory, Huntsman, Strongwell, and Chomarat). This paper highlights the development of the thermoplastic resin formulations, including an additive designed to control the peak exothermic temperatures. Infusion and cure times of less than 3 hours are also demonstrated, highlighting the efficiency and energy savings associated with manufacturing thermoplastic composite blades.
Relating Paris Law Fatigue Crack Growth in Composites with a Physics-Based Fatigue Model
The focus of this paper is to explore potential connections between the kinetic theory of fracture (KTF), a physics-based fatigue life prediction methodology, and the traditional Paris Law approach to prediction of fatigue crack propagation in composites. In previous research, KTF has been used primarily as a predictor of fatigue initiation. In this work, linear elastic fracture mechanics assumptions are used to advance a crack through a polymer material that is nominally epoxy. The results show that KTF-based crack propagation is compatible with the Paris Law description of fatigue life propagation. Furthermore, the KTF approach is shown to correctly predict crack growth trends with changing frequency, temperature, and R-ratio. These parameters directly influence the Paris Law coefficient. The Paris Law exponent is shown to depend entirely on the nature of the crack tip singularity.
Effects of Interface on the Mechanical Properties of Filled Polymer Hexagonal Honeycombs: A Comparison Between Different Designs
Polymer-filled hexagonal honeycomb structures have been extensively used as impact resistant materials. Though filled hexagonal honeycombs demonstrate excellent behavior under in-plane compression loadind, their tensile properties are poor. However, a material needs to be strong in tension as well as compression because it undergoes both tension and compression cycles under impact loading. The objective of this study is to determine the best designs with competing functionalities such as stiffness, strength and toughness for polymer filled hexagonal honeycombs. The hexagonal architectures have been designed by considering the cellular walls made of a stiff polymer such as PMMA while the filler material has been chosen as a soft, viscous polymer such as Polyurethane. Mechanical properties under tensile loading have been characterized for various design modifications within the hexagonal architecture in a finite element based study where a no-slip condition has been assumed between stiff-soft polymer interfaces. To determine the toughness and the compositeâ€™s failure properties, contact, and cohesive zones have been described across the stiff-soft polymer interfaces. Uniaxial tensile behavior for the designs varies depending on interface properties. The material property charts were developed for the design modifications to draw a comparison between their competing properties such as stiffness and toughness.
Manufacturing or Repairing Composite Parts and Components Using Laser Scanning Technology
Many aerospace and automotive parts are made from advanced composite materials for increased performance and weight savings. Composite tooling is used to manufacture composite parts when they are manufactured by the original equipment manufacturer, but this tooling is typically not available in an aircraft repair station or automotive repair shop. The authors have developed a method using laser-scanning technology to design composite tooling that could be used to manufacture replacement parts or repair damage to a structure. The scanning system is portable and could be used to scan components while they are on the aircraft or vehicle or when they are removed. The laser scanner will produce a solid model and CAD/CAM software will be used to develop the tool geometry and program tool paths for CNC machining.
A Novel Stress Analysis Method for Composite Z-Stiffeners Under Mechanical Loads
A closed-form analytical solution is developed for analyzing laminated composite beam with asymmetric Z cross-section. The explicit expressions for evaluating sectional properties such as centroid, shear center, equivalent bending/torsional stiffness and warping stiffness are formulated based upon modified lamination theory and taken into consideration of the structural deformation characteristics of beam with narrow section. The ply stresses of flanges and web laminates are computed for composite Z-stiffener under axial, bending, and torsional loads. The present results give excellent agreement with the results from ANSYSâ„¢. A parametric study of their centroid and shear center with various layup sequences was performed by using the developed solution. It is found that the sectional properties are not only dependent of structural configuration but also the laminate property. Moreover, these properties are only dependent of structural configuration if the entire Z-stiffener is made of the same family laminates regardless their ply orientation and stacking sequence. It is concluded that the present approach is a viable and efficient method for designing composite Z-stiffener.
Multiscale Structural Analysis of Honeycomb Sandwich Structure Using Mechanics of Structure Genome
A new multiscale modeling approach based on mechanics of structure genome (MSG) has been proposed to analyze honeycomb sandwich structures. On the one hand, MSG can be used to homogenize a honeycomb core as a homogeneous solid to get effective elastic properties without various boundary conditions (BCs). The results are compared with those obtained by using finite element (FE) based numerical homogenization method and analytical formulas. Then a though-the-thickness constitutive modeling can be conducted by MSG to model the whole honeycomb sandwich structure as a laminate for equivalent plate properties (MSG two-step). On the other hand, MSG can directly give constitutive modeling of the whole honeycomb sandwich structure as an equivalent plate to obtain the equivalent plate properties (MSG direct). The numerical case shows that the equivalent plate properties provided by MSG direct are more accurate than traditional approach when compared with the direct numerical simulation (DNS) and much more efficient than the DNS.
A Multiscale, Physical Criteria Based Approach for Composite Structural Assessment
The ultimate goal of structural assessment is to ensure that a given design is able to accomplish its mission without suffering a structural failure. Traditional structural analysis accomplishes this by using mathematical models to determine the maximum internal stress and strain state of the material and then compare that to measured critical values of the material to determine a margin-of-safety. Those critical properties are directly measurable for metallic materials. Yield and ultimate tensile stress and strain can be directly observed from simple coupon tests. The Von Mises yield criterion for example, provides a reliable, conservative criteria for evaluation of a material subjected to combined loading. Safety margins are based on comparisons to critical material properties. Except in rare cases, analysis and simulation do not attempt to predict catastrophic, two-piece failure.
Investigation of the Carbonate Rock Permeability Based on X-Ray Tomography Images
Digital rock physics (DRP) is a nondestructive method to study properties of rocks. Based on this approach, images of rocks are used for determining petrophysical properties such as porosity, permeability and thermal conductivity. In order to study permeability, the real rock microstructure is extracted from binarized rock images. The 3D imaging and segmentation software VCAT (released by V-CAD Program, RIKEN, Japan) has been used to transform medical computed tomography (CT) images to a meshed file that can be imported into commercial software ANSYS for simulating fluid flow and solving Navier-Stokes equations. The rock core samples contained of 10 carbonate pluges with the resolution of 187.5Î¼m. The novelty of this work is to characterize fluid flow properties through rock samples by use of tomography technique and numerical simulations. The full-size plug samples in the medical X-ray CT can be used for imaging, and it is the main advantage of using this method in DRP. However, an important problem with the CT images is their resolution, which may fail to detect those pores which are smaller than the selected resolution. Although in this resolution some pores are not distinguishable, some responses of these pores are mapped on the images based on partial volume effect theory. Based on the descriptions mentioned above, the values of permeability gained from numerical simulations were compared with experimental results of plugs. The computed permeability values had good correlation with the experimental results.
Calculating the Effective Thermal Conductivity of Carbon Nanotube Reinforced Polymer
The excellent properties of carbon nanotubes (CNTs) make them appropriate for polymer matrix which leads huge interest to nanocomposites. In this paper, the effective thermal conductivity of RVEs which were reconstructed in Mahdavi et al[1] is calculated. A computer code (MATLAB) is developed in such a way that it calculates the effective thermal conductivity of a nanocomposite using Mori-Tanaka method. In this procedure, some nanotubes of each RVE were chosen and they entered to an empty RVE that its polymer matrix is the nanocompositeâ€™s polymer. Then the effective thermal conductivity for this RVE was calculated, and this step was repeated with a new polymer matrix that its thermal conductivity is the effective thermal conductivity of the last step. These increments were repeated until to reach to the total number of nanotubes in the primary RVE, and the effective thermal conductivity in the last step is the effective thermal conductivity of primary RVE. Results show that the RVEs that had higher aspect ratio (same length, but smaller diameters) have higher effective thermal conductivity.
Evaluation of Glass Geometry on Deformation Behavior Particulate-Filled Composite Materials Using X-ray Computed Tomography
The deformation behavior of epoxy reinforced with different shapes and content of glass fillers was investigated through combination of X-ray computed tomography and 3D digital image correlation. Spherical, flake and rod shaped glass fillers were dispersed into a mixture of epoxy resin with three different mass fractions of 5%, 10%, and 15%. The samples were indented using hardness testing machine. In order to quantify the amount of deformation, the samples were scanned before and after the indentation test by using X-ray CT. The displacement and strain field underneath the indentation site for all filler types and mass fractions were measured through correlation of before and after scan data. It was found that hardness increases with increased amount of filler and rod shaped filler gave the highest hardness values followed by the flake filler; the composite with spherical fillers had the lowest hardness values. The displacement fields evaluated were in close agreement with the measured hardness values. Displacement and strain values were higher for spherical filler samples while the fiber filler samples led to lower deformation indicators.
Strain Partitioning in a Functionally Graded Interphase Based Composite Micromechanics Model
In order to improve the predictive accuracy of homogenized micro mechanics models the strain partitioning between the fibres and the matrix must be accurate. Early homogenization schemes are only applicable for elastic deformations of composites; however, it has become important to be able to predict the response of composite materials up to the point of failure to improve the design of lightweight structural composite structures. Approaches to improve the strain partitioning for homogenized schemes where the material undergoes elasto-viscoplastic response are discussed. For complex polymer materials models where the composite has an elastic fibre which is perfectly bonded to the matrix it is proposed that the fibre strain is related to the recoverable elastic strain of the matrix material. Simulation results are shown for a unidirectional carbon fibre epoxy composite at various strain rates under transverse loading.
In-Situ Interface Bond Strength Determination in Thick Adhesive Bonded Box Beam
A box-beam structure, consisting of adhesive-bonded spar caps and shear webs, is a primary load-bearing structural element in a large composite wind turbine blade. In evaluation of a wind turbine blade structure, a scaled box beam is often used to examine and determine the structural strength and failure modes of a full-scale wind turbine. In this study, a series of scaled composite-sandwich box beams were designed and fabricated for rigorous investigation of their strength characteristics under 4-point bending. Damage and failure modes observed during the bending tests of the box beams exhibited progressive development of local buckling of spar caps and final adhesive bond-line failure. Computational mechanics analyses revealed local nonlinear deformation and stress distributions in the box-beam spar caps, shear webs and thick adhesive bond layers. The results also indicated the presence of large normal tensile stresses acting at the interfaces of shear web and adhesive layer and the spar cap and adhesive. In this paper, an investigation of in-situ adhesive joint tensile strength was conducted. Adhesive joint specimens were taken from the box beams tested under bending. Due to the very small dimensions of the selected sample regions which consisted of a bonded spar-cap laminate-adhesive-shear web joint, tab extensions consisting of (0)n laminate were attached for proper load application. Initial tensile test results from the specimens without the gage-section reduction failed prematurely at very low stresses with very large scatter. Consequently, tensile specimens with dog-bone shaped gage section geometry were designed and tested. By varying the gage region locations in the specimen, the true adhesive bond failure occurred at shear web/adhesive and spar cap/adhesive interfaces. The in-situ bond strength of the composite laminate and the adhesive joint in the box beam was determined. The experimental results of the in-situ bond strength were used in analytical modeling and evaluation of the box-beam strength and failure mode prediction.
3-D Woven Preform Joint Testing to Support the D8 Double-Bubble Composite Fuselage Design
The D8 double-bubble aircraft features a wide fuselage to enable operational improvements from a twin-aisle seating arrangement as well as fuel efficiency improvements due to increased fuselage aerodynamic lift, a more favorable fuselage pitching moment, and in some variants the installation of rear fuselage mounted boundary layer ingesting turbofan engines. The wide fuselage outer mold line consists of a vertically joined dual-lobe fuselage cross-section. The double-bubble configuration introduces unique features of the primary airframe structure when compared with classical tube or horizontally joined multi-lobe fuselage configurations as the vertical joint cannot take advantage of a floor at the lobe intersection. The structural â€œYâ€- joint of the double-bubble fuselage is composed of the co-cured fuselage skin and 3-dimensional woven composite preform and a keel. In this paper, mechanical testing of the 3-D woven composite preform is setup to support the development of the D8 double-bubble fuselage â€œYâ€-joint. The challenges of a wide-body fuselage, the structural solution used on the D8 concept, and the testing involved to validate the effectiveness of the solution in a commercial application are presented.
ESI VPS Multiscale Performance Simulation for Laser Structured Composite-Metal Joints
An economic utilization of modern engineering materials can only be achieved through a combination of different materials depending on the local design requirements. This further leads to questions about the joinability of certain materials in a multi-material assembly. During the design of hybrid parts consisting of metallic alloys and fiber reinforced polymers (FRP) different joining techniques can be applied. Since the present study focuses on the application of an Automatic Tape Laying (ATL) process, the joint is established utilizing the adhesive bond generated during the consolidation process of the composite. Laser structuring of the metallic surface has been found to allow for a significant improvement of the adhesive properties. However, in order to optimize the manufacturing process in terms of efficiency, it is required to quantify this effect. To this end, a numerical multiscale approach has been developed and implemented in the commercial FE package ESI Virtual Performance Solution (VPS) [1]. It allows for the examination of effective interface properties and enables to relate directly the joining process to the mechanical performance of the joint.
Generation of Realistic Stochastic Virtual Microstructures Using a Novel Thermal Growth Method for Woven Fabrics and Textile Composites
Generating realistic 3D tow-level finite element (FE) models of textile weaves and impregnated textile composites poses a challenge because of the complexity of the 3D architecture and the need for achieving high quality finite elements and nonintersecting tow volumes. A common approach sweeps a constant tow cross-section along a smooth and continuous centerline that repeats over a unit cell length. However actual microstructures of dry fabrics and textile composites are often aperiodic and non-deterministic. In this study, we present a novel method to generate realistic virtual microstructures of fabrics and textile composites using a â€œthermal growthâ€ approach. This involves a series of orthotropic volumetric expansions and shrinkages of the tow cross-sections and centerlines that are artificially induced by prescribed thermal loads, along with mechanics-driven tow deformations in order to grow or form the tows into their final realistic configurations within the weave. Contact-pairs are defined between interlacing tow surfaces to prevent tow inter-penetrations. The final virtual microstructures are generated through a series of simulations executed using LSDYNA. Two case studies are presented. The first is a plain-weave Kevlar fabric used in protective structures. The second is an angle-interlock PIP-CVI processed C/SiC ceramic matrix composite used in high temperature structures. The virtual microstructures are validated against experimental microstructures obtained from SEM, optical, and microCT characterization. Relatively fine features are generated correctly, including variations in tow shapes and the distribution of spacings that are left between tows. This novel thermal growth approach to generate 3D tow-level meshes of weave architectures can be applied towards any 2D, 2.5D, and 3D woven, braided, and knit architectures. The generated virtual microstructures are at the core of Teledyneâ€™s ICME-based virtual testing toolset to predict the linear thermoelastic and non-linear thermostructural damage behavior of textiles, PMCs, and CMCs.
Worldâ€™s First Predictive and Validated Yarn-level FEA Modeling of the V0-V100 Probabilistic Penetration Response of Fully-Clamped Kevlar Fabric
This paper presents the worldâ€™s first fully validated and predictive capability to model the V0-V100 probabilistic penetration response of woven aramid fabrics using a yarn-level fabric finite element model. The V0-V100 curve describes the probability of complete fabric penetration as a function of projectile impact velocity. The exemplar case considered in this paper comprises of a single-layer, fully-clamped, plain-weave Kevlar fabric impacted at the center by two types of 0.22 cal projectiles: a 11-gr spherical projectile and a 17-gr FSP or fragment-simulating projectile. Each warp and fill yarn in the fabric is individually modeled using 3D finite elements. A probabilistic computational framework is developed to map in the experimentally characterized sources of statistical variability into the fabric model and then generate a numerical V0-V100 curve through a series of impact simulations at varying impact velocities. This paper also reports the experimental characterization data and its statistical analysis used for model input, viz. the Kevlar yarn tensile strengths, moduli, and inter-yarn friction, and the experimental ballistic test data used for model validation.
Comparison of the In-plane Shear Behavior of UHMWPE Fiber and Highly Oriented Film Composites
Highly oriented solid state extruded polyethylene (SSE-PE) film composites, an emerging form of ultra-high molecular weight polyethylene (UHMWPE), are a promising candidate material to enhance the ballistic performance of lightweight protective systems. It is hypothesized that the in-plane shear modulus for SSE-PE film laminates is an order of magnitude larger than for gel-spun polyethylene fiberbased systems. This is significant in that the in-plane behavior of UHMWPE composite laminates is widely known to affect ballistic performance. The focus of this work is to experimentally measure the laminate in-plane shear behavior of a commercially available UHMWPE fiber-based system (Spectra ShieldÂ® SR3136) and a film system (TensylonÂ® 30A) using tensile tests of specimens with a [Â±45Â°] layup. The results are used to draw conclusions of the difference of in-plane response between the fiber-based and film polyethylene material systems.
A New Class of Body Armor Protection System Based on Behind Armor Blunt Trauma Criterion
A new class of Shear Thickening Fluids (STF) is incorporated aramid fibers. The STF is a combination of nanosilica and calcium carbonate nanocrystals, combined with polyethylene glycol. The STF assimilation process into aramid fibers has been accomplished through a combination of ultrasonic dispersion and airbrush, especially developed for this study. Two types of ammunition were considered, i.e. 9mm Full Metal Jacket (FMJ) and .357 Magnum Jacketed Soft Point (JSP). The results demonstrated an enhancement close to 25% in ballistic penetration resistance, due combined performance of aramid fabrics impregnated with STF and the epoxy system. Furthermore, considering only the addition of STF to the aramid fabrics through the improved dispersion technique, an increase on performance close to 41% in ballistic performance was achieved. A normalized equation was proposed, which allows a comparative study between the effect of using non Newtonian fluids and the chest cavity trauma made by high speed projectiles.
Blast and Frag Response of Composite Metal Foam Armor Systems
This study analyses the use of composite metal foam (CMF) to mitigate the damage caused by blast and fragmentation threats. A major threat to military and civilians in war-torn regions is the increasing use of improvised explosive devices (IEDâ€™s). These explosives can cause damage to equipment and injure solders through blast wave propagation and fragment penetration. In order to minimize such injuries and damages, advanced materials with high-energy absorption capabilities are needed. In this study, steel composite metal foam panels with extra-ordinary energy absorption capabilities have been tested in various thicknesses to defeat 23 x 152 mm High Explosive Incendiary rounds (HEI). The incendiary round impacts an initial target (a thin aluminum plate) creating a blast wave and fragment particles of various sizes (small, medium and large) that will then impact the CMF panels positioned approximately 45 centimeters away from the target aluminum panel. In addition to the experimental approaches, the CMF system is modeled using IMPETUS Afea Solver for validation. Both the experimental and modeling results indicated that composite metal foam panels were able to defeat different sizes of fragments resulted from a 23 x 152 mm HEI round, with fragment speeds reaching up to 1500 m/s.
Simulation of High Velocity Armor Fragmentation and Human Tissue Response
The future of personal composite body armor to protect against impacts will inevitably be lighter, stronger, and more flexible to increase comfort for the user. Simulating the performance of the flexible materials requires modeling in finite element analysis using more advanced bio-compatible, bio-inspired composite materials for both the armor and the human soft tissue. These next generation of materials are highly hyper-viscoelastic with extreme hardening under high rate loading which is a current limitation for many numerical simulations. These limitations are addressed starting at the initial assumptions for continuum mechanics and general non-linear assumptions made for the viscoelastic materials. Applying the current built-in finite element code, an Hydrogel is tested via low velocity impacts in both experimental tests and numerical analysis. The results show that there is discrepancy between the two dynamic impact behaviors, whereby the force response of the real test is significantly longer in duration and with slower rise time compared to the numerical results. It is expected that in more extreme impact conditions that will be explored, these differences will diverge more dramatically and invalidate the finite element predictions. This shortcoming can only be prevented by introducing more strong non-linearity in the fundamental material properties and conditions in the impact model approximation.
Effect of Strike-face Material Strength on Ballistic Performance of Multi-ply Soft Armor
In this study, the impact-face material of a multi-ply soft armor system was varied to different ratios and tested for the effects on the ballistic performance. While it is known that the first few layers of multi-ply soft armor material typically fail inelastically near the system ballistic limit and can be replaced with a â€œsacrificialâ€ material with other more desirable properties, the extent to which the highperformance fabric can be replaced has yet to be quantified. Materials of different shear strengths, namely stainless steel mesh, MakrolonÂ®, and cotton were used as replacement material for 840d TwaronÂ®, and the hybrid panels were impacted by O1 tool steel RCC projectiles fired using a single stage smooth-bore gas gun.
Predicting Body Armor Back Face Deformation (BFD)
Understanding how personal protection systems deform during an impact event is paramount to their proper use in mitigating injury to the user. Most Kevlar fabric characterization relies on testing fabric suspended in the air; however, in use, these materials may respond differently due to the resistance from the backing material. The current NIJ standard relies on a clay backing material, and measuring the deformation after impact. This does not provide any time of flight information about the deformation response of these materials. In order to characterize the response of the multiple layer Kevlar samples, experimental testing was performed using clear backing materials that simulate human flesh. A clear backing material made of ballistics gelatin was used, allowing images to be obtained of the back of the Kevlar sample during the impact event. From these images, the maximum depth, as well as width and shape profile of the deformation can be tracked as a function of time. In this work a spherical projectile was fired into the Kevlar samples using a gas gun to determine the deformation vs time history. The parameters of the impact velocity, number of layers of Kevlar, and areal density of Kevlar were examined to determine the response on the deformation vs time history. Two types of Kevlar fabrics are used in various multiple layer configurations tested at a range of projectile velocities. It was found that the multi-layer areal density of a sample and the number of layers affect the deformation response significantly and independently. In addition, it was found that shapes of the back face deformations become more rounded as more fabric layers are added.
Multiscale Modeling of Carbon Fiber Reinforced Polymer (CFRP) for Integrated Computational Materials Engineering Process
In this work, a multiscale modeling framework for CFRP is introduced to study hierarchical structure of CFRP. Four distinct scales are defined: nanoscale, microscale, mesoscale, and macroscale. Information at lower scales can be passed to higher scale, which is beneficial for studying effect of constituents on macroscale partâ€™s mechanical property. This bottom-up modeling approach enables better understanding of CFRP from finest details. Current study focuses on microscale and mesoscale.
Laser Ablation of Automotive Composite Panels for Adhesive Strength after Environmental Exposure
Adhesive bonding of SMC composite materials traditionally requires scuff sanding pretreatment to achieve adhesive strength and fracture mode specifications. Scuff sanding is a manual operation that adds cost and generates undesirable dust. Pulsed laser ablation pretreatment was identified as an alternative method having the advantages of an automated, relatively low-cost surface pretreatment method. In contrast to conventional continuous lasers, modern pulsed infrared lasers (ï¬=1064 nm) have lower average power output but can deliver each shot within a very short period of time (~ 100-200 ns) resulting in high peak power output which can remove surface contaminants without the risk of heat damage. A feasibility study including environmental exposure was conducted to investigate the effectiveness of the laser ablation surface pretreatment for adhesive bonding of several SMC composites. Results show that with sufficient pulse laser energy, laser ablation pretreated substrates met the GMW 15921 specified average lap shear strength and failure mode requirements at room temperature and after water soak conditioning. Furthermore, laser ablated substrates exhibited comparable adhesive bonding performance compared to scuff sanded substrates. This suggests that laser ablation is a feasible alternative to sanding while offering the potential for automation and repeatability.
Comparative Study of Lightweight Glass Fiber and Basalt Fiber Sheet Molding Compound (SMC) Composites Containing Cellulose Nanocrystals
Cellulose nanocrystals are used in lightweight sheet molding compound composites made with either glass fiber (GF)/epoxy or basalt fiber (BF)/epoxy. The chosen approach for making lightweight SMC composites is replacing portion of the heavier components, i.e. GF or BF, with a small amount of CNC with the constrain that there is no compromise on mechanical performance. The specific properties of the lightweight composites containing CNC are compared with those of corresponding composites with no CNC to investigate the effectiveness of using CNC for light weighting for each type of micro-size reinforcement, i.e. GF and BF. We report that incorporation of 1.4-2 wt% CNC in the epoxy matrix can allow for reduction of the GF from 60 wt% to 44-48 wt% resulting in ~11 % lighter composites with no penalty on specific tensile, flexural and impact properties. In contrast, for the case of BF, addition of CNC does not increase the properties of the light weighted composites to the level of BF/epoxy with 60 wt% fiber content.
Integrated Numerical Simulation for SMC in Compression Molding Process
In this work, sheet molding compound (SMC), formed using compression molding process, was investigated by numerical simulation. Integrated numerical simulations, involving both computational fluid dynamics (CFD) and computational structural mechanics (CSM), were used to predict the deformation and flow behavior of SMC in a compression molding process. As part of this work, LS-DYNA, a commercially available FEA software, and Moldex3D were applied to investigate the CSM and CFD behavior respectively in compression molding. The rheological property of a polymer has both solid and fluid behaviors, which was shown as â€œFigure 1.â€ Given that, LS-Dyna has difficulty in handling the fluid behavior of the polymer, an integrated procedure between LS-Dyna and Moldex3D was developed where the elasto-plastic deformation was computed by LS-DYNA, and the viscoelastic fluid flow was simulated by Moldex3D in the compression process. The initial charge shape, temperature, stress tensor, and anisotropic material property were computed by LS-DYNA, then these physical variables were projected to Moldex3D to continue the compression molding analysis. The results of the flow behaviors and warpage showed that the proposed integration of LS-DYNA and Moldex3D was successful to simulate the complex behavior of SMC in compression molding.
Reduction of CO2 Emissions through Lightweight Body Panelsâ€”Design and Optimization of a SMC Liftgate for the Volkswagen Atlas
Lightweight strategies are an essential part of Volkswagenâ€™s overall initiative of reducing CO emissions and enhancing customers driving experience. Due to its low material cost combined with established manufacturing technology, steel is the most commonly used material for automotive exterior parts. Unfortunately, steel has a high density, resulting in a relatively low specific strength and stiffness. Glass fiber based sheet molding compound (SMC) provides properties which combine a lower density with specific strength and stiffness to offer great lightweight potential. In comparison to steel, a mass reduction of up to 30% in automotive parts, without limiting functionalities, is possible. By performing a comprehensive simulation study of material characteristics and manufacturing processes, lightweight construction using functional integration and part consolidation is feasible. Utilizing both advantages, it is possible to eliminate separate reinforcement parts (i.e. braces and mounting plates) to further reduce weight and cost. These measures reduce the number of parts, assembly operations, and the number of tools required, reducing overall direct and indirect costs and investments. The Composites Manufacturing & Simulation Center (CMSC) of Purdue University is performing, in cooperation with the North American Engineering & Planning Center of Volkswagen Group of America Inc., the transformation of the current steel liftgate of the Volkswagen Atlas into a lightweight SMC liftgate. The emphasis of the design process is manufacturing-informed performance where manufacturing process simulation is utilized to inform the ultimate performance based on the as-manufactured part. The design and optimization goals are to reach a weight target similar to aluminum, performance comparable to metal, and costs on par with conventional stamping technology. This project marks the first large and complex composite exterior component producible in high volumes for Volkswagen Group of America.
Prediction and Validation of Anisotropic Elastic Property of Injection Molded PA66/Carbon Fiber Composites Parts for Automotive Applications
Automotive parts made with injection molding of carbon fiber reinforced polymers (CFRP) have shown significant advantages in lightweight vehicle development. Featured with balance between performance and cost, injection molded CFRP components are suitable for various automotive components. However, most concurrent computer aided engineering (CAE) analysis of such components still adopts homogeneous and isotropic cards for composites materials. Often is the case that high safety factor have to be used during design practice for composites parts due to incapability of accounting for anisotropic material behavior, which limits the application of the CFRP materials. While numerous studies have demonstrated the prediction of anisotropic material properties for fiber reinforced polymers through fiber orientation prediction and micromechanics calculation, validation is rarely done on an actual automobile part with complex geometry, which constrains the confidence of design engineers to perform analysis with anisotropic material properties for CFRP parts. In order to bridge such gap, in the present study, a vehicle part is manufactured using PA66/carbon fiber composites through injection molding process. Tensile tests are performed on samples cut from different locations of the part. Injection molding simulation based on actual processing parameters is performed to provide the fiber orientation predictions. The anisotropic elastic properties throughout the parts are obtained through the combined usage of micromechanics and orientation tensor averaging procedures to provide the anisotropic elastic properties in the parts, which are varying from element to element. A mapping process is also developed to transfer properties between the different meshes used in processing simulation and structural finite element analysis (FEA). To compare with the experimental results and validate the prediction of anisotropic elastic properties, the part model in FEA is virtually cut at the same locations as the real experiment samples to provide tensile bar FEA models, which enables FEA simulated tensile tests. Good match is observed between the experiments and FEA simulations at coupon level tests, indicating the accuracy of predicted anisotropic elastic properties from prediction.
Mode I Interlaminar Fracture Toughness Characterisation of Carbon Fibre Reinforced Thermoplastic Composites
Delamination has always been regarded as one of the most critical failures in laminated composites. The growing interest in a more rapid, out-of-autoclave (OOA) manufacturing techniques for automotive applications such as compression moulding and thermoforming has prompted the engineering community to further investigate and understand the delamination resistance of carbon fibre reinforced thermoplastic (CFRTP) composites over the more conventional carbon fibre reinforced thermosetting (CFRTS) counterparts. In this study, the mode I (opening) interlaminar fracture toughness of two non-crimp fabric (NCF) biaxial (0/90Âº) carbon/thermoplastic composite systems with fibre volume fraction (FVF) of 50%; T700/polyamide 6.6 and T700/polyphenylene sulphide have been characterised. The mode I delamination resistance, for both materials was determined using the double cantilever beam (DCB) specimen. Following the experimental results and data reduction, the Mode I interlaminar fracture toughness were compared. Fractographic analysis was conducted using a scanning electron microscope (SEM) to characterise the microstructure of the failed specimens.
Mechanical Behavior of Bio-Inspired Composites with Sutural Tessellation
In nature, architectured composites with sutural tessellations are found in the armor systems of many fauna and flora species across all length scales. These composites are typically composed of relatively stiff building blocks articulated via compliant wavy suture interfaces, showing a remarkable jigsaw-puzzle-like sutural tessellation. Inspired by these interesting natural composites, bio-inspired designs of architectured composite plates with diamond-shaped building blocks and sinusoidal suture tessellations were developed. Finite element simulations were performed to study the influences of the waviness and the stiffness ratio of the suture tessellation on the overall stress-strain behavior of the designs. Suture tessellation was shown to be essential in determining the mechanical properties and behavior of the composite plates. The waviness and the stiffness ratio of the suture tessellation were found to be the most important geometric and material parameters to govern the mechanical behavior of the composite plates. The effective stiffness and Poissonâ€™s ratio of the architectured biomimetic composite plate can be tuned in a very large range by tailoring the geometry and material combination of the suture tessellation. Generally, for a certain stiffness ratio and volume fraction, when the waviness of suture tessellation increases, the effective stiffness increases. Interestingly, for cases with wavy suture tessellation and large stiffness ratio, negative Poissonâ€™s ratio can be achieved. These results provide a guideline for designing new composite materials for protection and energy dissipation.
Realizing Damage-tolerant Nacre-inspired CFRP
In this work, a nacre-inspired carbon/epoxy composite is designed, synthesised and tested. Analytical and numerical models are developed to design a tiled microstructure, mimicking the staggered arrangement of ceramic platelets in nacre and exploiting geometrical interlocks for crack deflection and damage diffusion. The designed pattern of tiles is then laser-engraved in the laminate plies. In order to increase the damage-spreading capability of the material, a thin layer of Poly-Lactic Acid (PLA) is film-cast on the interlaminar region. Three-point bend tests show how the nacre micro-structure succeeds in deflecting cracks, with damage diffusion being significantly improved by the addition of PLA at the interface between tiles.
Biofidelic Conductive Synthetic Skin Composites
Skin is the first point of contact of the human body with the outer environment, and influences the biomechanics of different organ systems in normal and diseased states. Wearable electronics such as fitness tracking equipment, motion sensing devices, and advanced wearables in prosthetics and orthotics are often used to quantify the interaction of the body with the environment during different physical activities, and improve health. These wearable equipment can be bulky and a source of discomfort to the human skin with prolonged wear. To date, very few flexible polymers have been developed which can conduct electricity and be used in wearable devices. In the current work, a novel conductive synthetic skin composite system was developed, which would be indispensable for integration into wearable technologies, and also allow the biomechanical testing of the human skin for different engineering and medical applications. The mechanical behavior of this polymer can be tuned to mimic the human skin from different locations of the body with varying stiffnesses, with a phenomenal degree of accuracy. The composite system is composed of short carbon fibers dispersed in a multi part silicone based matrix material. The volume fraction of the fibers were varied to control the mechanical and electrical properties of the composite. Uniaxial tensile tests were conducted to generate stress versus strain responses of the synthetic skin composites at different fiber volume fractions, and electrical measurements were recorded at different strains. Microscopy was used to understand composite fiber orientations in unstretched and stretched states, and its effects on the electrical conductivity of the material. Additionally, non-linear material characterization models were developed to characterize the composite variants. To the best of our knowledge, such an accurate synthetic skin composite system with tailorable electrical properties has not been developed; making this state of the art in bio mimicking and functionalization of the human skin.
The most recent progress in biocomposite textile and leather alternatives is using fungi to cultivate laminate materials, which are compostable and offer lower environmental impact. Until now, the majority of mycological materials created, which are derived from fungi, have been for applications which require rigidity, compressive strength and toughness as defining characteristics. The latest developments in the field of fungi-based materials is the focus on creating replacements for current polymer and composite materials, which are resourcedemanding and have complex processing needs, such as laminate fabrics in the textile and tanning industry. By creating biomimetic analogues to replace materials that have unfavorable environmental impacts, fungi-based substances can be used to develop materials that can offer the sustainability and disposal phase impact reduction that is desirable from bio-based composite materials. With increasing needs for materials which have less environmental impact, mycological materials can offer many advantages, including faster processing and manufacturing than animal-based materials. Due to fungi being able to digest a wide variety of organic substances, from agricultural wastes to crude oil-based synthetic materials, mycological materials have the potential to be an alternative to animalbased hides such as bovine leather and crude-oil based polymeric textiles. Innovations in the field of fungi-based textiles have shown fungi can be grown in layers to form composite laminates, in a matrix of reinforcing fibrous structures made from chitin, which can mimic other natural materials such as animal hides. Similar microstructures of a cellular material with fibrous regions were identified from scanning electron microscopy images, which may explain how mycological materials can form similar textures, morphologies and material properties to animal-based leather, despite being made from the significantly different protein structures of fungi.
Non-conventional Composite Laminates â€“ Clutch Laminates
This paper presents the design and manufacturing of novel non-conventional woven composite laminates coined as clutch laminates. Carbon/epoxy clutch laminates are manufactured using prepreg slit tape in an automated fiber placement (AFP) machine. The AFP process allows tailoring the material architecture resulting in innovative structures without the need for any loom. For instance, the thermal behavior of the laminate may be designed such that warping after curing is significantly minimized compared to a traditional asymmetric laminate. We study the thermal and mechanical stiffness behavior as an example where the clutch laminate lay-up pattern is represented with a simple smart meshing approach utilizing 2D shell elements where the tows are deposited onto the regions mimicking the AFP machine course. The results show that warpage for the clutch laminate is significantly smaller compared to the conventional asymmetric laminate. However, clutch laminates pose a significant challenge in predicting the deformation, damage and impact behavior.
Thermomechanical Behavior of Continuous Glass Fiber-reinforced Polyamide Composite Laminates
E-glass/polyamide laminates were fabricated using a hot press and the effects of temperature on static tensile behavior was studied both below and above the glass transition temperature of the material. The glass transition temperature was determined using DMA. Uniaxial tensile tests were conducted on [0]8, [90]8, [02/902]s and [04,904]s laminates at room temperature, as well as at multiple temperatures above and below the glass transition temperature. Finally, the Youngâ€™s modulus and strength of the specimens were evaluated versus temperature, fractography was performed on the fractured [0]8 samples and the effect of temperature on failure modes was investigated. [0]8 and [90]8 laminates displayed three reduction stages in modulus and strength versus temperature where the largest reduction was in glass transition region. However, [02/902]s and [04/904]s laminates displayed the largest reduction before the glass transition temperature. These experimental findings were compared with the rule of mixtures and classical lamination theory.
Effects of Adiabatic Heating on the High Strain Rate Deformation of Polymer Matrix Composites
Polymer matrix composites (PMCs) are increasingly being used in aerospace structures that are expected to experience complex dynamic loading conditions throughout their lifetime. As such, a detailed understanding of the high strain rate behavior of the constituents, particularly the strain rate, temperature, and pressure dependent polymer matrix, is paramount. In this paper, preliminary efforts in modeling experimentally observed temperature rises due to plastic deformation in PMCs subjected to dynamic loading are presented. To this end, an existing isothermal viscoplastic polymer constitutive formulation is extended to model adiabatic conditions by incorporating temperature dependent elastic properties and modifying the components of the inelastic strain rate tensor to explicitly depend on temperature. It is demonstrated that the modified polymer constitutive model is capable of capturing strain rate and temperature dependent yield as well as thermal softening associated with the conversion of plastic work to heat at high rates of strain. The modified constitutive model is then embedded within a strength of materials based micromechanics framework to investigate the manifestation of matrix thermal softening, due to the conversion of plastic work to heat, on the high strain rate response of a T700/Epon 862 (T700/E862) unidirectional composite. Adiabatic model predictions for high strain rate composite longitudinal tensile, transverse tensile, and in-plane shear loading are presented. Results show a substantial deviation from isothermal conditions; significant thermal softening is observed for matrix dominated deformation modes (transverse tension and in-plane shear), highlighting the importance of accounting for the conversion of plastic work to heat in the polymer matrix in the high strain rate analysis of PMC structures.
Degradation Due to Oxidation Around Voids, Cracks and Holes
High Temperature Polymer Matrix Composites (HTPMCs) exposed to oxidizing environments are subject to mechanical property degradation due to chemical changes in the resin. Long-term degradation rates due to oxidation in a composite structure are controlled by the damage evolution rates in the oxidized zones, which in turn are determined by the compositeâ€™s strength and toughness in these regions. The presence of voids, cracks, and holes in the structure alters the local oxidation zone morphologies. In this paper, we describe a methodology for prediction of Open Hole Tensile (OHT) residual strength of isothermally-aged IM7/(bis) maleimide quasi-isotropic composites under static loading. The composite oxidation profiles around holes are explained by monitoring the development and growth of damage through fluorescence imaging in conjunction with optical microscopy and X-ray computed tomography (CT) techniques. Preferred damage initiation directions are investigated for several combinations of the fiber orientations with respect to the mechanical loading direction and its influence on OHT. The paper presents comparisons of observed and simulated microstructure changes and correlations of damage states and strength predictions with experimental observations.
Modeling of Multiple-Type, Multiple-Site Damage in Composite Laminates Using Peridynamics Theory
The present study presented a numerical model for the tensile damage in fiberreinforced composites with an open hole, using a peridynamics theory. Especially, peridynamics theory can deal with the damage in the composites having arbitrary fiber orientations by setting the bond properties (bond constant and critical stretch) appropriately. We compared two approaches for the peridynamics analysis of fiberreinforced composite materials, proposed by Oterkus and Madenci [1] and Ghajari et al. [2] and discussed the advantages and disadvantages of the peridynamics modeling for the damage in composites with curvilinear fiber paths. Finally, we reproduced the multiple-type and multiple-site damage in multi-ply laminated composites with arbitrary fiber orientations, using peridynamics simulations.
Determining Elastic Properties from the Dynamic Response of Discontinuous Fiber Composites
An investigation into the effect of meso-structure variation on the dynamic properties of a plate made from unidirectional pre-impregnated discontinuous carbon fiber platelets was performed. A platelet generation code was implemented to make finite element models of flat plates comprised of platelets with random in-plane orientations. While the mode shapes were not particularly sensitive to orientation changes of the platelets, natural frequency was shown to have a frequency variance similar to a normal distribution for specific modes. Using the orientation tensor developed by Advani and Tucker [1], the local effective elastic properties were calculated and shown to have a correlation with natural frequency shifts. This method could be used in the future to inform modal tap testing results to determine local material properties of discontinuous fiber parts.
The Forchheimer Effect and Non-Darcy Flows in Liquid Composite Molding Processes
In liquid composite molding (LCM) processes, the infiltrating resin has been assumed to have low velocity and low Reynolds number, where Darcyâ€™s law can accurately describe the flow. In this creeping flow regime, viscous interactions are the dominant sources of pressure gradients. However, with the need to reduce cycle time and increase the production rate of composite parts there is considerable interest in the high pressure resin transfer molding (HP-RTM) process. The potential for high velocity resin flows in HP-RTM can result in an inertial pressure loss produced from the acceleration and deceleration of the fluid and a corresponding nonlinear relationship between pressure and velocity. This inertial pressure loss warrants the need for an additional second order term in Darcyâ€™s law commonly referred to as the Forchheimer Equation. In this study, previous work related to non-Darcy flows and the Forchheimer Equation were reviewed and criteria for defining the point at which Darcyâ€™s law is no longer valid were identified. To investigate non-Darcy flows during LCM processing, high flow rate experiments were conducted to measure the relationship between flow rate and pressure drop. The measurements were made with a test fixture used determine the in-plane saturated permeability of fibrous preforms. The study was conducted with tackified and non-tackified IM7-4HS preforms. Unfortunately, inertial effects and the transition from Darcy to non-Darcy flow could not be determined with the test fixture used in the present study due to leakage within the cavity of the test fixture due to high inlet pressure.
Process Modeling of Bonding Characteristics of Additively Manufactured Interlaminar Reinforcements in Layered Composites
Delamination is a common failure mode observed in fiber reinforced polymeric composites when subjected to various thermo-mechanical loads. Additional reinforcements at the interlaminar regions or in the through-thickness direction have shown to improve damage resistance and durability of these composites. Additive manufacturing, when used for custom-printing polymer reinforcements over carbon fiber prepregs prior to layup, has shown to increase the interlaminar shear strength and fracture toughness values. In this paper, the rationale behind the improvement in interlaminar properties due to printed reinforcements have been investigated by understanding the bond quality between prepreg resin and printed polymers. The factors affecting the bond quality have been explored by developing a validated three dimensional finite element model.
Finite Element Simulation of Stiffener Free Composite Panels with Carbon Nanotubes
This paper presents a finite element modelling approach for analyzing composite structure stitched to stiffeners using nanotubes with very large stiffness. The composite structures with stiffeners plays a significant role in modern day composite design in automotive, naval and aerospace industries. First a composite plate under pure shear buckling is analyzed for determining its buckling load and buckling mode using finite element model and validated against published data. Then an approach is proposed to eliminate the stiffeners by replacing them with carbon nanotubes with very large stiffness. The stiffness of the nanomaterial is determined using a homogenization approach based on the Representative Volumetric Element (RVE) using a unit cell subjected to unit loading in normal, transverse and shear directions. The stiffeners are replaced by inserting nanomaterial submerged into the epoxy material. The stiffness values determined form the RVE test are then implemented into the composite plate to produce similar buckling load and modes. Such structure designed with nanomaterial can be 3D printed with existing 3D printing technology.
End to End Process Simulation of the High Pressure Resin Transfer Molding Process
An end-to-end process simulation has been created to predict the high pressure resin transfer molding (HP-RTM) process. Material properties such as fiber orientations and resin properties are mapped throughout the process so that a performance model can be run on a part as it is actually manufactured, not just as it is designed. A B-Pillar geometry was used as a demonstration part and showed a difference in residual deformation of almost 20% when the as-manufactured material properties were considered.
CAD/CAM Tool for 3D Woven Textile Fabric Design
3-D weaving processes have been advanced within the past decade to manufacture composite components with complex macro- and micro-geometries. Complex composite components are not an assembly of simple unit-cells. At each machine step during the weaving process, the position of each warp yarn must be controlled individually by a Jacquard head. Designing the procedure for the weaving kinematics is labor-intensive. In this paper, a CAD/CAM computational tool is developed to 1) input and define fabric topology and 2) establish a link between fabric topology and weaving kinematics. At each step, the weaving position of the Jacquard head corresponding to each individual warp is calculated, exported in a commercial loom format, and sent to the Jacquard head controller. The 3-D fabric topology can be exported in a format which can be read by the Dynamic Fabric Mechanics Analyzer, the software applied to determine the micro-geometry of fabrics. The geometry of a fabric would thus be available for future mechanical property calibrations and simulations.
Investigation of Solid and Shell Elements for the Modeling of a Cross-ply Thermoplastic in Thermoforming
The feasibility of implementing a thickness-stretch shell element in combination with a user-defined material model in LS-DYNA for completing the simulation of the thermoforming of a cross-ply thermoplastic is investigated. For fabric-reinforced sheets, the primary mode of deformation during the preforming phase of thermoforming manufacture is in-plane shearing of the fabric with varying degrees of shear to conform to the geometric variations over the surface of the preform tool. The decreases in areal coverage that occur with changes in the local shear angle will induce local changes in thickness. During the consolidation phase, multiple preform layers are compressed in a set of matched tools, and the compounding of the thickness variations through the plies can adversely affect the uniformity of the pressure distribution on the composite part in the matched die tooling. Pressure variations over the surface of the part can then lead to incomplete consolidation of the ply stack. Although solutions for implementing thickness changes and contact of the general shell elements have successfully been employed, the challenge of through-plane compressive properties cannot be addressed with that approach. The simulation must accurately capture the thickness changes not only from shear deformation, but also from compression between tooling and other laminate plies. New formulations of solid and shell elements in LS-DYNA may be able to handle the aspect ratios of a general shell element without any compromise in the fidelity of the solution and should be able to model the through-plane compressive properties as well as thickening from shear deformation. A user-defined material model is combined with thickness-stretch shell elements, and LS-DYNA simulations of tension and shear testing of Dyneema HB80 are performed.
Optimal Paths for Steered Fibers in a Composite Laminate with a Stress Raiser
This study provides techniques used to model steered fibers while considering manufacturing constraints. An example of a flat laminate with a circular cut out is used to demonstrate the method of placing AFP tows around the hole. Using BÃ©zier curves it is possible to define the path of fibers in a parametric fashion. This helps in reducing the number of variables used to optimize the path so as to reduce stress concentration arising due to a circular cut out or other geometric discontinuities
Simulation of Gas and Resin Transport Mechanisms in Manufacturing Process of Composite Structures and Their Effect on Porosity
Porosity, as a manufacturing process-induced defect, highly affects the mechanical properties of cured composites. There are several contributing factors to the formation of porosity in composite structures, most notably entrapped air, bag leaks, and volatiles or off gassing. These sources highlight the multi-scale and multi-physics nature of the formation of porosity which make it such a challenging problem to solve. Understanding how these mechanisms contribute to the formation of porosity becomes a key tool in minimizing their negative effect. Experimental evidences show that local resin pressure history plays a major role in void growth during a cure cycle. Local reductions in resin pressure due to factors such as geometric features or cure shrinkage can lead to growth of existing voids and increase the likelihood of porosity becoming locked into the final part structure. Prediction of local resin pressure has been the focus of this approach to modeling the cure of composite structures. This study is focused on developing an efficient FE modeling framework to simulate the resin pressure and porosity (bubble) formation in a composite part. The case study presented here features a laminate with ply drop-off subjected to compaction pressure via a caul plate.
Prediction of Manufacturing Induced Residual Stress and Deformation of Composites Using Mechanics of Structure Genome Based Shell Theory
An efficient shell model based on mechanics of structure genome is introduced to predict manufacturing induced residual stress and deformation for composite laminates. This model utilize shell elements in commercial finite element codes to represent the composite laminate, which greatly reduces the computational cost compared with a direct numerical simulation (DNS) using 3D solid elements or accuracy loss compared with using smeared properties. In this study, a line through the thickness of the composite laminate is chosen to be the structure genome, separating the original 3D body into a 1D through the thickness analysis and a 2D shell analysis. Constitutive relations for the shell elements, considering the effect of residual stress and strain, are constructed. The tool is modeled using 3D elements with a relative coarse mesh and a contact interaction is applied between the tool and composite part, as commonly done in a DNS. Several case studies are presented with comparison between DNS and analysis using smeared properties.
Wrinkles, puckers and fiber bridging are among the major issues encountered in Automated Fiber Placement (AFP) process. These defects are all different manifestations of fiber misalignment. Residual stresses introduced in the tow during the deposition stage by the AFP head are the main driver for these defects. Tack between the deposited tape and the substrate is the resisting force against formation of such defects. Tack is a very complex phenomenon that is influenced by a variety of process parameters including temperature, head pressure and speed, as well as prepreg aging, moisture content and surface condition. A physics-based modelling framework for simulation of tack is developed in this study that allows for prediction of tack response. A model is also developed to simulate the deposition of the prepreg tape by the AFP head. It is shown that the model is capable of predicting AFP-induced puckers.
Process Modeling of Thermosetting Polymers Manufactured Using Fused Deposition Modeling
Achieving dimensional control for fused deposition modeling (FDM) of polymeric parts can be tedious and often involves inefficient trial-and-error experimentation through adjusting print parameters. These inefficiencies may be reduced through the use of process modeling. Understanding the fundamental effects of the print conditions will facilitate predicting part dimensions as well as strength, accounting for the degree of contact between roads, thermal history, and residual stresses. Modeling of additive processes is complex due to transient local boundary conditions and a continually increasing part volume throughout the printing process. Numerous previous studies in FDM have focused on varying experimental process variables such as print speed, extrusion, build temperatures, and print path to determine their effect on mechanical properties. Fewer efforts, however, investigate the complex thermo-mechanical history throughout the printing process or the driving mechanism in road-to-road bonding due to local re-melting of previously solidified material and diffusion. In this research, a 3D finite element model framework for predicting the thermal and residual stress history of an additively manufactured thermosetting polymer is presented. By incorporating material sub-models with the Abaqus FEA solver, the model is able to capture the cure kinetics during the printing process. The model takes a high-fidelity approach through the modeling and stepwise activation of individual roads to simulate the deposition process. Further, the geometry (i.e. cross-section) is varied to demonstrate a mechanism capable of including future research road-to-road bonding.
Automated Construction of Layer-by-Layer Finite Element Sub-Models of Damaged Composites Based on NDE Data
Composite laminate structures are usually modeled as a shell in finite element analysis tools for strength and stiffness determination. However, modeling for fatigue or degradation analysis often needs to be performed with layer-by-layer solid models, but building these models for nontrivial geometries can be extremely difficult, especially when trying to represent realistic defects. This paper discusses how the process of generating layer-by-layer solid finite element models, including insertion of defects, can be automated. We have developed a tool, Delamo, to automate the construction of such models. The tool provides an interface to a commercial solid modeling kernel (ACIS) and a commercial finite element analysis package (ABAQUS). It allows the solid model and finite element model to be built in parallel, layer by layer, starting with a mold, following the same assembly steps as the physical laminate. The bonding step determines the boundary conditions to be applied in the finite element model. Delaminations, determined from nondestructive evaluation (NDE) data can be inserted between layers as needed and are represented as unbonded regions. Potential delamination growth regions can be modeled with a cohesive layer or cohesive boundary condition. Fiber breakage in a layer will be represented by an internal boundary. Based on a mold and a sequence of layer construction and bonding instructions, the tool generates both a solid model and a Python script for ABAQUS that will generate a complete finite element model based on that solid model.
Experimental and Numerical Determination of the Thermal Conductivity Tensor for Composites Manufacturing Simulation
The in-plane and through-the-thickness thermal conductivities are fundamental input parameters for cure simulations of composite manufacturing processes such as vacuum assisted transfer molding (VARTM). The modeling of these conductivities is challenging due to the inherent anisotropy of composite materials, difficulty of the measurements during the cure process, and availability of the equipment. In general, cure simulations consider the homogenized thermal conductivities at lamina level and hence, micromechanics is often used to connect the individual matrix and fiber properties with the homogenized lamina properties based on microstructure, fiber volume fraction, and void content. This work presents the experimental determination and multiscale modeling of the thermal conductivity tensor of a textile VARTM composite as a function of the thermoset cure. Mechanics of structure genome (MSG) is used to compute the equivalent thermal conductivity tensor of the lamina as a function of the thermoset degree of cure. Two-step homogenization is iteratively performed coupling the evolution of the resin thermal conductivity with SwiftCompTM, a general-purpose multiscale constitutive modeling code based on the MSG. The dynamic response of the thermoset resin is measured by means of differential scanning calorimetry to characterize the cure kinetics and thermal conductivity. In addition, an in-house measuring apparatus is used to determine the through-the-thickness thermal conductivities of the dry fabric and cured composite part. The predicted through-the-thickness thermal conductivities of the cured composite are compared against experimental data. The MSG proved to be an efficient method to predict the evolution of thermal conductivity during curing process, and hence, helped bridge the gap by providing data that could not be measured directly with the present experiment.
Virtual Testing for the Mechanical Characterization of Cured Polymer Matrix Composites
The effect of the curing process on the mechanical response of fiber reinforced polymer matrix composites (PMCs) is studied by performing virtual testing at the microscale. Repeating Unit Cells (RUCs) are analyzed to determine the matrix dominated strengths of a thin lamina. Computational models are built using the Finite Element Method (FEM) within the commercial codes ABAQUS/Standard and ABAQUS/Explicit supplemented by user written subroutines. The curing process and the mechanical response of the RUC are modeled including the onset of damage and subsequent failure assuming that damage can initiate during the cure cycle if the internal stresses exceed prescribed values and that it can subsequently grow in the form of cohesive cracks. The transition from a continuum to damage/failure state is effected by using the Bazant-Oh Crack Band model, which preserves mesh objectivity. Results are presented for a randomly packed RUC which is first subjected to curing and subsequently to mechanical loading in transverse and shear directions.
Compression Molding of Discontinuous Fiber Composites, a Thermodynamics Approach to the Compaction Problem
Discontinuous Fiber Composites (DFCs) constitute a new class of highperformance molding compounds that enable the use of compression molding as the manufacturing process of complex components with high volume fraction of fibers. However, the use of DFCs is still limited, one reason being the lack of theoretical modeling and numerical analysis methodologies enabling their use at full potential. During compression molding, the polymeric resin is brought above its melting temperature and thus the material system consists in the cohabitation of a fluid and solid component. In this paper we adopted a macroscopic point of view that consists in depicting the material system as an open thermodynamic system through which the fluid can flow thus yielding a macroscopically compressible system for which a specific state, referred to as the compaction point, exists. The compaction point denotes the state at which no relative motion of fluid can occur with respect to the solid one and thus the thermodynamics system becomes incompressible. A stabilized mixed finite element method is presented that allows for the direct solution of the compaction problem in which all variables are treated as primary ones. This method yields a generalized saddlepoint problem that is known to be stable only for specific coupling of interpolation orders between the primary variables. Circumventing the latter is achieved via a stabilization procedure based on the use of the Algebraic Sub-Grid Stabilization (ASGS) method that yields a fully stable Galerkin weak mixed formulation for equalorder interpolation between all primary fields. This method proves beneficial to highlight the physical phenomena occurring in compression molding of fiber-reinforced media.
Numerical Analysis of the Effects of Cutting Parameters on the Sub-Surface Damage in Machining of CFRPs
Carbon Fiber Reinforced Plastics (CFRP) are widely used in the aerospace application. However, the machining induced sub-surface damage is critical to them. Because it would significantly reduce the reliability and the service life of the components. Particularly when the cutting angle is obtuse. The main purpose of this paper is to research the effect of the cutting parameters on the sub-surface damage in machining of CFRPs with obtuse cutting angle. And the numerical simulation is an effective method to study and observe the sub-surface damage under different cutting parameters. Despite some researches have been carried out on the effects of the cutting parameters on sub-surface damage through two-dimensional cutting models of CFRPs. The out-of-plane force also has influences on the change of the depth of the subsurface damage. And the generated cutting forces are unequal under different cutting widths during the cutting of the CFRPs. Therefore, the effects of the cutting width on the sub-surface damage still need to be studied. In this paper, a three-dimensional numerical model for the orthogonal cutting of CFRPs was developed based on the AbaqusTM. And the stress-strain relationship in the out-of-plane direction when machining CFRPs was considered in this model. Particularly, the mode I fracture in the transverse direction of the CFRPs was indicated by the damage factor of the tensile failure in the transverse direction to describe the sub-surface damage. The 135Âº cutting of the CFRPs under different cutting widths were simulated by this model, and the change of the depth of the sub-surface damage were analyzed. In addition, the simulation results were validated by experiments. The model provides an effective method for the prediction of the sub-surface damage, which is helpful for the optimization of the design of the tool.
Numerical Simulation with Experimental Validation of the Draping Behavior of Woven Fabrics
Woven fabric composites are extensively used in molding complex geometrical shapes due to their high conformability compared to other fabrics. Preforming is an important step in the overall process. In this step, the two-dimensional fabric is draped to become the three-dimensional shape of the part prior to resin injection. During preforming, the orientation of the tows may change significantly compared to the initial orientations. Accurate prediction of the tow orientations after molding is important for evaluating the structural performance of the final part. This paper investigates the fiber angle changes for carbon fiber woven fabrics during draping over a truncated pyramid tool designed and fabricated at the General Motors Research Labs. This aspect of study is a subset of the broad study conducted under the purview of a Department of Energy project funded to GM in developing state of the art computational tools for integrated manufacturing and structural performance prediction of carbon fiber composites. Fabric bending, picture frame testing, and bias-extension evaluations were carried out to determine the material parameters for these fabrics. The PAM-FORM computer program was used to model the draping behavior of these fabrics. Following deformation, fiber angle changes at different locations on the truncated pyramid were measured experimentally. The predicted angles matched the experimental results well as measured along the centerline and at several different locations on the deformed fabric. Details of the test methods used as well as the numerical results with various simulation parameters will be provided.
Analysis of Cure Dependent Residual Deformation in Thermosetting Composite
Constrained thermoset deformation has been shown to exhibit cure history dependence. The practical significance of this problem is due to the necessity to ensure dimensional stability and tolerance in composite structures especially with curvilinear geometry. During manufacturing of curved composite laminates, cure shrinkages produce so-called spring-in or spring forward effect, as a result of material thermo-elastic anisotropy. The residual cure deformation in a thermosetting composite laminate was considered by subjecting bi-lamina strip samples, composed of [0/904] plies, to cure cycles with different heating profiles. This allowed to separate and study different sources of shrinkage in thermosetting composites, namely chemical resin shrinkage, which occurs due to cross-linking reaction, and thermal shrinkage or expansion, result of applied change in temperature. The significant dependence of the residual strip deflection was found to relate to (i) interaction between chemical and thermal shrinkages and (ii) resulting resin cure strain rate, which affected elastic modulus development in the composite. The constitutive material model of composite during cure was developed and applied to reduce residual deformation during the manufacturer recommended cure cycle (MRCC) by enabling different interactions between thermal and chemical shrinkage. Consequently, the modified cure cycle (MCC) was proposed allowing to further reduce the amount of residual deformation in bi-lamina laminate prior to cooling. The presented approach of establishing the constitutive cure model of composite provides an insight of how to control the laminate final shape by selecting the parameters of the cure cycle. Consequently, cure cycle can be used to mitigate the adverse effects of the residual deformation on the performance of laminated composites.
Thermoviscoelasticity in Extrusion Deposition Additive Manufacturing Process Simulations
The Extrusion Deposition Additive Manufacturing (EDAM) process, more commonly known as Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF), is a promising new manufacturing method for high temperature tooling and molds. However, nowadays this process is still largely empirically calibrated. Based on the required capabilities for determining residual stresses and the final deformation state of printed parts and their effects on the part performance, there is a large interest in the development of process simulations. To this end, a set of simulation tools has been implemented in Abaqus/Standard through a user subroutine suite at Purdue University to model the EDAM process. This work extends these simulation capabilities to include thermoviscoelastic material properties. An incremental linear thermoviscoelastic model is presented for an anisotropic material and implemented in Abaqus with a UMAT user subroutine. The thermoviscoelastic properties of a 50wt.% carbon fiber reinforced Polyphenylene Sulfide (PPS) were characterized in relaxation experiments and the implemented model was verified using the experimental data. To assess the significance of crystallization informed thermoviscoelastic material properties, process simulation results were compared for an autoclave tool. Here, the outcomes of the thermoviscoelastic analysis were compared with a simulation considering temperature dependent elastic properties. The results of this comparison indicate that it is essential to capture viscoelasticity that is informed by crystallization in order to predict realistic stress levels for subsequent performance analyses. In addition to the stresses, the resulting final deformations differ significantly for both analyses as well. The different internal stress levels after deposition and the crystallization informed material stiffness in the thermoviscoelastic analysis are the main causes for this deviation.
Compression Resin Transfer Molding (C-RTM) Simulation Using a Coupled Fluid-solid Approach
Composite materials are being used at an increasing rate in the automotive industry due to their superior mechanical properties at low densities leading to lightweight components. Continuing this trend requires the identification of manufacturing processes for these components that have short cycle times along with delivering reproducible parts. The modeling of these processes is essential in order to prevent the excessive experimentation currently required to develop the process parameters. One of the manufacturing processes currently under consideration is compression resin transfer molding (C-RTM). Recently, General Motors and ESI, NA Group have been working together in developing a state of the art computational tool for process simulation of composites in a project supported by the Department of Energy (DOE). This section of work on the development and validation of a simulation tool for the CRTM process is under the broad scope of the above DOE project. Current development for C-RTM simulation introduces two new technologies embedded into the current PAM-RTM solver: - Fluid-Solid mechanics coupling to determine the preform deformations during the compression-injection process, and - An Inter-Penetrating Mesh providing the ability to handle the vanishing gap during the closing of the mold. This paper will focus on the development and validation of the proposed simulation software using resin flow experiments in a truncated pyramid tool manufactured at General Motors Research and Development Labs.
Delamination Analysis of CFRP Laminates Exposed to Simulated Lightning Current Considering Pyrolysis Reaction
Carbon fiber reinforced plastic (CFRP) composites have been applied to the primary aircraft structures to improve fuel efficiency because they have superior specific strength and stiffness. Lightning that occasionally strikes flying aircraft produces structural damage. Once the CFRP structures are damaged, inspection and repair are required. To improve the robustness and damage resistance against lightning strike, it is important to elucidate the damage mechanism of CFRP laminates exposed to lightning strike. In this study, simulated lighting current (impulse current) testing was conducted to elucidate the damage behavior of CFRP laminates. Carbon fiber / toughed epoxy prepreg (IMS60/133; Toho Tenax Co. Ltd., Japan) laminates, which had a quasiisotropic stacking sequence ([45/0/-45/90]4s), were evaluated. Simulated lightning current testing were conducted using an impulse current generator (Otowa Electric Co. Ltd., Japan) in National Composite Center, Nagoya University, Japan. After the impulse current testing, ply lifting caused by the thermal decomposition of matrix resin and carbon fiber breaks were observed on the surface of the specimen. In addition, delamination within approximately 1.5 mm (10 plies) from the surface was detected using ultrasonic scanning. Coupled thermal-electrical analysis, heat transfer analysis, and stress analysis using cohesive elements were carried out using a commercial finite element analysis software (ABAQUS ver.6.14, Simulia; Dassault Systemes, France). A thermal decomposition model, which was estimated experimentally using thermogravimetric analysis results, was incorporated into ABAQUS with user-subroutine. The delamination of CFRP laminates exposed to impulse current was simulated successfully. As a result, it was revealed that the reduction of interlaminer strength (fracture toughness) due to the progress of thermal decomposition accompanying heat
Fatigue Life Prediction of GFRPs under Random Ocean Current Loading
A comprehensive fatigue characterization of Glass Fiber Reinforced Plastics (GFRPs) under random ocean current loading is performed. A procedure to calculate random loading from the time history data of ocean current velocity is described. Two sources of randomness were included in the calculation, namely, i) turbulence in ocean current and ii) wake flow behind the support structure of the Ocean Current Turbine (OCT). The random ocean current velocity was first converted into hydrodynamic load and subsequently to tensile stress of the OCT blade using Blade Element Momentum (BEM) theory and Finite Element (FE) method. The random ocean current loading had high stress ratio (R), which was inherently different from that of the wind loading. The random ocean current loading was applied to GFRP coupons using a materials testing machine. A code was developed with LabView and was interfaced with the testing machine to implement random loading. Fatigue experiments were then conducted under this random loading. Experimental results indicated that fatigue life increased exponentially with the decrease in mean velocity. Fatigue life of GFRP was also predicted by using Constant Life Diagram (CLD) and Palmgren-Minerâ€™s rule. CLD was developed from S-N curves of various stress ratios between R = 0.1 and 0.8. A good agreement was observed between the experimental and the predicted fatigue lives when the mean velocity was high. Comparing with traditional Goodman diagram, it was found that results obtained from CLD prediction were more conservative. Details of random load calculations, fatigue experiments, and life prediction methodologies are described in the paper.
When an Interface Crack Occurs in a Composite?
An efficient method is developed to estimate the load level at which a crack occurs on the constituent fiber/matrix interface of a composite subjected to any load. Only the original properties of the fiber and matrix documented in advance or measured independently are required for this estimation, in addition to the transverse tensile strength of a unidirectional (UD) composite made from the same constituent system. Stress concentration factors (SCFs) of the matrix in the composite are crucial for this purpose. Such an SCF cannot be defined following a classical approach, and the SCFs with the perfect and cracked interfaces are derived respectively. Letting the predicted transverse tensile strength of the UD composite with an initial perfect and later cracked interface be equal to the measured counterpart, a critical Mises stress of the matrix at which the crack occurs is obtained. When a Mises stress of the matrix in any other composite made of the same constituents approaches the critical value, the interface cracking load is determined accordingly. Given a fiber and matrix system, whether any further interface modification is necessary and how much potential exists for doing such can be easily understood.
Damage Initiation in Unidirectional Fiber Reinforced Polymeric Composites Under Shear Loading
This work studies the initiation of the first failure event in unidirectional composites subjected to axial (longitudinal) shear loading. Two failure modes are investigated, depending on whether the dilatational or the distortional component of the strain energy density reaches its threshold. The nonuniform fiber distribution is described in terms of â€œdegree of nonuniformityâ€. This degree of nonuniformity is proposed as a quantitative measure of manufacturing accuracy. The nonuniformity 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. A parametric study is conducted to study the effects of fiber volume fraction, degree of nonuniformity and matrix stiffness on the damage initiation.
Free-Edge Strain Localization of Cross-Ply Laminates with Different Lamina Arrangements
Earlier studies indicated that a failure of a laminate is initiated from a location where there is Free-edge due to a geometrical discontinuity. Numerous efforts have been made to elaborate these free-edge effects using analytical and numerical approaches, nevertheless, only few experimental works have been recorded. Considering the recent advance in Digital Image Correlation , an experimental work is conducted to investigate the effect of free-edge on the failure mechanism of cross-ply laminate under uni-axial tensile loading. The full-field strain including small strain localizations at the free-edge of the specimen are extracted for three groups of cross-ply laminates with distinct stacking arrangements. From the results, it is observed that the variation in the location and arrangement of plies have a direct influence on the quantity and size of the matrix/transverse cracks generated.
Failure Prediction of Countersunk Composite Bolted Joints Using Reduced Order Multiple Space-Time Homogenization
This paper presents the computational modeling and prediction of fastened composite joints using the Eigendeformation-based reduced order homogenization (EHM), as part of the Air Force Research Laboratoryâ€™s Composite Airframe Life Extension Tools for Assessing the Durability and Damage Tolerance of Fastened Composite Joints. The multiscale modeling approach is based on computational homogenization theory, which computes the effective stress-strain behavior of the composite by solving a boundary value problem defined over unit cells of the composite microstructure. The key feature of the proposed modeling approach is that nucleation and propagation of damage within the composite are tracked at the microstructure, and no macroscale phenomenological failure criterion is used [1]. To reduce computational cost, this approach evaluates the microstructure problem by employing a reduced approximation basis, expressing the microstructural variations by numerical Greenâ€™s functions, computed prior to macroscale analysis. Three experimental test setups are considered for prediction that consist of static tension and compression tests of open hole, filled hole, and single shear bearing configurations. Motivated by blind prediction results the model is enhanced by: (a) recalibration of the damage parameters to better fit the input composite properties consistent with the curing process used in these experiments; and (b) incorporation of a better modeling methods to improve the post-failure behavior of the constituents for bearing-dominated failure cases.
Initial Investigation of Time Dependency on Failure Curvatures of FlexLam High Strain Composites
Using the column bend test (CBT) developed at Opterus R&D Inc. by Tom Murphey, effects of instantaneous and long-term bending stress rupture where investigated for the high strain composite (HSC) architecture known as FlexLam. The CBT method was selected for its ability to induce a pure moment into the test specimen while maintaining a reduced stress concentration at the clamp interface, which is critical for examining the curvature effects on these thin plate specimens at high strain states. In this investigation, two different types of tests were conducted in order to develop an understanding of the time dependent failure curvature behavior of HSC coupons. Both tests involved laminate samples with varied fiber orientations (0, 90, and +/-45 degree). The first test was to quasi-statically compress samples until failure was observed in each sample. This data was required to establish an average instantaneous failure curvature. The samples were then stowed at a constant curvature corresponding to eighty percent of the curvature observed at rupture for a long period to allow for stress relaxation within the sample. Resulting damage from the stowage was characterized into different categories based on the severity of each failure ranging from cosmetic blemishes to catastrophic failure of the laminate. Further empirical and analytical investigation of time dependent stress rupture of FlexLam HSCs will be presented in part two of this investigation.
Intra-Inter Crack Band Model (I2CBM) as a Prediction Tool for Open Hole and Filled Hole Failure Analysis of Composite
Intra-inter crack band model (I2CBM) is a unified approach for modeling various intralaminar and interlaminar progressive damage and failure events in polymer matrix composites. The progressive damage and failure formulation used in this method accounts for various crack planes, modes and their mutual interactions as observed in experiments. By carefully selecting the material properties, 3D elements in I2CBM formulation can be adapted for modeling individual lamina elements or to model the interface delamination elements. The input parameters used for the I2CBM predictions are either obtained from coupon level tests or calibrated against relevant laminate level test data. A path for communication between the intralaminar and interlaminar failure mechanisms are enabled in the model to overcome some of the limitations associated with homogenized element modeling and also helps to capture complex interactions in a physically correct manner. A non-local crack tracking method is added to the analysis tool to keep a global record of the development of matrix cracks in the model and it ensures a certain spacing between adjacent cracks as observed in experiments. The above two features equips the analysis method to be able to capture various failure modes observed in composite laminates including the growth of splitting cracks in 0 layers within a laminate. These capabilities are validated against open hole tension and filled hole tension test data of 27 ply and 36 ply laminates with countersunk holes. The effect of bolt pretension on the filled hole failure analysis is also presented and the failure mechanisms are discussed.
A Numerical and Experimental Approach to Characterize the Failure Modes of Hexagonal Cell Honeycomb Core Under In-Plane Loading
In the present study, the failure modes of a hexagonal cell honeycomb core under in-plane loading are investigated. It is shown that, under tensile loading, the node bond failure and ribbon fracture are the main causes of the honeycomb core failure when the core is loaded along the transverse and ribbon directions, respectively. However, under compression loading, the ribbon fracture dominates the failure mode along both directions. The development of a failure envelope for a honeycomb core material is demonstrated using a combination of experiments and simulations. The failure envelope is cast in terms of the core normal strains along the principal material directions. The failure envelope can be used as a design tool to determine the suitability of the core material for sandwich structural applications.
Predicting Adhesive Bond Performance Based on Initial Dielectric Properties
In recent years, there has been a widespread growth in the application of composite materials particularly in the Aerospace and Automotive sectors. This is because composite structures are generally comparatively light in weight and provide corrosion and wear resistance as compared to metals or ceramics. Due to the strict failsafe philosophy of the aerospace industry, the certification approach for current practice in joining composite materials is to thicken the joining areas and to use numerous fasteners which in turn increases the weight and stress concentrations in the structure. The use of adhesive bonding can improve the stress distribution between the composite materials / dissimilar materials and can contribute to a lighter structure. However, there much investigation is yet to be done in this discipline to predict the bond strength and performance using non-destructive evaluation methods. This paper will focus on an approach to study the mechanical as well as the dielectric properties of an adhesive bond. The dielectric testing is done by using Broadband Dielectric Spectroscopy (BbDS), wherein the dielectric characteristics of the material are analyzed in a wide frequency spectrum. The data obtained by this technique are used to demonstrate the charge transport, the combined dipolar fluctuation, and the effects of polarization occurring between the boundaries of materials. The continuous modifications of the dielectric spectra are due to the changes in the electrical and structural interactions between the particles, shapes, and orientations of the constituent phases of the morphological structure of the material system. Information about the morphologies, impurities/contamination or interaction of the dissimilar surfaces of the pristine bond can be obtained from the initial BbDS properties. The dielectric properties for adhesively bonded composites with different surface adhesion properties have shown promising evidence of predicting the final mechanical performance of the bonded material system. The success and limitations of this approach will be discussed, and needs for continued investigation identified
Progressive Failure Analysis of a Composite Lamina Using Puck Failure Criteria
This paper focuses on the progressive failure analysis of a composite at lamina scale using two different material property degradation models in an open hole specimen under uniaxial tension. Puck failure criterion is selected to detect the onset of damage. The ABAQUS user defined material subroutine UMAT was developed to apply the failure criteria and degradation models. A partial discount method and a gradual stiffness degradation is implemented in ABAQUS environment. The damage initiation and progression obtained from the proposed model is compared with the observed experimental results from digital image correlation. The comparative study confirmed that the simulation results were in good agreement with the experimental results.
Adhesive Energy Between Sylgard and Inclusions: Experiments and Model Calibration
There are well established classical experimental and analytical studies to analyze the bonding between polymers and rigid particles. On the other hand, most numerical studies of fracture in composite materials use particle/matrix interface cohesive laws and parameters extracted from the cohesive energy at the macroscale level. Such an approach enables good results when the mechanical response is compared to macroscale experiments but in high energy composites where localization of energy dissipation by delamination and friction may lead to initiation of hot spots the particle/matrix cohesive law is of extreme importance. In this work systematic experimental and numerical studies are performed to establish a cohesive law of the composite interface.
Overview of Database Generation for Development of Bolted Composite Joint Fatigue Methods
In anticipation of service life extension for airframes in the United States Air Force (USAF) with composite materials, Air Force Research Laboratory (AFRL) is currently developing various tools for durability and damage tolerance (DADT) assessment of bolted composites joints. Under AFRLâ€™s Composite Airframe Life Extension (CALE) Project 2 program, parametric and progressive damage analysis (PDA) tools were developed for such purpose. As part of the tools development under Project 2, over 150 single shear bearing (SSB), open- and filled-hole tension and compression (OHT/FHT and OHC/FHC) and tension bearing-bypass (BB) specimens were tested in static, constant amplitude (CA) fatigue, and CA block fatigue loading to establish a database for the semi-empirical parametric model while also providing opportunities for iterative maturation of the PDA tools through many prediction/calibration cycles. Stress ratios, R, in the range of -1.0 to 0.1 were used in CA and CA block fatigue tests as one of the goals for the program was to develop tools capable of variable fatigue stress and amplitude. All specimens were fabricated from toughened 350oF-cure carbon/epoxy unidirectional material and consisted of three thicknessesâ€”0.143â€, 0.191â€, and 0.239â€â€”from [44/44/12] laminate family. The fastener used was a Â¼â€ diameter stainless steel 100o flush head rivet pin that is commonly used in production. The tests were performed at both Northrop Grumman Corporation (NGC) and AFRL. These tests provided data on 1) static response, 2) hole elongation (%e) vs number of cycles (N), 3) joint stiffness (k/ko) vs N, and 4) residual strength and/or S-N with respect to diameter-to-thickness ratio (D/t), R, fastener preload, and fatigue load profile (i.e., CA vs CA block spectrum). This paper will provide an overview of the various test methods, summary of key results from the trial phase of the 150+ tests, and the effects of these test results on the remainder of the program testing approach.
Damage Modeling in Ultra Short Fiber Tailorable Feedstock Composite Materials
The objective of this study is to provide detailed investigation of stiffness, strength, and failure mechanisms in ultra-short carbon fiber polymer composite materials. The reinforcement phase in traditional chopped fiber composites generally consists of glass fibers 10 Î¼m in diameter with lengths on the order of 15 mm in length. Ultra-short carbon fiber composites with lengths of 3 mm and diameters of 3 to 8 Î¼m show promise as a versatile inexpensive reinforcement, yet failure of morphologies which bridge particle-like and fibril behavior are not yet well explored. 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. Discrete damage and progressive failure are simulated using both micromechanical element deletion schemes and the Extended Finite Element Method (XFEM) for mesh-independent fracture and failure analysis. The XFEM method allows splitting of an element to represent discrete damage, while inserting a cohesive law to describe post-cracking behavior. Careful parametric analysis will elucidate the effects of various microstructural factors, including: fiber-matrix interface effects, fiber diameter, chopped fiber length, uniform fiber alignment / misalignment, fiber spacing, and the constituent properties of both fiber and matrix. Trends in microstructural effects on material performance will be carefully analyzed to provide guidance for processing targets to achieve desired properties in tailorable feedstock.
Temperature Evolution in Carbon Fiber/Bismaleimide Matrix Composites under Continuous-Wave Laser Irradiation
Comprehensive experimental and computational studies have been conducted to accurately measure and predict temperature evolution in polymeric composite material subjected to laser irradiation. Plain-woven 16-ply composites with T650 carbon fibers and bismaleimide matrix were used as a substrate subjected to laser irradiation using a continuous wave, ~1.0692 Î¼m wavelength laser with a flat-top profile and a nominal beam diameter of 3.0 cm. The spatial and temporal evolution of temperature for the front (laser-exposed side) and back surfaces of the composite specimens were monitored and recorded with spatially calibrated infrared (IR) cameras. Additionally, internal temperature evolution was measured with thermocouples embedded in the midplane of the composite specimens. A nonlinear transient finite element (FE) analysis, based on conductive, convective, and radiative heat transfer, was conducted using a commercial FE software package, ANSYS, to predict the temperature history during heating (laser exposure) and cooling (after exposure). For all investigated laser powers (5.0 - 13.8 W), the numerical prediction demonstrated considerable agreement with experimental IR camera and thermocouple data.
Discrete Damage Modelling of Delamination Migration in Clamped Tapered Laminated Beam Specimens
Laminated composite materials are used in stiffness critical applications and contain various stress concentration features such as holes, tapers and stiffener attachments. It is these areas which often determine the load carrying capacity of the structural elements and thickness scaling of critical parts. A representative subelement is studied in this work with the aim of evaluating the ability to predict the mechanical response of such critical regions by using discrete damage modeling. Clamped Tapered Beam Specimen (CTBS) designed by Advanced Composite Project Team at NASA LaRC is considered. This specimen allows studying the matrix crack initiation from pristine condition as well as delamination initiation and evolution including migration from one interface to another. Regularized eXtended Finite Element Method (Rx-FEM) is employed for analysis. The Rx-FEM allows modeling the displacement discontinuity associated with individual matrix cracks in individual plies of a composite without regard to mesh orientation by inserting additional degrees of freedom in the process of the simulation. The propagation of the mesh independent crack is then performed by using Cohesive Zone Method (CZM). A simultaneous experimental program was conducted at NASA LaRC and used for the analysis validation. The predicted results showed good agreement with experimental data for ply level transverse strength parameter obtained by using three point bend test method, whereas the results obtained by using a lower value resulting from tensile testing of 90o coupons resulted in under-prediction of the peak load. The distance between the migration location and the load application point is consistent with experimental data.
Three-Dimensional High Fidelity Progressive Failure Damage Modeling of NCF Composites
Performance prediction of off-axis laminates is of significant interest in designing composite structures for energy absorption. Phenomenological models available in most of the commercial programs, where the fiber and resin properties are homogenized, are very efficient for large scale structural analysis, but lack the ability to model the complex nonlinear behavior of the resin and fail to capture the complex load transfer mechanisms between the fiber and the resin matrix. On the other hand, high fidelity meso-scale models, where the fiber tows and matrix regions are explicitly modeled, have the ability to account for the complex behavior in each of the constituents of the composite. However, creating a finite element model of a larger scale composite component could be very time consuming and computationally very expensive. In the present study, a three-dimensional mesoscale model of non-crimp composite laminates was developed for various laminate schemes. The resin material was modeled as an elastic-plastic material with nonlinear hardening. The fiber tows were modeled with an orthotropic material model with brittle failure. In parallel, new stress based failure criteria combined with several damage evolution laws for matrix stresses were proposed for a phenomenological model. The results from both the meso-scale and phenomenological models were compared with the experiments for a variety of offaxis laminates.
Two Modified Compressive Strength Criteria for Unidirectional Composites
The compressive strength of unidirectional composites is strongly influenced by the elastic and strength properties of the fiber and matrix phases, as well as by the local geometrical properties, such as fiber volume fraction, misalignment and waviness. In this present investigation, two microbuckling criteria are examined against a large volume of measured data of unidirectional composites taken from the literature. The first criterion is based on the compressive strength formulation based on the buckling of Timoshenkoâ€™s beam. It contains a single parameter that can be determined according to the best fit to various types of polymeric matrix composites. The second criterion is based on buckling-wave propagation analogy based on the solution of an eigenvalue problem. Both criteria provide closed-form expressions for the compressive strength of unidirectional composites. We propose modifications of the two criteria by a fitting approach, for a wide range of fiber volume fractions, applied to four types of unidirectional composite systems. Furthermore, a normalized form of the two models is presented after calibration in order to compare their prediction against experimental data for each one of the material systems. The new modified criteria are shown to match a various types of unidirectional composite systems, and can be employed as practical failure criteria in the engineering design of laminated structures.
It was previously shown that stacking unidirectional carbon fiber composite helicoidally with small angles can result in improvements in terms of maximum load resistance against transverse load. Through simulations and experiments on 19 ply helicoidal and cross-ply configurations, it was concluded that one of the reasons 19 ply single helicoidal configurations outperform common cross-ply configuration is because the helicoidal laminate has a single large delamination at middle of the through thickness under transverse load, previous study has shown the merging of the fiber/matrix damage initiates at bottom of the plate and the large delamination layer causes load drops which decreases maximum load. helicoidal outperforms cross-ply because the fiber/matrix damage initiating from the bottom surface in helicoidal plate has to grow further to reach the delamination layer located at middle and will not cause load drop until final failure where multiple large delamination in cross-ply plate will cause multiple load drops. This allows a large increase in stiffness and maximum load bearing capability for helicoidal configuration. Following on from this finding, further experiments have been conducted to study the performance of helicoidal configurations. Experimental results show the location and numbers of delamination can be controlled through ply orientation, which further improves the performance of helicoidal laminates.
Intra-Laminar Damage in Carbon Fiber-Reinforced Composites: Experimental Observations and Model Validation
The intra-laminar damage during quasi-static loading of Carbon Fiber-Reinforced Plastics (CFRPs) leads to degradation of ply mechanical properties. The possible intralaminar damage mechanisms are fiber failure, matrix cracking, and fiber/matrix debonding. In addition to experimental investigation of these mechanisms, modeling methodologies have been developed to predict the degradation of ply properties, accounting for (some of) these mechanisms. One of these methodologies is proposed by LadevÃ¨ze and LeDantec [1]. This model requires a set of input parameters, which are obtained through mechanical tests on laminates with prescribed lay-ups. In the current study, we extract these input parameters for an aerospace-grade CFRP, with detailed explanations. Beside the conventional parameter identification procedure, the parameters are linked to the physical progression of damage (in terms of crack densities), which is measured in the in situ images taken during the tests.
Oscillating Crack Migration in Mode I Fracture Specimens
Damage propagation in [90Â°/0Â°10/90Â°]s double cantilever beam (DCB) specimens, previously studied experimentally, was shown to exhibit an oscillating, trapezoidal fracture pattern throughout the central two 90Â° plies. Interface delamination between 0Â° and 90Â° plies tended to periodically migrate through the 90Â° plies, producing the oscillating damage pattern. Additionally, the experimental study showed significant toughening of the [90Â°/0Â°10/90Â°]s DCB specimens when compared with a standard 0Â° DCB of the same material. Both standard 0Â° and [90Â°/0Â°10/90Â°]s DCB specimens were modeled in the present work. Damage was modeled using a regularized formulation of the eXtended-Finite Element Method (Rx-FEM) to represent transverse matrix cracking and a standard interface cohesive zone method to represent interface delamination. A series of plane-strain models were produced to examine the effects of interfacial ply strength and fracture toughness properties. Qualitatively, the oscillating migrating crack pattern was reproduced. However, in all cases the spacing of the crack migrations was larger than observed. Additionally, the large toughening region observed experimentally was not reproduced. Of the various cases examined, it was found that increasing interfacial Mode I fracture toughness and reducing load step size decreased migration spacing the most effectively. Slight increases in toughness were observed, however, it is concluded that the primary mechanism for increased toughness is a 3D, fiber bridging phenomenon as described in the experimental effort.
Preliminary Examination of Ply Cracking and Delamination in a Tapered Chevron Notch Laminate Subjected to Tensile Loading
New composite airframes have integrated design features for improved performance not necessarily related to structural efficiency. In many cases, such as on the wing, the panels are an integral part of the primary structure, and thus accurate strength predictions of these specially notched laminates are crucial for platform structural integrity management. Experimental and numerical studies were conducted to determine the effect of these specially shaped notches on the laminate tensile strength and in particular how sub-critical damage evolved to final failure. High definition video and high-speed video with contrast enhancement and thermal stress analysis were used to characterise failure mechanisms and measure the full field stress. The results showed that significant stress concentrations exist around the notch root leading to sub-critical damage evolution. The overall strength, however, was similar to an open-hole notch specimen. The work provides an experimental and numerical foundation for the further development of strength prediction techniques that model directly the onset and progression of sub critical damage and predict failure.
Fatigue Simulation for Damage Propagation in Composite Structures
The capabilities of newly-implemented Abaqus procedures for delamination propagation in composite structures subjected to fatigue loading are investigated by examining the response of double cantilever beam (DCB) tests. Two-dimensional and three-dimensional models are at first evaluated under quasi-static loading conditions using the virtual crack closure technique (VCCT). The results show good agreement between the shapes of the predicted and experimental delamination fronts. Then, fatigue analyses are performed using two fatigue procedures: direct cyclic fatigue (DCF) and simplified fatigue (SF). It is found that, when modeling the response of DCB tests, the SF and DCF procedures correctly recover the Paris law propagation rate. The analysis with the SF procedure appears to require 50 times less computational time than DCF. However, the fatigue delamination front is characterized by sharp edges. Therefore, additional modeling features such as gradual nodal release may be required before the residual lives of more complex postbuckled structures can be predicted with confidence.
XFEM Analysis of Progressive Damage and Failure in CFRP Laminates under In-Plane and Out-of-Plane Loading Conditions
The aim of this study is to examine the capability of quasi-3D extended finite element method (XFEM) approach under in-plane tension, compression, and out-ofplane loadings. This study is based upon the previous works by the authors [1-3] but damage models are updated to predict longitudinal failure, matrix crack, and delamination under both of tensile and compressive loadings. The validity of proposed methodology was tested against the open-hole tension/compression (OHT/OHC) and quasi-static indentation (QSI) tests. Predicted results are compared with experiments in terms of nonlinearity, internal damage, and strength.
Mixed Mode I/II Damage Initiation and Evolution in 3D-Printed Soft Interfacial Layer
Inspired by biological sutures in nature, biomimetic suture specimens with different waviness and material combination were designed and fabricated via multi-material 3D printing. The damage initiation and evolution of the material in the wavy suture layer is the key to determine the strength and failure mechanisms of the biomimetic sutures. Due to the complicated geometry, the suture layer is under a mixed mode I/II loading. To explore the influences of geometry and material combination on the damage initiation and evolution of the soft suture layer under mixed mode I/II loading, based on the concept of virtual internal bonds (VIB) theory, a hyperelastic softening constitutive model was developed and numerically implemented in ABAQUS via subroutine VUMAT. Mechanical experiments were performed on the 3D printed suture specimens, and two failure modes were observed: tooth breakage mode and interface failure mode. The failure mode transition was found to be jointly determined by the geometry of the suture layer and the material properties in it. It was found that when the tooth tip angle increases and/or the strength of the material in the interfacial layer decreases, the failure mode of the suture can transit from tooth breakage mode to interfacial material failure mode. The proposed hyperelastic softening model could accurately predict the damage initiation and evolution in the interfacial layer without pre-assuming any failure criteria.
Multiscale Sensitivity Analysis of Failure in Composite Materials Using an Interface-Enriched Generalized FEM
A computational framework is developed to model the failure of composite materials with complex interphase boundaries. The problem of interest in the present study pertains to the transverse failure of fiber-reinforced composites with an emphasis on capturing the debonding of the fiber-matrix interfaces using a cohesive zone model (CZM). The Interface-Enriched Generalized Finite Element Method (IGFEM) is used to model efficiently the geometrical features of the microstructure obtained from reconstructed images of actual composite samples using meshes that do not conform to the microstructure. For cases where fibers are closely packed, a two-interface element is presented, which allows a single finite element to contain multiple cohesive surfaces. We introduce a nonlinear material sensitivity formulation to quantify how variations in the interfacial cohesive zone properties affect the evolution of the transverse stressstrain response.
Comparative Study of Progressive Damage Prediction of a Countersunk Bolted Joint Using Continuum and Discrete Damage Modeling Approach
Two approaches have been developed to perform blind and recalibrated analyses of composite laminated plates with an open and a filled countersunk hole subjected to remote static loading. The first approach is based on a continuum damage mechanics (CDM) model, with the application of LaRC04 as the damage initiation criterion and an energy driven softening model to describe damage evolution. Failure event is determined elementwise based on a mesh characteristic length based on the crack band model in order to ensure its energy dissipation independence of the mesh size. Because of the presence of the localized stress concentration near a countersunk hole, fiber orientation dependent matrix cracks can be initiated and the resulting intensified stress field can promote delamination initiation and propagation at a ply interface. To enhance the modeling fidelity for discrete damage characterization at a critical region of a countersunk hole, the second approach based on a hybrid modeling strategy is applied where the inclined portion of the countersunk hole is characterized by CDM and its straight portion of the hole is described by a discrete crack network (DCN) model using a phantom paired crack modeling approach. A comparative study of both approaches is performed through the static failure prediction of the open and filled countersink hole tension specimen. Both the applicability and accuracy of the hybrid approach are demonstrated via its application for static failure prediction of open and filled countersink hole specimens subjected to compression.
Enhancement of Delamination Resistance with Threaded and Anchored Z Pins in Composite Laminated Structures
Z-pinning is a technique where composite laminates are typically reinforced with metal or composite fiber pins inserted vertically into the laminate. In the present work, novel approaches to the current z-pinning technology are investigated to increase delamination resistance of the laminate. Using threaded pins for reinforcement increases mechanical interlocking between the pin and the laminate as well as the epoxy-pin contact area thus delaying delamination. When smooth metal pins are used for reinforcement, anchoring their ends on to the surface of the laminate before curing delays delamination through pin gripping. Experiments were conducted to compare pin pullout strengths under tensile loading conditions on reinforced laminates manufactured from IM8-epoxy prepregs. Test results showed increase in pullout strengths and thus the potential for increase in delamination resistance when threaded or anchored pins were used. In addition, fracture toughness and delamination resistance under mode-I loading were investigated by performing experiments on DCB specimens. Results indicate superior delamination resistance properties for both threaded pin and anchored pin inserts.
Simulating Matrix Crack and Delamination Interaction in a Clamped Tapered Beam
Blind predictions were conducted to validate a discrete crack methodology based on the Floating Node Method to simulate matrix-crack/delamination interaction. The main novel aspects of the approach are: (1) the implementation of the floating node method via an â€œextended interface elementâ€ to represent delaminations, matrix-cracks and their interaction, (2) application of directional cohesive elements to infer overall delamination direction, and (3) use of delamination direction and stress state at the delamination front to determine migration onset. Overall, good agreement was obtained between simulations and experiments. However, the validation exercise revealed the strong dependence of the simulation of matrix-crack/delamination interaction on the strength data (in this case transverse interlaminar strength, YT) used within the cohesive zone approach applied in this work. This strength value, YT, is itself dependent on the test geometry from which the strength measurement is taken. Thus, choosing an appropriate strength value becomes an ad-hoc step. As a consequence, further work is needed to adequately characterize and assess the accuracy and adequacy of cohesive zone approaches to model small crack growth and crack onset. Additionally, often when simulating damage progression with cohesive zone elements, the strength is lowered while keeping the fracture toughness constant to enable the use of coarser meshes. Results from the present study suggest that this approach is not recommended for any problem involving crack initiation, small crack growth or multiple crack interaction.
Effect of Cooldown Induced Fiber/Matrix Interfacial Disbond on Transverse Failure in Composites
Despite efforts to improve the overall quality of composite materials, the presence of fiber/matrix interfacial defects seems inevitable. In composites with high fiber volume fraction, the small inter-fiber spacing can lead to development of high tensile radial stress at the fiber/matrix interfaces on cooldown from a high cure temperature. This stress can cause failure from defects at the interfaces earlier before any mechanical loads are applied. In the present paper, we study further progression of cracking from a preexisting disbond (debonding crack) that has been formed by thermal cooldown on remotely applying transverse tension to the composite. In the finite element model, a local region of hexagonally packed fibers embedded in a homogenized composite is analyzed. The cooldown induced disbond is assumed to initiate at the location where tensile radial stress resulting from cooldown is the highest. Energy release rate of the debonding crack is calculated by the Virtual Crack Closure Technique (VCCT). Upon loading, it is found that the debonding crack tends to grow towards the symmetry plane normal to the loading direction. Furthermore, this crack is found not to kink out of the interface until it has fully propagated past the symmetry plane. As a result, further growth of the cooldown induced disbond as well as the potential kinking process are found to be the same as when the disbond initiates due to applied transverse tension.
Void Modelling and Virtual Testing of Prepreg Materials from 3D Image Capture
Voids remain the most prolific manufacturing defect, and while the reduction in mechanical performance due to voids is well understood, the initial size, distribution, and evolution of voids during the manufacturing process is not well understood. In this study, the ply-ply interface of a laminate was replaced by a ply-glass interface and a high-resolution surface scanner was used to capture the surface topology of prepreg samples during the manufacturing processing. The size, shape, and depth of voids were captured and compared to a numerical method used to describe the evolution of the voids throughout processing. The model captures the void reduction trend during heating but some disparity remains in the final void size. The final experimentally measured void shape was used to predict the fracture toughness performance of the laminate. A finite element model was constructed from the surface images and the crack growth behavior was investigated using a multi-scale model of a double cantilever beam test. The fracture toughness and the stick-slip crack growth behavior of the samples were captured by the model. The continuing aim of this work is to advance both the experimental characterisation techniques and modelling capabilities in-order to deploy virtual performance testing from process simulation.
Reduction of Voids in VARTM Composites by Magnetic Compaction of Preforms before Infusion
The quality and performance of composites fabricated by vacuum assisted resin transfer molding (VARTM) are primarily characterized by their void content, often induced by the resin flow during impregnation. In this study, the effect of compacting 18-ply random mat and plain weave fabrics in a vacuum bag before the infusion was investigated as a possible low-cost method to substantially reduce or eliminate the process-induced voids in VARTM. The compaction of the fabrics was achieved by applying a set of Neodymium Iron Boron (NdFeB) permanent magnets on a vacuum bag lay-up. Laminates without magnetic compaction were also fabricated using identical parameters to demonstrate the effectiveness of this method. The impregnation of the compacted fabrics became much slower and took almost twice as long compared to the uncompacted parts. The final thickness of random mat and plain weave reinforced epoxy laminates did not change and remained at approximately 4 mm and 2.7 mm, respectively. The measurements indicate a relaxation of the compressed mat as the resin front progressed along the lay-up. Most interestingly, however, the void volume fractions of the compacted random mat and plain weave laminates were reduced to the very low 0.90% and 0.99%, respectively, from the relatively high 5.7% and 2.7% level observed for the uncompacted laminates. Finally, the flexural strength and modulus of the compacted random mat and plain weave laminates slightly improved compared to those manufactured by traditional VARTM. For instance, the flexural modulus of compacted woven/epoxy laminates is almost 6% higher than that of the uncompacted laminates.
Inverse Modeling of Compressive Response in Presence of Defects
In this study we look at defect induced compressive strength reduction in thick unidirectional carbon composite material. Test results from controlled defect size coupons are used to guide the study. A numerical model was developed to replicate the tested specimens and validated against the test results. Using the Latin Hypercube Sampling method random input parameters were generated to study the correlation of input parameters to the compressive response.
Effect of Interfacial Defects on Mechanical and Electrical Properties of Composite Materials Fatigue
Formation of defects during the manufacturing, adhesive bonding, and repair process can be very detrimental to the performance of a composite structure. Specifically, interfacial defects can severely affect the durability of fiber reinforced laminated composite materials and their joints. Despite progress in detection of defects (micro-scale void and micro-to-macro scale disbond) using popular nondestructive methods, significant challenges still remain in quantifying the distribution of defects and the resulting loss of properties. The objective of this study is to explore the effects of interfacial defects on the strength of adhesively bonded lap joint made of laminated composite substrate and epoxy adhesive. Nano-micro-scale interfacial defects (surface features and impurities) can cause a weak joint called â€œKissing bondâ€ where the substrates may remain in contact, but unable to provide the required bond strength. This is often difficult to characterize and a major concern for reliability. The proposed concept relies on the fact that heterogeneity at the material interfaces changes charge polarization inside a material when subjected to an AC electric field and this fundamental material behavior can be quantified using Broadband Dielectric Spectroscopy (BbDS) technique. Both bulk and spatial distribution of dielectric properties can be measured nondestructively to represent the state of weak interface. Surface contamination which causes weak bond can be identified prior to bonding and hence potentially allowing for corrective measure. Results show that changes in dielectric properties (e.g., complex impedance, permittivity, capacity) can be utilized as a quantitative measure of the residual or remaining strength of a weak joint under tensile loading.
The Combined Effect of Fiber Distribution Irregularities and Matrix Voids on Failure Initiation in Polymer Matrix Composites
The aim of the current work is to study damage initiation and propagation in fiber- reinforced polymeric composites. A systematic analysis is carried out to analyze the effects of manufacturing induced defects such as random distribution of fibers and presence of voids in matrix on the damage initiation in unidirectional composites under transverse tension. This study focuses on the resin infusion process and the defects hence formed. Upon infusing resin, the initial fiber configuration undergoes perturbation and results in a random distribution with regions of resin rich areas and fiber clusters. In addition, micro-voids (between the fibers in a bundle) and macro- voids (between fiber bundles) are formed. Representative Volume Elements (RVEs) are constructed to capture essential fea- tures of the composite microstructure that determine the local stress fields and hence damage initiation and propagation. Stress analysis of the RVEs is conducted using Abaqus FEA software. A novel methodology is put forward to generate random dis- tributions of fibers that would simulate different levels of perturbations of the fibers from initial (dry bundle) positions during the manufacturing process resulting in dif- ferent configurations of fiber clusters. An embedded RVE approach has been adopted in a finite element model to calculate the stress fields to avoid artificial effects of the RVE boundary. Damage initiation is then monitored using a previously proposed energy based criterion for cavitation induced fiber/matrix debonding in polymers. Subsequent damage in the form of crack formation by coalescence of the debonding cracks is also analyzed by the energy based approach.
Study of Defect Coalescence in Heterogeneous Material Systems Using Broad Band Dielectric Spectroscopy
Composite materials are essential for many modern applications, including airplanes and cars, energy conversion and storage devices, medical prosthetics, and civil structures. Detecting the initiation, growth, accumulation, and coalescence of micro-damage in these heterogeneous materials and predicting the onset of component failure using conformal Broadband Dielectric Spectroscopy (BbDS) is a promising area of ongoing research. Recently, the authors have developed the critical path concept and Heterogeneous fracture mechanics concept that depicts the effect of defect nucleation, growth, coalescence, and fracture plane development and correlation of these damage mechanisms to change in dielectric response respectively. Current research applies those concepts to detect the damage mode and the critical fracture path (conduction path) that leads to eventual failure. Also, the use of BBDS to detect the weak adhesion bonding is proposed, where the change in dielectric properties as a function of the frequency of the applied potential through the thickness of the sample is measured in the vicinity of the weak region. It is observed that at low frequency the gradient in the potential indicates a spike which marks the charge concentration around the imperfect region.
ONR High Fidelity Database and Validation Protocols for Structural Failure Mode Characterizationâ€”Static Open Hole Tension and Compression
The objective of the High Fidelity Database and Validation Protocols for Structural Failure Mode Characterization project sponsored by ONR (N00014-14-C- 0128) is to provide the required material characterization data and associated guidelines to effectively benchmark computational analysis methods. Open hole tension (OHT) and open hole compression (OHC) specimens comprising layups of hard, medium, and soft configurations were tested in room temperature dry (RTD) conditions to threshold loadings. In-situ and post-test inspection techniques were used to characterize and chronicle the developing internal damage state, and the results were compiled to create unique damage maps by which computational analysis techniques may be benchmarked. The purpose of this paper is to present a selection of the program damage maps and computational analysis predictions for OHT and OHC at RTD. Analysis was performed using the Regularized Extended Finite Element Method. Advanced inspection using in-situ radiography and post-test computed tomography identified a key splitting mode associated with the 0o embedded ply. The strain relief mechanism was found to be a significant physical point of comparison between method prediction accuracy and formulated capability based on analysis method class.
Application of In-situ Computed Tomography for Validation of Open Hole Fatigue Model Predictions
The objective of the High Fidelity Database and Validation Protocols for Structural Failure Mode Characterization project sponsored by ONR (N00014-14-C-0128) is to provide the required material characterization data and associated guidelines to effectively benchmark computational analysis methods. The development of fatigue damage within composite specimens was characterized using in-situ computed tomography to capture damage initiation and propagation on a layer-by-layer basis. Specimens (24-ply, quasi-isotropic) were tested in 50,000 cycle blocks of OHTT (R=0.1) up until 250,000 cycles from 20-90% of ultimate static strength. DIC and insitu CT scanning were performed to capture residual stiffness and internal damage state associated with increasing cycle number. The interaction of in-plane ply damage and delamination modes was observed to most severely develop for fatigue loadings above 60% UTS at 50,000 cycles. Once reaching runnout at 250,000 cycles, the specimens were subsequently tested in quasi-static tension to ultimate (two-piece) failure and the residual stiffness and strength were recorded. Notably, it was determined that a correlation exists between increasing damage state and increasing residual strength. From inspection, the 0o embedded plies exhibited large splits at the hole edges that increased in length as the loading cycles increased. Two specimens were additionally tested to ultimate two-piece failure in fatigue at 90% of ultimate load to estimate a percent of life for the specimens tested using in-situ CT. The number of cycles to failure were 727,495 and 842,298, which indicates that the earlystage matrix-dominated fatigue damage in the laminates was captured.
ONR High Fidelity Database and Validation Protocols for Structural Failure Mode Characterizationâ€”Program Highlights
Computational progressive damage and failure analysis (PDFA) methods for composite structural applications offer a significant pathway towards reduced reliance on structural testing. Industry, universities, government, and software vendors are developing improved PDFA method targeted towards improving the predictive capacity for composite structural performance. Thus, an increasing number of analysis tool are available. However, the assessment of technical maturity of the tool is left to the end user. This assessmentrequires a high level expertise in the methods and tools. Additionally, establishing technical maturity requires identification and characterization of the individual failure modes relative to a particular application, which necessitates advanced test and inspection to provide benchmarking data. Currently, no standard verification and validation guidelines exist for composite PDFA methods, nor does the supporting high fidelity test and inspection database. The High Fidelity Database and Validation Protocols for Structural Failure Mode Characterization project sponsored by ONR (N00014-14-C-0128) is filling the gaps by developing the necessary validation protocols with a supporting test database that can be used by PDFA methods developers and end-users. A series of tests were designed to capture individual failure modes as well as failure mode interactions in typical structural laminates (IM7/5320-1 tape). The program details and preliminary findings are presented in this paper.
An Efficient Material Model to Simulate the Effects of z-pin Reinforcement
Interlaminar delamination has been identified as a dominant damage mode for laminated composites subjected to impact. Even small impact energies can cause a major drop in residual strength. In order to overcome delamination, through the thickness reinforcement (TTR), such as z-pinning and stitching, have been used frequently and have shown substantial improvements in delamination resistance and residual strength of laminated composites. Modeling the impact experiments using finite element simulations would be a useful tool to predict performance of z-pinned composites. Considering the small size of individual z-pins (less than 1 mm) relative to the size of the laminate, modeling detailed geometric features of z-pins is computationally expensive. In this work, a two-stage strategy for efficient simulation of pin reinforcement is proposed. In the first stage, a small scale, detailed single pin geometric model is analyzed to capture different failure modes of the pin. Results from this detailed simulation are then used to develop an equivalent material model for z-pin reinforcement using discrete cohesive surfaces. This eliminates the need for detailed computational models including the pin geometry. This method is implemented to estimate the load-displacement response and delamination area of a laminates subjected to impact. The comparison with experimentally observed results shows good correlation.
Machine Learning for Acoustic Emission Signatures in Composite Laminates
Development of new composite material systems and structures require detailed understanding of failure mechanisms and failure progression. The identification of damage initiation on a damage-mode basis is essential for validation of progressive damage analysis techniques. Damage progression is the industry pathway towards expanding the composite design space to limit weight and increase confidence in structural performance. The challenge in validating these techniques is that the initial damage events predicted by analysis occur at a scale that are smaller than the defects measured by standard laboratory level inspection techniques (x-ray, ultrasound, digital image correlation). Of the techniques, acoustic emission has been found to be one of the most sensitive methods for identifying the onset of damage. However, characterizing the nature of the damage event from the acoustic emission signal has been challenge for many years. This paper describes a method for classifying acoustic emission signals that applies machine learning to a feature called the frequency distribution function. The method is demonstrated on composite laminates specimens and compared to observations from in-situ testing inspection techniques.
Impact and CSAI Analysis and Validation Using High Fidelity Test and Inspection Data
Currently, Progressive Damage and Failure Analysis (PDFA) methods are being developed to simulate the damage state as a result of impact and predict the residual strength of composite laminates. To date, these methods have shown success in predicting either the damage state or residual strength, but generally not both. The refinement of the PDFA methods to achieve success in both is limited due to availability of relevant test data with the resolution required to characterize damage state as a result of impact. In order to fill this gap the High Fidelity Database and Validation Protocols for Structural Failure Mode Characterization project sponsored by ONR (N00014-14-C-0128) is developing an extensive set of data to benchmark Compression Strength After Impact (CSAI) predictions using PDFA methods. The database provides high fidelity experimental and post-test inspection for a family of common laminates used in structural applications. The following laminates (10/80/10, 25/50/25, and 50/40/10) of IM7/5320-1 were impacted in accordance with the ASTM D7136 standard at 175 and 350 in-lb. During impact, high speed 3D Digital Image Correlation (DIC) was recorded of the back face deformation to determine the time-displacement history, deflection profile, and the onset of first ply cracking for two impact energies. At the completion of the test, the permanent deformation and damage state were measured by advanced Computed Tomography (CT) providing a ply-by-ply view of damage from the impact event. In this paper, the application of high fidelity experimental and posttest inspection data is demonstrated with a PDFA method.
DIC Data Driven Methods to Verify Simplifying Assumptions and Increase Confidence in Material Properties of Laminated Composites
Over the last six years, the University of Texas Arlington Advanced Materials and Structures Lab (AMSL) has been developing a number of Digital Image Correlationbased and computed tomography-based advanced measurement techniques for material characterization as well as effective computational analysis methods to verify simplifying assumptions and increase confidence in material allowables for laminated composites [1]. DIC data-driven methods developed at AMSL include (1) Short-Beam Shear (SBS) Method, and (2) Small- Plate Twist (SPT) Method â€“ simultaneously measuring nonlinear shear material properties in the 1-2, 1-3, and 2-3 material planes. The methodology to generate nonlinear stress-strain response has been objective enough so one can start from measuring the strain and calculating the stress independent of the strain measurement and use iterative FEM-based stress Updating (FEMU) after the initial geometric stress approximation to generate the material stress-strain relations free of ad hoc assumptions. The DIC data-driven modeling methodology developed at AMSL has been verified by Boeing under the High-Fidelity Experimental and Analytical Characterization of Input Properties for Progressive Damage Analysis Methods. In particular, lamina shear stress-strain curves have been generated in this work using SPT and SBS methods for a material that has no basis for a transverse isotropic approximation. SBS/SPT testing relying on DIC resulted in excellent-quality nonlinear stress-strain curves for 1-2 in-plane shear, and 1-3 as well as 2-3 interlaminar shear (ILS) behavior. It is worth noting that to this date SPT has been the only method to fully measure 2-3 ILS stress-strain curves including the nonlinear near-strength regime. This work, a collaborative effort between AMSL and the Boeing Company, presents some of the most recent results and ongoing activities related to DIC datadriven modeling.
Effects of Hybridization on the Impact Performance of Flax-carbon Fiber Composites
This paper presents an experimental investigation of the effect of interlayer hybrid fiber dispersion on the low velocity impact response of the carbon-flax/epoxy hybrid laminates at low carbon volume fractions. Five different hybrid laminates with similar carbon to flax weight ratio but varying stacking sequences were fabricated. Three impact energy levels were selected to produce a non-perforated and a perforated damage state in the hybrid laminates for study. The results showed that the stacking sequence with all the stiffer carbon plies at the impact face does not necessarily lead to enhancement of the impact properties of the hybrid composite. However, shifting the carbon plies to the back face of the plate led to a significant improvement in the damage initiation and the energy absorption behavior compared to the non-hybrid flax composite with similar thickness.
High Fidelity Simulation of Low Velocity Impact Behavior of CFRP Laminate
In this paper, finite element model which simulates damage extent of CFRP laminate subjected to low velocity impact (LVI) is proposed. It is well-known that face-on LVI against the CFRP laminate can significantly degrade the material performance, although it leaves only barely visible impact damage (BVID). Hence, it is quite important to precisely predict damage of CFRP laminate caused by LVI. The validity of the model is demonstrated by comparing experimental and numerical results. Experimental impact response (i.e. load-history curve) and damage extent of CFRP laminate were obtained from drop-weight test and non-destructive inspection (such as C-scan, radiograph and X-ray CT), respectively. Numerical results were obtained from finite element (FE) analysis done on Abaqus/Explicit 2016, performed using JAXAâ€™s supercomputer system, â€œJSS2â€. Proposed model in this paper was implemented using user subroutine VUMAT. The FE model of the CFRP laminate is lamina-level, therefore a laminate is divided into each lamina, and lamina is divided into continuum shell elements. In the present model, each damage mode is modeled in each manner. Fiber damage is modeled by smeared crack model (SCM). In-plane ply cracks are modeled by the enhanced continuum damage mechanics (ECDM) model, which is composed of continuum damage mechanics (CDM) and SCM. Pre-peak nonlinear behavior caused by micro cracks, and post-peak softening behavior caused by matrix cracks are modeled by CDM and SCM, respectively. Delamination between laminae is modeled by cohesive behavior based on the contact formulation. The numerical results obtained from the present model show good agreement with experimental results. Regarding the delamination shape and its size, numerical results and experimental results agree very well. The validity of the present model was demonstrated.
Impact Performance and Damage Behavior of Composite Sandwich Structures in Arctic Condition
In this work, the impact behavior of composite sandwich structures at low temperature condition is investigated and explored. Composite sandwich structures are commonly used in naval vessel constructions, due to their light-weight and high bending stiffness. The state-of-the-art Instron CEAST impact machine, equipped with an environmental chamber, is employed to perform impact testing on composite sandwich panels at low temperature range. The objective of this work is to study how low temperature affects the impact behavior and damage response of composite sandwich structures, with a longer overarching aim to develop new sandwich construction with enhanced impact damage tolerance for future naval applications in arctic environment. Results from this study reveal that composite sandwich structures experienced greater damage when impacted at low temperature (-70Â°C) compared to the same specimens impacted under same test condition at normal room temperature (23Â°C). The extent of damage is clearly evident by post-mortem observation. Impact history curves are also analyzed to understand the dynamic behavior of composite sandwich structures during the impact event. Force-time graphs indicate a lower peak force for low temperature specimens. This implies that the maximum strength of the composite sandwich structure is significantly reduced and it is easier to cause damage at low temperature condition. Force-time and energy-time data also delineate the roles played by energy absorption of the carbon/fiber epoxy face sheet and the PVC foam core during the impact event. At low temperature, the face sheet absorbs less energy, due to reduced material strength; while the foam core absorbs most of the energy by foam crushing mechanism. At normal room temperature, the face sheet absorbs considerable amount of energy before failure and the foam core absorbs the remaining energy with lesser extent of damage.
Implementation of a Tabulated Failure Model into a Generalized Composite Material Model Suitable for Use in Impact Problems
The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites under impact conditions is becoming critical as these materials are gaining increased use in the aerospace and automotive communities. The aerospace community has identified several key capabilities which are currently lacking in the available material models in commercial transient dynamic finite element codes. To attempt to improve the predictive capability of composite impact simulations, a next generation material model is being developed for incorporation within the commercial transient dynamic finite element code LSDYNA. The material model, which incorporates plasticity, damage and failure, utilizes experimentally based tabulated input to define the evolution of plasticity and damage and the initiation of failure as opposed to specifying discrete input parameters such as modulus and strength. The plasticity portion of the orthotropic, threedimensional, macroscopic composite constitutive model is based on an extension of the Tsai-Wu composite failure model into a generalized yield function with a nonassociative flow rule. For the damage model, a strain equivalent formulation is used to allow for the uncoupling of the deformation and damage analyses. For the failure model, a tabulated approach is utilized in which a stress or strain based invariant is defined as a function of the location of the current stress state in stress space to define the initiation of failure. Failure surfaces can be defined with any arbitrary shape, unlike traditional failure models where the mathematical functions used to define the failure surface impose a specific shape on the failure surface. In the current paper, the complete development of the failure model is described and the generation of a tabulated failure surface for a representative composite material is discussed.
Enhancement of Impact Performance of AS4/3501-6 Carbon/Epoxy Composite Laminates by Nylon-66 Nanofiber
A 32 ply quasi-isotropic AS4/3501-6 carbon/ epoxy base and three interleaved laminates with Nylon-66 nanofibers mats of areal density 0.5, 1.5, and 2.5 g/m2, that translate to 0.23, 0.69, and 1.15% increase in ply weight were impact tested. Impact heights ranged from 50 to 260 mm, which resulted in impact energy range from 2.82 to 14.67 J. The threshold impact force (PTh) for the 0.23, 0.69, and 1.15% interleaved laminates increased by 8, 42, and 45%, respectively. The threshold impact energy (ETh) increased by 16, 89, and 97% for the same three interleaved laminates. Then the damage growth rate for the three interleaved laminates was reduced by 11, 32, and 48%, respectively. Compression after impact test (CAI) results showed that the compression strength reduction rate was reduced by 0.3, 7, and 34% for the three interleaved laminates compared to the base laminate.
High Speed Thermal Imaging on Ballistic Impact of Triaxially Braided Composites
Ballistic impact experiments were performed on triaxially braided polymer matrix composites to study the heat generated in the material due to projectile velocity and penetration damage. Quantifying the heat generation phenomenon is crucial for attaining a better understanding of composite behavior and failure under impact loading. The knowledge gained can also be used to improve physics-based models which can numerically simulate impact of composites. Triaxially braided (0/+60/-60) composite panels were manufactured with T700S standard modulus carbon fiber and two epoxy resins. The PR520 (toughened) and 3502 (untoughened) resin systems were used to make different panels to study the effects of resin properties on temperature rise. Ballistic impact tests were conducted on these composite panels using a gas gun, and different projectile velocities were applied to study the effect on the temperature results. Temperature contours were obtained from the rear surface of the panel during the test through a high speed, infrared (IR) thermal imaging system. The contours show that high temperatures were locally generated and more pronounced along the axial tows for the T700S/PR520 composite specimens; whereas, tests performed on T700S/3502 composite panels using similar impact velocities demonstrated a widespread area of lower temperature rises. Nondestructive, ultrasonic C-scan analyses were performed to observe and verify the failure patterns in the impacted panels. Overall, the impact experimentation showed temperatures exceeding 525 K (485Â°F) in both composites which is well above the respective glass transition temperatures for the polymer constituents. This expresses the need for further high strain rate testing and measurement of the temperature and deformation fields to fully understand the complex behavior and failure of the material in order to improve the confidence in designing aerospace components with these materials.
Impact Response of a Randomly Oriented Fiber Foam Core Sandwich Panel
It has been shown that glass fiber reinforced polyurethane foam (GRPUF) can haveimproved mechanical properties under specific strain rates and fiber volumes. But its performance as a core in a composite sandwich structure has not been fully investigated. This study explored different manufacturing techniques for the GRPUF using existing literature as a guideline. Its mechanical properties were determined using a three-point bend test and compared to unreinforced polyurethane foam. Flexural bend test showed that introducing chopped glass fibers made the GRPUF samples stronger yet more brittle. While quasi-static compression tests showed a decreased in performance with GRPUF. Both results can be attributed, in part, to inconsistent densities and void content resulting from the manufacturing process. Future work will include integration of the GRPUF in a foam core sandwich panelto investigate its response to low and high velocity impact of up to 103 s-1 strain rates. The ultimate goal is to explore a cost-effective alternative to existing, cost prohibitive, composite sandwich structures.
Visualization of Fiber/Matrix Interfacial Shear Debonding Mechanism at High Rate Loading
The interfacial properties of fiber reinforced composites are widely studied due to their importance in controlling the desired material strength. However, the effects of loading rates on interfacial crack initiation and propagation at the interfaces are yet to be studied systematically. Thus, visualization of the debonding event in real time facilitates the study on the dynamic crack initiation and propagation mechanisms at the fiber-matrix interface. In this study, the debonding between the fiber and the matrix is recorded by synchronizing a modified tension Kolsky bar with high-speed synchrotron X-ray phase contrast imaging (PCI) technique. The microbond and pullout methods were utilized to study the interfacial shear debonding mechanism of S-2 glass fiber and carbon fiber composite Z-pin with SC-15 epoxy at pull-out velocities of 2.5 and 5.0 m/s. Both S-2 glass fiber and Z-pin at 2.5 and 5.0 m/s experienced a catastrophic interfacial debonding. Z-pins revealed relatively high peak debonding forces compared to S-2 glass fiber at both velocities. Furthermore, the peak debonding forces for both materials were higher for higher velocity.
Isogeometric Analysis for the Prediction of Damage and Residual-Strength in Laminated Composite Structures
The concept of the Isogeometric Analysis (IGA) is adopted in the framework of structural analysis, where the Kirchhoff-Love theory is used for the development of efficient shell elements for the simulation of full scale components. The proposed multi-layer modeling approach consists of the representation of composite laminates at the level of individual plies, or group of similar plies, connected through cohesive interfaces. The novel cohesive interface formulation, which is specifically developed to be used in combination with thin shell elements, is equipped with a Mixed-Mode Cohesive Model (MMCM) in order to predict the onset and the propagation of the delamination. The interlaminar damage, such as matrix cracking, fiber breaking and pullout, are modeled at the lamina level by degrading the pristine elastic properties of the material. This methodology introduces mesh-dependency and damage localization. A nonlocal damage model is therefore investigated in order to alleviate the dependency of the solution with respect to the adopted discretization. The formulation is validated through the correlation with an experimental test from the literature. The model aims to simulate the damage caused by a low-velocity impact on a flat composite coupon. The richness of the isogeometric design-toanalysis concept is then illustrated. A commercial NURBS-based CAD software is used to create the multi-layer model of a reinforced composite panel typical of aerospace structures. The IGA analysis framework is then used to predict the damage caused by a mid-energy impact. It is found that the higher-order continuity of the NURBS shape functions translates into a better representation of the strains, which drive the intralaminar damage model, and of the displacement jumps in the area of the delamination front, which drive the MMCM. Therefore, IGA simulations remain stable and accurate at low number of DOFs even when severe intralaminar damage and delamination occurs.
Computed Tomography Informed Composite Damage State Model Generation
During an impact event on composite structures a complex damage state is created involving matrix cracking, delamination, and fiber damage. For multi-directional composite laminates, conventional Non-Destructive Evaluation (NDE) techniques such as Ultrasonic C-Scan are generally limited to characterizing damage as a projected overall envelope (through-transmission), or as a series of damage planes that are partially hidden by planes closer to the transducer (one-sided pulse-echo mode). Alternatively, X-ray Computed Tomography (CT) imaging provides non-destructive high fidelity damage information that accurately displays layer-by-layer damage states and matrix damage in a way that can be made compatible with implementation into high fidelity modeling. A method has been developed to convert detailed 3D damage state information into composite relevant delamination, matrix cracking and fiber damage model representation. Based on geometric information, delamination information can be implemented into a computational model by excluding points within the delaminated zone from the initial bonding set in a ply-by-ply model, Matrix damage can be implemented as element sets with degraded initial matrix properties, and fiber breakage can be implemented as element sets with degraded fiber properties. Composite specimens subjected to low velocity metal tip impact were CT imaged and the resulting data were converted to model relevant data. Potential implementable data includes delamination, matrix damage, and measured indentation profile.
Electrical Resistance-Based Damage Sensing of Carbon Fiber-Reinforced Composites Using Simulation-Based Low-Velocity Impact Damage
In this paper, finite element models were developed in ABAQUS for studying the influence of simulation-based low-velocity impact damage on the 4-probe electrical resistance of carbon fiber-reinforced polymer matrix (CFRP) laminates. The specimens in the current study were 16- and 32-ply AS4/3501-6 laminates with quasi-isotropic layup. Electrical resistance in the CFRP laminates was evaluated using a 4-step procedure. First, FE models were created in ABAQUS for simulating low-velocity impact using a quasi-static loading approach. Second, the stress results were imported into Matlab, and delamination analysis was performed using approach developed by de Moura and Marques [1]. The delamination predictions were plotted onto a discretized grid for each interface between plies with dissimilar fiber orientations. Third, 4-probe electrical FE models were developed in ABAQUS for specimens before and after impact using the concept of effective conducting thickness. The electrical conductivity was reduced in the through-thickness direction for damaged elements in order to represent the impact-induced delamination. Forth, a direct current of 10mA was applied to the electrical models in order to determine the influence of the simulation-based lowvelocity impact delamination on 4-probe electrical resistance using top and oblique measurement planes. For the top measurement plane, source and sensing electrodes were placed in a single line on the top surface of the specimen. For the oblique measurement plane, positive source and sensing electrodes were placed on the top surface, whereas the negative source and sensing electrodes were located on the bottom surface of the specimen. Generally, the oblique resistance measurement was more sensitive to the impact damage compared to the top resistance measurement. In addition, the resistance of the 16-ply specimens was more greatly affected by the delamination compared to the 32-ply specimens. Electrical resistance predictions were compared to the experimental data.
Energy Absorbing Damage Mechanisms in Progressive Crushing of Composite Tubes
Progressive crush is an important mechanism by which the kinetic energy associated with an impact event is dissipated. Unfortunately, the mechanisms governing the progressive-crush response of advanced composite materials are not well understood. Also, many of these materials are known to exhibit responses that are sensitive to loading rate. Therefore, understanding the influence of impact velocity and laminate design on the crush response is critically important for modelling the crashworthiness of a given composite structure. Laminate design of composite structures can be tailored by placing the fiber in the direction of the applied load. For axial progressive crush the fiber path can play a different role by promoting certain failure or energy absorbing mechanisms. To quantify this by experimentation, tubes were chosen as the primary structural component and different geometries (circular and square cross-sections) and laminate architectures were then chosen with the goal being to initiate different damage mechanisms. Three different layups were designed so that their failure modes corresponded to the three broad classifications (fragmentation, fiber splaying, and brittle fracture) that are typically observed during axial crushing. Tests were conducted using a servo-hydraulic test machine in closed-loop mode at quasi-static rates (less than 100 mm/s) and in open-loop mode for dynamic rates (up to 6000 mm/s). High speed video was used during testing, in conjunction with postmortem inspection, to identify the damage mechanisms associated with the different test configurations. Results showed that circular cross sections were more efficient than square cross sections when comparing similar test variables. Quantitative
Impact Response in Polymer Composites from Embedded Optical Fibers
This study investigates the feasibility of using embedded optical fibers in polymer matrix composite laminates to characterize delaminations caused by low-velocity impacts with energies between 30 J and 50 J. Impact damage can occur in composite structures during manufacture, in-service, and during maintenance. Because of their small size and light weight, optical fibers can be embedded in composite structures during the manufacture of composite parts, allowing the structure to be monitored for impact induced delaminations without being removed from service. In this study, optical fibers are embedded in a grid configuration at four selected locations (one-third from impact surface, midplane, two-thirds from impact surface, and farthest ply from impact) in thick autoclave-cured graphite/epoxy laminates. Low velocity impact testing is performed at four energy levels. Manufacturing procedures for embedding the optical fibers within the composite laminates are investigated. The strain distribution from the optical fibers is correlated with ultrasonic C-scans of the laminates in which they are embedded. Results indicate that embedded optical fibers can provide post-impact strain responses and delamination area from each embedded site within the impacted laminates.
Experimental Study on High-velocity Impact Damage Behavior of Carbon Fiber Reinforced Composite Laminates
Carbon fiber reinforced composite laminates are widely used in the field of aerospace. Service environment of all kinds of aircraft makes the airframe vulnerable to impact loads. In this paper, the damage behavior of laminates under impact loading was studied by experimental method of gas gun impact tests. Taking into consideration the different impact conditions that include attitudes, shapes, velocity and others of the projectile during the tests, we summarized the basic failure process and typical damage modes. The main mechanism of the different fracture modes formed in the period of early penetration was obtained by comparing the damage morphology. Through this work, we will achieve understanding of the interaction between shape of projectile and damage behavior, which is of great importance for the simplification of analysis methods for impact problems.
Crack Propagation from a Circular Defect in a Unidirectional CFRP Composite under Dynamic Tension
A single-ply unidirectional IM7/8552 carbon fiber reinforced plastic (CFRP) material with artificially introduced circular defects is subjected to dynamic tensile loading using a modified Kolsky tension bar. A high-speed X-ray phase contrast imaging method is integrated with the Kolsky bar setup to record the crack initiation from the defects and subsequent propagation in the material in real time during the tensile loading. The tensile loading was applied along the fiber direction (0Â°) of the specimens. Shear failure of the matrix and axial splitting along the loading/fiber direction (0Â°) were observed to initiate from the edge of the artificial circular defects. The dynamic tensile loading history during the crack propagation was recorded using a piezoelectric load cell.
Peridynamic Modeling of Impact-induced Delamination in Laminated Composites
Peridynamics (PD) is a nonlocal form of continuum mechanics theory. It has advantages in simulating damage and fracture problems. In this paper, a twodimensional bond-based peridynamic model has been extended into three-dimensional for laminated composites. In the modeling, both the critical bond stretch and critical bond compression have been used to describe the damage of the interface of laminates. The proposed PD model is then employed to study the failure of a laminated composite plate under out-of-plane impact loading. The matrix damage and delamination have been simulated in the plate with a good match with the experimental results.
Modeling the Impact Response of Woven Composites in Arctic Conditions
The objective of the research presented in this paper is to investigate the response of woven composites at room and arctic temperatures. Influence of repeated impact is also explored under these conditions. Next, a stratified macro-mechanical model within the finite element method (FEA) framework is developed that is capable of predicting the dynamic impact response of woven composites at low velocities at room and under arctic temperature. The linear elastic properties for the material are first determined by inverse calculation, by matching the the linear region of the force-time graph determined experimentally. Damaging capability in the inter laminar regions is introduced through cohesive laws. An uniform temperature profile on the material is applied to simulate the extended exposure of low temperature on the composite, followed by impact loading to determine the impact response and extent of damage in the material.
Visualization of PBX Response to Impact Loading
The safety and reliability of energetic materials (EM) is of great importance in many industries including mining, oil drilling, and weapons. The understanding of the critical loading conditions and mechanisms that drive the detonation of EM is crucial to ensuring that they are consistently safe and reliable. In this study impact experiments were performed on Plastic-Bonded Explosive (PBX) samples, which are composites consisting of HMX (cyclotetramethylene tetranitramine) crystals inside a Sylgard 184 binder, using a single stage, smooth bore, light gas gun. Impact velocities varied, but were around 420 m/s. High-speed x-ray phase contrast videos were recorded during the experiments and then analyzed to characterize crystal behavior under the given experimental conditions. The videos showed two main responses the crystals exhibited when impacted. These are fracture and expansion. The crystal expansion is thought to be the result of chemical reaction in the crystal. The samples do not have enough mass or length to reach detonation, but hot spot formation and chemical reaction are still possible and of interest.
Peridynamic Modeling of Damage in Laminated Composites Reinforced with Z-pins Due to Low-velocity Impacts
In this study, a new approach for predicting damage and specific failure modes in laminated fiber reinforced composites is presented. The new method is based on the peridynamic theory and models individual plies, and represents fiber and matrix materials in each ply explicitly. These features enable analysis of laminates with arbitrary fiber orientation in a convenient manner. Additionally, a new failure mode identification algorithm has been developed and implemented. Instead of the conventional peridynamic damage parameter, the new algorithm works with individual broken bonds, which makes identification of different failure modes including matrix cracking, fiber breakage and delamination straight-forward and unambiguous. The new peridynamic approach is demonstrated by considering low velocity impact damage on composite laminates with and without translaminar reinforcements. The translaminar reinforcement technique considered in this study is z-pinning; two different geometric configurations of z-pins are explored. The impact testing and the post-impact non-destructive evaluations with ultrasonic c-scans are performed at the Air Force Research Laboratory to characterize of the delaminations. The impact tests on different samples are simulated using the current peridynamic approach. The predicted impact damages are compared against the experimental measurements. The new approach is shown to capture low velocity impact damage both quantitatively and qualitatively.
Matrix and Confinement Influence on the Dynamic Behavior of Fiberglass
Woven polymer matrix composites (woven PMCs) have the potential to offer mechanical and multifunctional improvements over traditional engineering materials. At the same time, the complex interplay between the fiber-matrix architecture during dynamic loading is not fully understood, and thus cannot be optimized for damageresistance in design. The focus of this work is to aid in understanding the role of the thermosetting matrix, and the influence of bounday conditions on damage mechanisms under acceleration-driven and high strain-rate loading events. To do so, we investigate two types of commercially available woven glass cloth composites with varying thermosetting resins under the conditions of dynamic compression at uniform high strain rate, and high-speed impact. The fiberglass variants of interest include commercially available Garolite (plain-weave) of either epoxy or melamine resin. Results indicate that under dynamic compression at nominally uniform strain rate, both types of composites exhibit an increase in strength over equivalent quasi-static loading conditions when unconfined, but can either increase or decrease compressive strength by adding bi-axial confinement, dictated by the orientation of the weave and ply layup with respect to the load when confined. Additionally, 0.8 mm thick fiberglass sheets impacted between 1 to 1.5 km/s with a 1.6 mm diameter nylon 6/6 sphere resulted in the melamine resin composite damage zone twice the size of the epoxy, with epoxy reaching its ballistic limit more readily than the melamine variant. It is suggested that the melamine matrix fiberglass composite acts more like a net, stretching out the damage over a larger area as the matrix breaks away from the weave, freeing the fibers to stretch during inelastic deformation and attenuating overall perforation.
MD-CF: A Molecular Model to Predict the Structure and Properties of PAN Based Carbon Fibers
Carbon fibers (CFs) are increasingly the reinforcement of choice for many highperformance composites, due to their high stiffness and strength, combined with their low density. However, while commercial fibers can approach the modulus of ideal graphite, high-strength fibers achieve no more than 10% of the ideal strength. Strength is limited by the presence of defects in the CF and thus, the development of highstrength fibers requires establishing relationships between processing, microstructure and properties. To contribute to the development of this knowledge base, we developed a molecular model, MD-CF, that uses a combination of kinetic Monte Carlo (kMC) and molecular dynamics (MD) to simulate the processes of carbonization and graphitization of PAN-based CFs; importantly, the molecular-level simulation predicts the development of microstructure during processing. The MD-CF method has been used previously to capture the cross-sectional microstructure and stiffness of carbon fibers and was shown to generate structures with experimentally observed features such as hairpins and graphitic sheets. In this paper, we extend this model to three dimensions to capture the fiber microstructure along the fiber axis, a key step towards the prediction of longitudinal modulus and strength. The model starts with ladder-like molecular chains that are broadly accepted to be the result of the stabilization of PAN precursors and uses kMC to describe the chemical reactions leading to the formation of graphitic sheets, and MD to describe the relaxation of the system during the process of cure. We find that the density of simulated structures ranges from 1.55 to 1.70 g/cc and is in excellent agreement with commercial PAN-based fibers. In addition, the predicted longitudinal moduli are in reasonable agreement with recent experiments. However, the transverse compressive moduli are an order of magnitude higher than experimental values, as a result of crosslinking in the transverse direction, preventing sliding between graphitic sheets.
Structural Evaluation of Complex Subcomponents Manufactured by Large Scale Extrusion Deposition of Carbon Fiber Reinforced ABS
The large scale extrusion deposition process has recently been commercialized in the form of additive manufacturing machines such as the BAAM from Cincinnati Inc., which enables layered production of chopped fiber reinforced thermoplastic polymer systems. The application of this process to the automotive market is being pursued by Local Motors to enable rapid design to market cycles and build on-demand capability for a wide range of vehicle styles from a distributed network of micro-factories. This endeavor will require an understanding of the structural performance of components with complex geometries, where the properties are known to vary as a function of printing conditions, as well as orientation and shape. In order to rapidly build a database of structural performance, a beam specimen with outer dimensions of 914 x 305 x 64 mm3 was selected to be subjected to bending, torsion, and shear loading with full field displacement measurement by digital image correlation (DIC). A total of 25 different specimen configurations were designed including variation of print geometry, reinforcement by foam filling, and overwrapping with advanced composites. This work documents the results of the study and presents the relative performance of the structures in addition to the approach to simulation that is being initiated to serve as a framework for efficient design of structures manufactured in this way. The results are presented as a table of structural efficiency including stiffness and strength as functions of mass for each design. Additional results will also be provided in the form of surface displacements from DIC; future work will utilize this data to better optimize simulations for predicting structural performance of articles manufactured by this process.
Tensile Properties of Carbon Fiber: Single Filament Vs Tow Based Testing
In the work presented here, single filament PAN carbon fibers (~7 micron diameter) are tested using a highly precise MTS Nano UTM tensile test machine. Current practice of ISO or ASTM methods involve testing tensile properties of carbon fibers using an infused tow of fibers. It is a current norm that carbon fiber companies report tensile strength and modulus (normal, intermediate, and high) based on measurements using full tow of fiber impregnated with some resin type, usually an epoxy system. For example, ASTM D4018 standard allows for the fiber tow to be collimated, impregnated with resin, and tensioned while keeping a fiber volume fraction in the range of 35% to 60%. This process can be completed two different ways: manually or automated. The manual method allows for the strand of a certain length and weight to be saturated in a resin bath and pulled through a die and attached to a tensioning rack. An automated setup for large volume applications typically involves passing the fiber tow through a resin bath, through multiple rollers (to help align the fiber) and wound on a tensioned spindle. After the strands are cured, they are tabbed using a standard gage length of ~150 mm. The present study considered how the mechanical properties measured using single carbon fibers translate to those obtained using a 24k tow based testing. Since tow based testing is largely dependent on morphology of the tow, interfacial shear strength between carbon fiber and resin, and the specific type of resin used, and relative tension of filaments within a tow, the motivation of this paper is to develop a fundamental understanding of translation of single carbon fiber mechanical properties to a resin reinforced composite tow. We are also developing a simple and repeatable test procedure for tow based properties without resin infusion. Weibull statistics of single filament based results are considered in the discussion.
Frontal Polymerization of Dicyclopentadiene
Fabrication of thermosetting polymers is restricted by the long cure cycles required for the full cross-linking of monomers. For example, high temperature performance epoxy resins require several hours of curing at elevated temperatures to obtain a robust polymer network. An oven or autoclave is typically used to provide the required energy for the polymerization, resulting in the bulk curing of the monomers. A promising alternative to the slow and long bulk curing is frontal polymerization, in which the monomers undergo a self-sustaining exothermic polymerization reaction once triggered by a thermal stimulus. Essentially, the monomers polymerize locally at the front of the exothermic reaction wave, leading to the rapid cure of the material. In this paper, we evaluate the frontal polymerization of the thermosetting polymer dicylcopentadiene under a variety of thermal triggering conditions. We show significantly faster cure times compared to typical epoxies as well as dramatically reduced energy required for curing.
Manufacturing-Informed Design for Injection Over-Molding
A fundamental tenant of materials engineering is the relationship between manufacturing, microstructure, and performance. Manufacturing simulation enables true performance prediction as well as tooling design and process design. This paper presents work completed on process simulation for an injection over-molded part emphasizing the need for manufacturing simulation to accurately predict performance. Accurate design for performance must consider the influence of manufacturing. Predictions of the injection over-molding process simulation for preform deformation, flow behavior, fiber orientations, effective properties, and dimensional stability are presented, and the implications on tooling and design process are discussed. Common concerns in injection over-molding manufacturing like weld lines, dimensional stability, interface temperatures, and air traps are all captured in the manufacturing simulations which will then inform design. The process simulation is connected to manufacturing and performance design, and a recommended process simulation workflow is presented.
Cure Monitoring and Characterization of Epoxy/Amine Networks, Modified with 1-Butyl-3-Bethylimidazolium Tetrafluoroborate Ionic Liquid
In the present work, electrochemical impedance spectroscopy (EIS) was employed for cure monitoring of bisphenol A/F based epoxy resin- polyoxypropylenediamine hardener systems modified with various concentrations of 1-butyl-3- methylimidazolium tetrafluoroborate ([BMIM]BF4) ionic liquid (IL). These systems exhibited phase separation induced by an increase in the molar mass through the curing of the resin. The progress of the curing process was accompanied by a steady increase of impedance for all types of samples, as expected, due to the increasing viscosity of the medium for increased degree of cure. The results were compared to those of unmodified resin-hardener systems. The sensitivity of EIS as a cure monitoring technique increased dramatically with the addition of the IL. The cure kinetics of the epoxy resin was characterized by differential scanning calorimetry and the data were used to create a model for comparison with the EIS results. The change of the ionic conductivity during curing as a function of IL concentration and the effect of IL loading on mechanical properties of the resin were also studied. We demonstrate that [BMIM]BF4 loading of 5wt.% increased the room temperature ionic conductivity of the fully cured product by an order of magnitude with only a 2.5% reduction in compression strength from that of pure epoxy.
Analyzing the Effect of Energy Dissipation on Thermo-Mechanical Responses of Viscoelastic Fiber Reinforced Composites Using Finite Element Method
Carbon fiber reinforced composites (CFRCs) are preferred materials used in the aerospace industry for high performance load carrying applications. The polymer matrix in CFRCs is a viscoelastic material and its mechanical properties vary with time, temperature and applied external loads. Experimental work suggests that polymers and polymeric composites, subjected to high-frequency cyclic loading, can generate enormous amount of heat from the energy dissipation which softens the polymer and accelerates failure. Current research efforts on the cyclic response in CFRC focus on understanding the macroscopic (overall) performance of composites, i.e., number of cycles to failure for a given frequency and loading amplitude. A systematic understanding on the formation of heat generation and its effect on the mechanical properties of the constituents in composites, and microscopic responses of composite is currently lacking. Changes in the micromechanical field variables (strain, stress, temperature) of CFRCs during cyclic loading can be crucial in understanding failure in composites. This study attempts to provide a detailed understanding on the effect of energy dissipation due to the viscoelastic nature of polymer on the overall mechanical responses of CFRP composites subjected to cyclic loading. Finite element (FE) analyses on the deformations of CFRP composites under various boundary conditions and loading histories are presented. A thermo-mechanical viscoelastic constitutive model is used for the polymer which is defined using a material subroutine, in ABAQUS FE code. In addition, voids are added to the CFRC models to account for manufacturing imperfections and their influence on the field variables and macroscopic behaviors are investigated.
Influence of Crack Distribution on the Responses of the Homogenized Unidirectional Composites
In this research, the influence of the number of cracks and the location of a crack on the responses of composites is studied. The representative unit cells are divided into two groups. In the first group, one crack, two cracks, three cracks, and five cracks, respectively, are implemented in individual RUC, respectively. In the second group, a crack is placed at different locations in four separate RUCs. Each RUC is subject to compression, tension in the x direction, compression, tension in the y direction, and shear deformation. The results show that cracks propagate in some RUC under tension and cracks propagate in some RUCs under compression. The interaction can be observed from the cracks that are in close distance. The results show the crack number and crack distribution has significant influence on the responses of crack evolution inside microstructures.
Comparative Analysis of the Elastic Behavior of a Lamina Containing Wavy Fibers
In this paper we review and compare some of the analytical methods which have been proposed to predict the elastic behavior of a lamina containing wavy fibers. The earliest attempt in this regard may be traced to the work of Tabaddor et al. [1] who provided an explicit, but approximate, stress-strain relation of a lamina containing a wavy fiber and subjected to uniaxial loading. Their derivation is based on a closed-form equation worked out earlier by Tarnopolskii et al. [2] for the average longitudinal Youngâ€™s modulus of a lamina with a sinusoidal fiber. A major discrepancy between the results in [1] and more recent ones obtained by more exact, albeit numerical, approaches has been observed. The source of this discrepancy is identified to be a faulty assumption in [1]. We also review briefly methods based on complementary strain energy which may incorporate material nonlinearities into the already geometrically nonlinear response of a lamina with a wavy fiber. Finally, a novel method based on a simple idea is briefly discussed. It yields within the framework of infinitesimal deformations, an exact and explicit relation between stress and strain.
The stress field is derived analytically for a three-phase elastic material in which a spherical inclusion is surrounded by another inclusion with different material properties outside which lies an unbounded medium subject to a uniform strain at infinity. The derived stress expressions are novel and in closed form which can supersede the work by Christenson and Lo. A computer algebra system is essential for the analysis without which it is not possible to handle the amount of algebra involved. The stress field inside the innermost inclusion can be used for calculating the effective elastic stiffness of composites. It is also possible to extend the analysis to n-phase (layered) materials.
Order-Reduced Solution of the Nonlinear High-Fidelity Generalized Method of Cells Micromechanics Relations
The High-Fidelity Generalized Method of Cells (HFGMC) is one technique for accurately simulating nonlinear composite material behavior. The HFGMC uses a higher-order approximation for the subcell displacement field that allows for a more accurate determination of the subcell stress/strain fields at the cost of some computational efficiency. In order to reduce computational costs associated with the solution of the ensuing system of simultaneous equations, the HFGMC global system of equations for doubly-periodic repeating unit cells with nonlinear constituents was reduced in size through the use of a Petrov-Galerkin-based Proper Orthogonal Decomposition order-reduction scheme. A number of cases were presented that address the computational feasibility of using order-reduction techniques to solve solid mechanics problems involving complex microstructures.
Randomization Algorithm for the Micromechanical Modeling of Fiber-reinforced Polymer Matrix Composites
The randomized nature of fiber orientations on the micromechanical level greatly affects the stress and strain distributions in composites which ultimately determines the initiation of different possible failure mechanisms. This paper presents a new algorithm for generating randomly packed Representative Unit Cells (RUCs) with high volume fractions to act as a preprocessor in Finite Element Analyses (FEA) for predicting mechanical properties and failure mechanisms in Polymer Matrix Composites (PMCs). The proposed algorithm first packs fibers within the RUC hexagonally, then, shifts them randomly through the domain while preventing overlap amongst neighboring fibers. Inputs of this algorithm are: fiber volume fraction, vf , fiber diameter, df , and total number of fibers, nf . Square RUCs of dimension LRUC are created in a time efficient manner.
Micromechanics Modeling of Unit Cells Using CUF Beam Models and the Mechanics of Structure Genome
A novel approach for the micromechanics analysis of composite structures is developed using refined beam models and the mechanics of structure genome (MSG). The MSG provides a tool to obtain the complete stiffness matrix of general composite materials by asymptotically minimizing the loss of information between the original heterogeneous body and the sought homogenized body. The constitutive information is in this manner extracted from the representative volume without the need of ad-hoc assumptions and in one single loading step. The local fields are then straightforwardly recovered using the same unknowns of the original homogenization problem, with no need of additional analyses. This work proposes the use of higher-order beam models based on the Carrera unified formulation (CUF) to solve the micromechanics problem by means of MSG. The fibers, or equivalent constituents, are discretized along the longitudinal direction with beam elements and the unknown variables are expanded over the remaining two local coordinates making use of Legendre-class polynomial sets, denoted to as Hierarchical Legendre Expansions (HLE). In addition, non-local expansion domains with curved boundaries are defined to capture the exact shape of the constituents independently of the refinement of the model. In this sense, the quality of the approximation is controlled by the polynomial order of the beam model, which is introduced in the analysis as a user input, and the size of the computational problem can be reduced for many typical microstructures with no loss of accuracy.
Micro-Mechanical Modelling of the Entanglement of Carbon Fiber Tows
Carbon fiber reinforced plastics (CFRPs) are the focus of many research efforts for light weight, high stiffness applications. While CFRPs generally yield more desirable properties than their metal counterparts, variations in the microstructure can lead to problems such as early failure. For example, voids in the matrix provide points of failure initiation in the matrix. The existence of voids is often attributed to nonaxialities in the fiber packing structure, which block the matrix from passing through certain regions of a tow. These non-axialities arise through the processing and handling of tows during manufacturing, where compression of a tow or friction on the outer fibers leads to fibers becoming entangled into one another. It is evident that composite materials cannot be fully utilized without a sound understanding of the manufacturing
Interphase Mechanics in Fatigued Carbon Fiber Composite Materials
The mechanisms of fatigue damage in carbon-fiber-reinforced-polymer (CRFP) composites are governed by complex material damage accumulation behaviors which depend on the reinforcement orientation and loading conditions. A common failure mode is destruction of the fiber-matrix interface through which loading is transferred through shear strain from the matrix to the fiber. It is believed that a mechanically distinct interphase region exists between the fiber and matrix, and that this region is degraded in fatigue loading. In this research, the fiber, matrix, and interphase behavior is investigated for unidirectional IM7-8552 specimens of the laminate [013] mechanically tested in three-point bending to failure. The thermodynamic and quasistatic mechanical properties are presented from an investigation via nano-indentation and atomic force microscopy (AFM). Room temperature and elevated testing at various frequencies was used on neat resin as well as [013] fatigued and control specimens.
A Novel Architectured Polymer Composite for Near Isotropic Characteristics and Improved Combined Stiffnessâ€”Damping Properties
A novel micro-architecture is proposed to design multifunctional composite with simultaneous stiffness and damping capabilities. The micro-architecture utilizes the hexagonal symmetry in order to manifest in-plane isotropy in the properties. The micro-architectured composite is made of circular gear and triangular shaped stiff material and a layer of viscoelastic material in between them. The gear and triangular shaped building blocks is designed to utilize the interlocking mechanism for them to transfer load through the interlock under tensile load. The combination of the stiff and the viscoelastic material in the composite yields to both high-stiffness and highdamping, which otherwise are competing in engineering materials. Numerical experiments are performed in order to characterize the design and quantify the performance parameters of the micro-architectured composite using PMMA as the stiff materials and PU as the viscoelastic materials. Quasi-static tensile behavior and damping performance under low frequency load is predicted using finite element analysis. The thickness of the PU layer is varied to investigate the effect of PU volume fraction. The simulation results show that with approximately 7 percent volume fraction of PU the composite retains 55 percent stiffness of PMMA. With the increase in PU volume fraction (layer thickness), the damping of the composite increases and the stiffness decreases. The composite achieves 10 and 50 percent damping of PU with only 7 and 30 volume percent of PU, at 10 Hz loading frequency. The combined performance (i.e. multiplication of stiffness and damping) is found to be close to Wang-Lakes line, which is notable for a polymer-polymer composite.
Micromechanical Modeling of Particulate-filled Composites Using X-ray Computed Tomography
Glass fillers with different shape and mass fractions were considered to demonstrate the wide range of applicability of micro-CT imaging. It is shown that micro-CT technique can be used to create realistic RVEs for two-phase particulatefilled composite materials removing the need to use artificial computer algorithms to distribute and orient 3-D inclusions randomly. Solution methods are proposed to the encountered problems during reconstruction of micro-CT imaging. In addition, effective properties are calculated and contributions of different parameters on effective elastic properties are evaluated. Three different shapes of glass fillers (spherical, flake, and fiber) and filler mass fractions (5%, 10%, and 15%) were introduced to epoxy resin. Voxel-based RVEs were created from binary segmentation of images taken from micro-CT. Geometry-based RVEs were created after reconstruction of voxel-based RVEs to eliminate the stepped-like appearance of nonorthogonal interfaces. These RVEâ€™s were then used in the finite element analysis to find the effective mechanical properties such as Youngâ€™s modulus, shear modulus, Poissonâ€™s ratio of the samples. In order to assess the numerical findings, compression tests were performed.
Critical Comparison of 12 Micromechanics Models for Stiffness and Strength of UD Composites
Any micromechanics model developed to predict the stiffness (elastic properties) of a composite is also applicable to the prediction of a composite strength, as long as the homogenized stresses in the matrix are converted into true values by virtue of its stress concentration factors (SCFs) obtained recently. In this paper, the predictability of 12 most well known micromechanics models for stiffnesses and strengths of unidirectional (UD) composites is assessed. The measured elastic and strength data of the 9 UD composites adopted in three world-wide failure exercises (WWFEs) are used as a benchmark in this assessment. It is shown that Bridging Model exhibits overall the best accuracy in prediction of both the stiffness and strength of a composite.
Quantification and Classification of Continuous Ceramic Fiber Reinforced Ceramic Matrix Composites Microstructures
Advance ceramic reinforced ceramic matrix composites (CMCs) materials exhibit hierarchical internal structure with rich details at multiple length scales of interest. These details of the materialsâ€™ internal structure are referred to as the microstructure which dictates the overall performance of the material. In this work, we show that a rigorous statistical quantification of the material internal structure (i.e. microstructure) is possible for CMCs. In particular, a 2-point spatial correlation function is shown to capture the spatial heterogeneity for the two different class of microstructures generated using Uniform and Gaussian distribution. Additionally, it is demonstrated how these stochastic representations of the distributed filaments can be imported into a finite element framework to simulated transverse cracking strength. The damage in these simulations in modeled using a novel Regularized eXtended Finite Element Method (Rx-FEM) that predicts damage initiation and propagation without a priori defining where that damage occurs.
A Novel Atomistic Modeling Technique for Poly-crystalline Nanoporous Metals
In this study, a novel technique, which is mainly based on the Voronoi tessellation method representing randomly oriented three-dimensional network of ligaments for the atomistic modeling of poly-crystalline nanoporous metals, is presented. In this regard, by the utilization of the proposed methodology atomistic models of polycrystalline nanoporous metals consisting of randomly oriented ligaments with nonuniform mass distribution along the ligament axis can be generated, which in turn enables researchers to control both length and diameter of the individual ligaments within the porous network. Furthermore, the atomistic models of poly-crystalline nanoporous materials can be simply employed for the mechanical, thermal and acoustic characterization through molecular dynamics (MD) simulations. In general, proposed modeling technique consists of five steps. In the first step, a modeling process, which has been already used in literature for the modeling of poly-crystalline bulk metals, is followed. In this step, Voronoi tessellation technique is utilized for the purpose of generating the granular structure. In the second step, in order to define the volumetric region for the ligaments of the nanoporous structure, the process for the generation of a new Voronoi tessellation is initiated. In the following step, the coordinates of randomly distributed points are determined in a controlled way to be employed in the establishment of the Voronoi tessellation, which results in randomly oriented and intersected line segments. Then, the line segment representation of the Voronoi tessellation is transformed into a unique volumetric region by using spherical regions. Finally, the atoms external to the volumetric region are removed from the poly-crystalline bulk metal resulting in a poly-crystalline nanoporous structure. In conclusion, the ultimate goal of the study is to generate atomic coordinates that can be employed for the MD simulations of randomly organized poly-crystalline nanoporous structures. Furthermore, the thermodynamic stability of poly-crystalline nanoporous metal is also investigated to validate the feasibility of the atomistic model.
Multiscale Modeling of PEEK Using Reactive Molecular Dynamics and Micromechanics
Polyether ether ketone (PEEK) is a high-performance, semi-crystalline thermoplastic that is used in a wide range of engineering applications, including some structural components of aircraft. The design of new PEEK-based materials requires a precise understanding of the multiscale structure and behavior of semi-crystalline PEEK. Molecular Dynamics (MD) modeling can efficiently predict bulk-level properties of single phase polymers, and micromechanics can be used to homogenize those phases based on the overall polymer microstructure. In this study, MD modeling was used to predict the mechanical properties of the amorphous and crystalline phases of PEEK. The hierarchical microstructure of PEEK, which combines the aforementioned phases, was modeled using a multiscale modeling approach facilitated by NASAâ€™s MSGMC. The bulk mechanical properties of semicrystalline PEEK predicted using MD modeling and MSGMC agree well with vendor data, thus validating the multiscale modeling approach.
Thermal Conductivity of Graphene Nanoplatelet/Carbon Fiber/Cycloaliphatic Epoxy Hybrid Composites: Multiscale Modeling
Graphene nanoplatelet (GNP)/Carbon fiber/cycloaliphatic epoxy (CE) hybrid composites are computationally modeled using a multiscale modeling approach to predict the thermal transport behavior. Equilibrium molecular dynamics simulations are performed to obtain the in-plane and out-of-plane thermal conductivities at the molecular level for GNP/cured CE with different levels of GNP dispersion, namely, 1, 2, 3, and 4 layer(s) of graphene. The thermal conductivities are randomized by arithmetic averaging. The thermal conductivities for 1 to 8 wt% of GNP are predicted using micromechanics analysis code based on generalized method of cells (MAC/GMC) and compare well with experiments. The effective thermal conductivities of the hybrid composites are also determined by using MAC/GMC and compared with experimental specimens. The axial thermal conductivity of the hybrid composite model slightly increases due to the amount of graphene. However, the transverse thermal conductivity is predicted to be improved up to 132% relative to no GNP loading.
Modeling of Glass Fiber with Surface Cracksâ€”A Molecular Dynamics Simulation Study
Theoretical strength of glass fiber is much higher than the experimental values. It is believed that nano-meter size surface cracks, which are developed on the fiber surface during manufacturing and later on handling, deteriorate fiber strength. In the paper, effects of surface crack on the mechanical properties (modulus and strength) of glass fiber are studied through reactive all-atom molecular dynamics (MD) simulations. Surface cracks of different length are created by deleting atoms. Two types of reactive force fields â€“ ReaxFF and Tersoff, are considered to assess their accuracy and computational expense. Simulation results indicate that surface cracks have no effect on glass fiber modulus, however, fiber strength is significantly affected by surface crack. Presence of surface cracks reduces fiber strength. With the increase in crack length, strength decreases and MD derived strength-crack length response is in good agreement with theoretical prediction.
Reaction-induced Phase Separation of Thermoset/Thermoplastic Mixtures by Coarse-grained Molecular Dynamics Modeling
This proceeding paper reports our recent study on the reaction-induced phase separation during the cure of thermoset/thermoplastic polymer mixtures using coarsegrained molecular dynamics simulations. Chemical reactions during the curing process of the thermoset are explicitly modeled using the MDPoS method extended to a coarse grain description of thermosets. The effects of composition and conversion degree on phase separation domain size growth are investigated. We find that the scaling phenomenon of domain size growth with phase separation time, observed previously in simple liquid mixtures and linear polymers, applies to reaction-induced phase separation, and the power-law exponent of growth rate depends on composition but not on the conversion degree.
Atomistic Composite Simulations with Modular Software Tools on nanoHUB.org
The use of polymers and composites is growing significantly and enabling emerging technologies. Molecular dynamics simulations of this class of materials plays an important role in our understanding of the fundamental physics of these systems. These techniques are also being used to guide the design of novel materials with customized properties. Simulating amorphous systems at molecular scales is often more challenging than simulating other materials due to the wide range of relaxation timescales, often exceeding those possible by direct MD simulations. The creation of well- relaxed structures is challenging and, though advanced methods have been designed to address this problem, not all are easily available or accessible. Many researchers use suboptimal approaches. In addition, the use of closed- source or proprietary software and the lack of molecular structure repositories can lead to unreproducible results. We are developing a framework for molecular simulations of polymers and amorphous systems. The tools we develop are universally accessible and useful to the community for cloud computing via nanoHUB. The framework consists of a stateof- the-art molecular builder, a parallel MD engine with stencils that enable efficient structure relaxation and property characterization, atomistic visualization, and postprocessing codes, all accessible from Jupyter notebooks running on nanoHUB. These notebooks can also access databases of force fields and molecular structures. Users do not need to download or install any software nor provide compute cycles. Expert users can extend the Python code in the notebooks or download it and run it in their own resources.
Effect of Volume Fraction of Graphene Nano Platelet on Fracture Characteristic of Graphene/Epoxy Composite Using Reactive Force Field
The fracture behavior of multilayer graphene nano-platelets in epoxy composite under tensile loadings was characterized using a computational molecular dynamic simulation and the Reactive Force Field. The investigation covers the approach for developing the molecular structure of graphene/epoxy composite, and the method for relaxing the systems. A uniaxial tensile deformation was employed for evaluating the fracture behavior of the model. The estimated results showed that fracture strength, fracture strain, and the Youngâ€™s modulus significantly increased with increasing layers of graphene in epoxy and the fracture occurs at the onset of yielding for all the models evaluated
Uncertainties of Parameters to Predictions of Polymer Properties by Molecular Simulations
We quantify uncertainties in the prediction of thermos-mechanical properties of polymers using atomistic simulations. We quantify uncertainties with respect to the choice of force field used to describe atomic interactions, chain molecular weight in the samples, and data analysis method in the prediction of glass transition temperature Tg, density and thermal expansion coefficients (CTE) of Poly(methyl methacrylate), PMMA. The PolymerModeler at nanoHUB was used to build amorphous samples with two different chain lengths and annealing MD simulations where performed two widely used force fields DREIDING and PCFF. In order to extract the properties of interest from the raw simulation data we use two methods: i) a bi-linear fits to the high and low temperature regions, extracting the Tg as the intersection between the fits and the CTEâ€™s from the slopes of each line and, ii) a hyperbola to fit the entire range of data and Tg is obtained as the intersection of the two asymptotes of the resulting hyperbole, each asymptote slope gives the value of the thermal expansion coefficient. We find that our results give accurate predictions, glass transition temperature is in good agreement once the correction for experimental data is considered; a significant Tg at higher temperatures is found with the pcff force field compared with Dreiding, as well for densities at room temperature. The analysis method used was found to influence the prediction of CTEâ€™s, but also the estimate of Tg.
Molecular Structure of PAN-based Carbon Fiber Precursor
The molecular structure of the Polyacrylonitrile(PAN) precursor is known to critically affect the microstructure, and consequently properties, of the final carbon fibers (CFs). Despite the importance of PAN-based carbon fibers, molecular modeling has not been attempted on these precursors and the only structural information available is from models proposed based on XRD experiments. We use molecular modeling to predict the molecular structure of spun PAN fiber precursors and their properties. Our goal is to predict low energy, fully relaxed structures and compare the resulting structures with experiments. The first step in the procedure is to generate a family of helical, rod-like isotactic and atactic isolated chain structures by building chains with a fixed torsional angle ranging from 5ï‚° to 175ï‚°. The second step is to pack the individual chains into a simulation cell following a closed-packed hexagonal arrangement and fully relax the cells using isobaric and isotherm molecular dynamics (MD) simulations. We find that chains built using torsional angles between 120ï‚° and 145ï‚° result in the lowest energy structures in the condensed phase. Furthermore, the simulated structure factors showed excellent agreement with experimental data in both peak positions and relative intensities.
Molecular Dynamics Study of Carbon Nanotube/Epoxy Interfaces Using ReaxFF
Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites. In pursuit of our research goal, we first seek to obtain the elastic properties of the nanocomposites. Properties at interface between CNT/polymer are critical to determine mechanical, electrical, and thermal properties of these nanocomosites. In the present study, we have used reactive force field (ReaxFF) to study the interfacial properties of CNT/EPON 862-DETDA nanocomposite system. Because molecular-level failure events can play a significant role in epoxy mechanical behavior, the ReaxFF can be used as an ideal tool for MD simulations involving crosslinked epoxies. Pull out simulations are performed to characterize the CNT/polymer interfacial interactions. Pull out energy is used to calculate the interfacial shear strength of CNT/polymer nanocomposite.
Elastic Constants of Carbon Nanotube Reinforced Polymer Nanocomposites
Typical electronics packages are assembled by integrating various parts on printed circuit boards (PCB). Traditional interconnect materials in electronics packages are not suitable for DoD electronics because in many DoD extremely transient conditions, mechanical failures of the whole packages invariably occur due to interconnect junction failures. The long-term objective of the research is to computationally investigate the effect of high strain rate loadings on the thermal and mechanical damage/failure of carbon nanotube reinforced polymer nanocomposites, while retaining their electrical functionality. In pursuit of our research goal, we first seek to obtain the elastic response of the nanocomposites. In particular, carbon nanotubes (CNTs) are dispersed in polymer matrix in a random fashion. In the present study, a two-dimensional network of CNTs spread in polymer matrix is statistically generated using Matlab code. This Representative Volume Element (RVE) is further processed into a Finite Element Model (FEM). Abaqus is employed to evaluate the elastic constants such as Youngâ€™s modulus, Poissonâ€™s ratio, and shear modulus for this nanocomposite. Further, Halpin-Tsai equations are used to compare the values obtained from the finite element analysis.
Electrical and Mechanical Properties of a Biocompatible Carbon Black Reinforced Thermoplastic Elastomer for Sensing Applications
The relationship between pressure-sensing capability, carbon black (CB) content, and tensile strength of a CB-reinforced poly(styrene-isobutylene-styrene) (SIBS) thermoplastic elastomer was investigated. Inexpensive pressure sensors have excellent potential in both implantable and external (stick-to-skin) biomedical applications. Due to its excellent biocompatibility and commercial use in biomedical implants, SIBS is an excellent host material for the conductive CB reinforcement. Applications include stickto- skin sensors to supplement peripheral nerve function for anesthetized patients, stroke victims, or diabetic patients suffering from peripheral neuropathy. Five specimens ranging from 0% to 25% CB content by weight were fabricated through a combination of high-shear mixing, ultrasonication, and solvent casting. Electrical properties were explored using a custom-built apparatus capable of measuring resistance as a function of applied pressure. Results indicated that the 15% CB specimen performed significantly better than other loading percentages; it produced the largest variability in resistivity within the studied pressure range, along with a 13% increase in tensile strength. A direct relationship between pressure sensitivity (resistivity range) and tensile strength appears to exist in specimens near or at the percolation threshold, whereas otherwise, an inverse relationship is present. This initial research represents a step toward understanding interactions between the competing design goals of CB content, mechanical strength, and retained biocompatibility. The results of the 15% CB specimens show particular promise, as pressure sensitivity was maximized without sacrificing tensile strength. Further, initial results showed no sign of toxicity from SIBS/CB on human stem cells after a seven-day incubation period. In conclusion, these results support the potential use of SIBS/CB composites in biomedical sensing applications without sacrificing tensile strength or biocompatibility.
Unidirectional Carbon-fiber Prepreg with Embedded Sacrificial Fibers for Three-dimensional Microvascular Network
Unidirectional carbon-fiber/epoxy prepreg with embedded sacrificial fibers was manufactured and used to fabricate microvascular fiber-reinforced polymer (FRP) composites. A laboratory scale prepregger was designed and built for the continuous processing of resin impregnation, incorporation of sacrificial fibers, and fiber winding. The sacrificial fibers were annealed at elevated temperature during the prepregging process to obtain a good alignment with the neighboring carbon-fibers. A laminated FRP composite was manufactured from the prepreg by a hot-pressing technique. Sacrificial fibers from the separate lamina were fused due to the compaction and temperature of hot-pressing process. Three-dimensional microvascular networks were revealed within a laminated composite when the lattice of sacrificial fibers was vaporized at 200 Â°C. This is the first demonstration of multi-dimensional microvascular networks embedded within an FRP composite using a prepreg processing technique.
Comparative Study of the Piezoresistivity of Free and Medium-constrained Carbon Nanotube Yarns: Effect of Slippage
Carbon Nanotube (CNT) yarns have gained increased attention in structural health monitoring due to their multifunctional properties. They exhibit a unique change in their resistivity when subjected to mechanical strain. This piezoresistive response can be tapped for sensing purposes. The objective of this study is to determine experimentally the piezoresistive response of CNT yarns that are embedded in a medium while subjected to tension, and compare it with that of the free or unconstrained CNT yarns. The constraint is achieved by embedding the CNT yarns in epoxy to produce coated CNT yarns and fiber monocomposite beams. Tensile tests were performed on the coated yarn while the monocomposites were subjected to both uniaxial and bending tests. The curves of the constrained CNT yarns are fairly linear and produce higher gauge factors than that of the free CNT yarn. Consequently, the piezoresistivity of the constrained CNT yarns is higher than that of the unconstrained CNT yarns. This difference between them may be explained by the lack of the effective slippage, fiber unraveling and subsequently, Poissonâ€™s effect of the CNT yarn when integrated in the polymer. The composite samples tested under bending showed a higher gauge factor than under uniaxial tension. CNT yarns that were dipcoated in the polymer showed high fiber impregnation due to the yarns porous structure. However, they also showed improved strength, more linearity and higher piezoresistive response but lower strain to failure when compared to the uncoated CNT yarn.
Foil Strain Gauge Sensors of Piezoresistive Carbon Nanotube Yarn: Fabrication and Calibration
Carbon nanotube yarns are micron-scale fibers comprised by tens of thousands of carbon nanotubes in their cross section and exhibiting piezoimpedance characteristics that can be tapped to sense strain. This paper presents the details of novel foil-based strain gauge sensor configurations comprising carbon nanotube yarn. The strain gauge sensors were designed considering parametric studies that maximize the sensitivity of the sensor to mechanical loading. The fabrication details of the strain gauge sensors that exhibit the highest sensitivity, based on the modeling results, are described including the materials and procedures used in the first prototypes. Details of the calibration of the strain gauge sensors are also provided and discussed in the context of the electromechanical characterization when bonded to metallic specimens. This characterization included studying their response under monotonic and cyclic loading. It is shown that these strain gauge sensors consisting of carbon nanotube yarn are sensitive enough to capture strain. Previous modeling results indicated the potential of the strain gauges to exhibit gauge factors higher than those of metallic foil strain gauges.
Piezoelectric Homogenization of the Macro-Fiber Composite with the Mechanics of Structure Genome
A convenient and robust way to convert an ordinary structure into a smart one is to adhere a piezoelectric macro-fiber composite (MFC) patch to it. Although the commercialization of the MFC has facilitated numerous sensor and actuator applications, analysis of host structures has mainly relied on homogenized constants that were obtained with various assumptions on the field variables. Here, the newly discovered mechanics of structure genome (MSG) is used to assess the effect of assuming uniform-fields, plane-stress assumptions, and homogeneous layers. First it is shown that MSG can maintain the accuracy of 3D piezoelectric finite element analysis (FEA) but without the need for nine analyses and complex boundary conditions for a full piezoelectric stiffness matrix. With a 3D unit-cell we show how recent reports of the effective in-plane properties of the full MFC have been underpredicted by as much as 30% and transverse shear moduli have been overestimated between 100% and 300%. The implications here are that analyzing host structures with MFCs can proceed with higher accuracy with the effective properties that are obtained without the aforementioned assumptions. The versatility of MSG is demonstrated with a 1D plus 2D homogenization that has improvements in accuracy and completeness over comparable approaches that utilize the homogenous layer assumption. This approach in MSG may provide the best compromise between simplicity and accuracy for performing optimizations and/or case studies on system level performance. Finally, we verify that the ubiquitous uniform-field assumptions are good for homogenizing the fiber layers. However, the plane-stress assumption taken by many researchers at the lamina level should be avoided since it leads to underpredictions of the transverse layer properties by at least 11%. This is especially important when modeling complex systems with many MFCs, off-axis applications, or load-bearing MFCs.
PDMS/CNF Nanocomposite Sensor Array for Pressure Sensing
Pressure sensing is a field that is rapidly advancing as driven by vast applications including biomedical devices, industrial automation, human-machine interfacing, and skin-like electronics. Recently significant amount of effort has been reported to develop new piezoresistive strain sensor made from polymers with carbon nanofillers. The widely used nanofillers with high electrical conductivity include carbon nanotubes, carbon nanofibers, and other carbon nanoparticles. However, most previous research focused on the improvement of material properties for sensing applications. Limited work balanced the sensor design and material innovation for real time pressure sensing. This paper presented a piezoresistance based pressure sensor array using flexible and stretchable polydimethylsiloxane (PDMS) polymer/carbon nanofiber (CNF) nanocomposite. In order to increase the sensitivity of the proposed sensor array, each sensing unit was optimized by selecting the appropriate shape and adjusting critical geometrical parameters using both finite element analysis and optimization algorithms. Considering different pressure range, two different set of sensing unit geometries were designed for the best performance in the pressure range of 50.100, and 150 psi maximum pressures. The developed sensor arrays were connected using a mesh grid approach. Preliminary sensor array experiments were tested to prove the concept. Finally, the developed pressure sensor arrays were characterized under both static and cyclic load conditions to demonstrate its sensor capability.
Computational Simulation of Composite Structural Batteries
Structural battery composites are multifunctional materials that store electrical energy while also bearing structural loads. These materials are particularly applicable to electric automobiles, aircraft, spacecraft and other energy demanding and weight sensitive applications. The auxiliary electric charge from the material will provide power for onboard electronics and extend the range without decreasing the range or the payload capacity. Current structural battery designs include solid-state battery technology with woven carbon or glass fiber composites as a structural reinforcement. The computational finite element method (FEM) was used to simulate various material reinforcements to assess the optimal material selections and designs. Full scale panel laminates were analyzed using a 3 point bend test configuration to determine the flexural stiffness and strength of each structural battery laminate configuration. Fabrication artifacts due to the vacuum bagging composite fabrication technique were assessed to determine the necessary gasket material needed to maintain structural integrity. In addition, plain weave glass fiber/epoxy and unidirectional carbon fiber/epoxy tape were compared to determine the optimal material for interstitial layers between the multiple battery cells in a single panel. For the case of a gasket around the battery layers in the panel, a parametric study was developed to determine the effects of different possible gasket materials. In addition, a micromechanics approach was utilized to analyze the conductivity of carbon fiber/epoxy composites for possible use in the solid-state battery for improved energy density and structural performance. Incorporation of high conductivity epoxy as a matrix in a carbon fiber/epoxy composite significantly increases the overall electrical performance of the composite to the point that integration into the solid-state battery unit cell is possible.
Fiber Optic Sensor Embedment Study for Multi-Parameter Strain Sensing in Smart Composites
Fiber optic sensor (FOS) is commonly used for large scale structure monitoring systems for their small size, noise free and low electrical risk characteristics. Embedded fiber optic sensors (FOSs) lead to micro-damage in composite structures. This damage generation threshold is based on coating material of the FOSs and their diameter. Normally, embedded FOSs are aligned parallel to reinforcement fibers to detect over-strain of the structure. This linear positioning of distributed FOS fails to provide all strain parameters. We suggest novel sinusoidal sensor positioning to overcome this issue. This method tends to provide multi-parameter strains in a large surface area. The effectiveness of sinusoidal FOS positioning over linear FOS positioning is studied under both numerical and experimental methods. This study proves the advantages of sinusoidal positioning method for FOS in composite materialâ€™s bonding.
Multiscale Investigation of Micro-Cracking at Composite Laminate Free Edge
To investigate the effect of microscopic features on initial micro-cracking at the laminate free edge, a two-scale finite element (FE) modelling approach is proposed. It consists of a meso-scale model to capture the laminate stacking sequence and the global stress field under a given loading condition, and a micro-scale model to predict the local constituent level stresses at the free edge. The two models are coupled oneway through a strain localization rule. A procedure to determine the boundary conditions for micro-scale FE models containing a free edge has been developed. The model developed this way was used to examine the 90/90 interface in [25N/-25N/90N]S composite laminates. The effects of thermal and tensile loading were investigated separately to understand the influence of interlaminar microstructure on micro-scale stresses at laminate free edges during manufacture and under mechanical loading. The results agreed with the trend of free edge pre-cracks and progressive damage observed in experiments.
A Multiscale Approach for Fiber Damage Prediction of Composite Open-Hole Tension Coupon under Fatigue Loading
Since fiber-reinforced polymer (FRP) composites are inherently anisotropic and display a wide variety of failure mechanisms, predicting their fatigue behavior is a challenging task. Prediction of fatigue in FRP composites demands progressive damage analysis tools that account for constituent physics of the problem. In this work a fatigue failure methodology based on the kinetic theory of fracture (KTF) is developed that uses the physics of the matrix constituent to track damage in both laminar and inter-laminar regions. Fiber failure is predicted using the maximum principal stress theory. Matrix and fiber damage mechanisms are implemented together to predict the overall progressive fatigue behavior of the composite. Using this methodology, finite element simulations of an open-hole coupon subjected to tension-tension fatigue loading were conducted. The resultant residual stiffness and the damage location, type and amount inside the plies with number of fatigue cycles were benchmarked against published experimental data.
Characterizing the Multiscale Elastic/Plastic/Rupture Response of Short Fiber Filled Plastics
To facilitate progressive failure structural simulation of short fiber filled injection molded parts, Autodesk has developed multiscale modeling methodology and software to seamlessly link the results of injection molding simulation with subsequent nonlinear multiscale structural response simulation. The key features of the methodology include: 1) Automated mapping of the predicted fiber orientation distribution onto the finite element mesh that will be used for the nonlinear structural response simulation, 2) Enhancement of the structural response simulation with a multiscale, progressive failure, constitutive model for short fiber filled plastic materials that accounts for plasticity and rupture of the matrix constituent material, resulting in a composite material that exhibits an anisotropic, nonlinear response that accounts for the observed differences between a tension dominated response and a compression dominated response, and 3) A robust, automated material characterization process that uses a minimal amount of simple uniaxial tensile and compression test data from the short fiber filled plastic material to fit the parameters of the multiscale, progressive failure, constitutive model. The injection molding process tends to produce specimens that exhibit a significant spatial variation of the fiber orientation tensor commensurate with the flow characteristics of the injection molding process. For relatively thin injection molded parts, the fiber orientation tensor exhibits a particularly strong variation in the thickness direction. Consequently, the typical uniaxial tension and compression specimens that are used to characterize the material model are more correctly viewed as structures rather than simple test coupons. In this paper, we propose an enhanced material characterization procedure that correctly accounts for the typical observed variation of the fiber orientation tensor through the thickness dimension of the test specimens that are used for material characterization. In this enhanced characterization process, the uniaxial tension and compression test data is supplemented with flexural test data to permit the material model coefficients and the fiber orientation tensor distribution to be determined simultaneously from the available test data.
Component-Wise Method for Macro-Meso-Micro Modeling and Failure Analysis of Composite Structures
The work assesses the advanced capabilities of the Component-Wise approach (CW) to model composite structures, with particular attention paid to the failure index evaluation. The CW has been recently proposed as an approach to model complex structures with far fewer degrees of freedom than classical shell and solid finite elements. The CW exploits refined beam models based on the Carrera Unified Formulation (CUF) to model each component of the structure. The classical finite element 1D approach is used along one direction. On the other hand, the unknown variables are expanded over the remaining two local coordinates via Legendre polynomials, referred to as Hierarchical Legendre Expansions (HLE). Also, non-local expansion domains with curved boundaries are defined to capture the exact shape of the constituents independently of the refinement of the model. Maximum stress criterion and Tsai-Wu criterion are used to evaluate the failure indices and identify critical portions of the structure for different modeling strategies using CW approach. Numerical results are provided for different configurations and compared against the standard 3D finite element models. The paper highlights the efficiency and accuracy of proposed framework against the conventional finite element modeling technique.
A Multiscale Progressive Damage Analysis for Laminated Composite Structures using the Parametric HFGMC Micromechanics
A nonlinear multiscale damage analysis framework, which is based on the parametric High Fidelity Generalized Method of Cells (HFGMC) micromechanics, is developed to predict the mechanical response of laminated composite structures, with and without notches, subjected to tensile and compressive static loading. A refined hexagonal-array repeating unit-cell (RUC) HFGMC micromodel for the unidrectional composite microstructure is discretized into subcells to represent explicity the fiber and matrix phases at the microscale level. The material behaviors of the fiber and matrix constituents, are directly prescribed. The fiber is considered to behave as an elastic transversely isotropic material. The nonlinear material for the matrix phase is represented by the J2-deformation plasticity theory. The predicted HFGMC overall in-plane shear and longitudinal moduli of a single composite lamina together with a compressive strength equation are applied to determine the micro-buckling compression failure of a lamina. To determine the failure of an individual composite lamina due to the fiber, matrix, or in-plane shear failure modes, the local strain field within each subcell within the RUC in conjunction with the strain invariant failure theory (SIFT) criterion are employed. Once failure is detected within a subcell, the (cell-extinction-damage) CED damage modeling approach is employed to remove the considered subcell from the hexagonal RUC. This framework is implemented as a user material subroutine within standard displacement layered-shell FE models of laminated composites. When failure of RUC representing an individual layer is determined, the corresponding material point is removed from the thickness at the global surface integration point of the shell finite element. The properties and damage parameters of the composite constituents are determined by proper calibration of the axial, transverse, and in-plane shear responses of unnoteched laminates. Damage analyses for both un-notched and notched laminated structures are presented and compared with available experimental results
Including the Effects of Weave Draping Within Multiscale Simulations
This paper will present a general methodology by which weave draping manufacturing simulation results can be utilized to include the effects of weave draping and scissor angle in a structural multiscale simulation. While the methodology developed is general in nature, this paper will specifically demonstrate the methodology applied to a truncated pyramid, utilizing manufacturing simulation weave draping results from ESI PAM-FORM, and multiscale simulation using Altair Multiscale Designer (MDS) and OptiStruct. From a multiscale simulation perspective, the weave draping manufacturing simulation results will be used to develop a series of woven unit cells which cover the range of weave scissor angles existing within the part. For each unit cell, a multiscale material model will be developed, and applied to the corresponding spatial locations within the structural simulation mesh. In addition, the principal material orientation will be mapped from the wave draping manufacturing simulation mesh to the structural simulation mesh using Altair HyperMesh mapping technology. Results of the coupled simulation will be compared and verified against experimental data of the same available via General Motors (GM) Department of Energy (DOE) project.
Multiscale Finite Element Modeling of Carbon Fiber Reinforced Polymeric Composite Containing Void Defects
The research demonstrates the implementation of a multi-scale modeling approach for predicting the mechanical effects of porosity on carbon fiber reinforced polymer composite. Two load cases are singled out for analysis which emphasize matrix dominated mechanical response, and therefore highlighting the effects of porosity in the matrix. Microscopy is used to determine an average, idealized void morphology in both distribution and size, which in the present case gives way to ellipsoidal voids located between and within individual lamina. Macro-scale parent models of ASTM test coupons for each loading case are first used to acquire local maximum gage section strains. These strains are then applied as boundary conditions to a second set of micromechanical models containing the desired void morphology. Local strains and stresses can then be computed as a result of said local defects and or other explicitly modeled heterogeneities at the fiber level. The current work focuses on the predicted strength knockdown of simple unidirectional laminates using maximum strain criteria, and Hashin failure criteria for homogeneous-smear property approaches. These values are obtained for simulated loading cases only, but implemented in such a way as to make use of empirical results at a later stage. It is found that the Hashin damage model is particularly unsuited to deal with variations in void morphology in terms of predicting failure. This is due to the basic fact of elasticity in where stress concentrations are largely governed by inclusion aspect ratio (sharpness) and not position or size. However, the overall multiscale methodology is robust enough to allow application of alternative point-wise failure criteria and or the addition of progressive damage models at a later date.
Comparison of Multiscale Method of Cells-Based Models for Predicting Elastic Properties of Filament Wound C/C-SiC
Three different multiscale models, based on the method of cells (generalized and high fidelity) micromechanics models were developed and used to predict the elastic properties of C/C-SiC composites. In particular, the following multiscale modeling strategies were employed: Concurrent multiscale modeling of all phases using the generalized method of cells, synergistic (two-way coupling in space) multiscale modeling with the generalized method of cells, and hierarchical (one-way coupling in space) multiscale modeling with the high fidelity generalized method of cells. The three models are validated against data from a hierarchical multiscale finite element model in the literature for a repeating unit cell of C/C-SiC. Furthermore, the multiscale models are used in conjunction with classical lamination theory to predict the stiffness of C/C-SiC plates manufactured via a wet filament winding and liquid silicon infiltration process recently developed by the German Aerospace Institute
Tailoring of Flaw Length-Scale for Mode I and Mixed-Mode Toughness Enhancement in Composite Airframes
Motivated by previous research performed by the authors investigating Mode I toughness enhancement using nanographene in EPON 862 epoxy, this article aims to investigate to cases: (a) the changes in mixed mode fracture properties of a thermoset polymer (EPON 862) reinforced with hydrogen passivated nanographene platelets (HP-NGPs) and, (b) Mode I fracture properties of EPON 862/IM7 unidirectional laminate with and without dispersed HP-NGPs. For case (a), mixed mode fracture experimentation was performed using an asymmetric four-point bending specimen on baseline (0 wt%), 0.1 wt% and 0.5 wt% HP-NGP reinforced EPON 862 polymer. Three different mode mix (KII/KI) ratios (0.78, 1.53, 117) were used to obtain an experimental fracture envelop encompassing pure Mode I to Mode II. Remarkable increase in the fracture envelop both in Mode I (~3 times) and Mode II (~2.5 times) was observed with only 0.5 wt% of HP-NGP. For case (b), Double Cantilever Beam (DCB) experiments were used to obtain the fracture toughness of the unidirectional IM7/EPON 862 laminates with the HP-NGP reinforced matrices as in case (a), in accordance with ASTM D5528. Significant increase (~100%) in resistance to crack propagation in DCB specimens was observed for 0.5 wt% HP-NGP reinforced system in comparison to baseline samples. A brittle to ductile transition at the crack tip due to a nanoscale size effect is postulated as the reason for the toughness increase.
Electrospinning Approach for the Improvement of Mechanical and Shielding Properties of Nanofiber Mats
Electrospinning has been successfully used to develop aligned micro- and nanofiber mats for lightweight reinforcement of composite materials and structures. Electrospinning systems can be oriented either vertically, utilizing gravity-assisted electric field material deposition, or horizontally, utilizing electric field with little effect on gravity. A horizontal apparatus has been designed and fabricated for improved micro- and nano-fiber 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 multi-functional applications. The micro- and nano-fiber mats produced with the set up were scanned under Scanning Electron Microscope (SEM). These images show successful deposition of micro- and nano-fibers with alignment comparable with current electrospinning systems. System enhancement is in work to enhance alignment of deposited fibers through additional electromagnetic controls.
Effect of Nanocellulose on the Properties of Glass Fiber/ Nanocellulose/Unsaturated Polyester Resin Composites
Glass fiber/nanocellulose/unsaturated polyester interfacial adhesion was explored as a bottom up approach to determine the optimum method of adding nanocellulose in glass fiber-unsaturated polyester composites. The interfacial adhesion, in the form of interfacial shear strength, was determined using microdroplet debonding procedure. Two approaches were followed to introduce the nanocellulose in the glass fiberunsaturated polyester system: add the nanocellulose as a coating on the glass fibers or add the nanocellulose directly in the unsaturated polyester matrix. Different types of nanocellulose (cellulose nanocrystal and cellulose nanofiber) and surface functionalization of nanocellulose were employed in this study. Preliminary results showed an improvement of interfacial adhesion with cellulose nanofiber coated glass fibers and nanocellulose in the polyester matrix.
Searching for Optimum Piezoelectric Nanomembranes
For decades piezoelectricity was associated to ceramic materials. It was only recently that a class of materials piezoelectric polymers emerged. This work focuses on piezoelectric polymers, in special Poly (vinylidene fluoride) (PVdF) and its copolymers (PVdF-HFP and PVdF-TrFE), and how the electrospinning process affects its piezoelectric behavior. To be able to improve, even more, the phase formation, which is the phase responsible to the piezoelectric behavior, multi-walled carbon nanotubes functionalized by surfactants (SDBS) were introduced to the polymeric solution. Fiber morphology and spatial dispersion plays an important role into phase formation. Moreover, the CNT addition to polymeric solution lead to a decrease on fiber diameter around 34% and an increase 24% into phase formation. The SDBS effect was more effective at CNTs high concentrations (0.6% m/m), which somehow deactivate the saturation limit (0.15% m/m). Finally, the optimal nanomembrane configuration is highly dependent of the electrospinning operational parameters, but the CNT concentration of 0.15% m/m seems to serve as a nucleation site.
Scale-up Study and Demonstration of Highly Aligned and High Concentration CNT Reinforced Composites for Aerospace Applications
The objective of this research is to design, fabricate, and demonstrate scalable manufacturing processes, aerospace-quality and advanced structural and multifunctional properties of nanocomposites using commercially available carbon nanotube (CNT) yarns and sheets and aerospace qualified resin matrix. The goal is to achieve extra-high modulus, and adequate interfacial shear property with scale-up sample sizes (4"Ã—4"Ã—0.1"). Different from current low CNT content (<5 wt%) composites and micrometer-scale sample size efforts, we will study and demonstrate effective approaches to manufacture aerospace-quality structural composite materials of ~60 wt% CNT concentration and achieve sizable demonstration samples with a high degree of alignment and packing for potential structural applications. We have developed a unique manufacturing process to produce highly aligned, and high concentration CNT yarn composites by stretch-assist CNT filament winding, heatassisted multiple-step stretching and curing under tension processes, which are potentially easy to scale-up and are low cost with utilization of commercially available CNT materials. CNT purification was also performed to eliminate residual catalyst and amorphous carbon and to facilitate the removal of manufacturing added chemicals during CNT synthesis. CNT surface treatment, such as chemical functionalization and plasma etching were further studied to improve the CNT and resin interfacial bonding. Cytec 5250-4 BMI resin and an autoclave process were used to produce the large size test panels. Tensile testing and three-point bending tests were conducted, as well as SEM failure analysis. This is the first attempt to scale-up manufacturing of high CNT loading and alignment test panels and conduct ASTM standard level tests.
Effect of Nozzle Geometry on Melt Flow Simulation and Structural Property of Thermoplastic Nanocomposites in Fused Deposition Modeling
Fused deposition modeling (FDM) is a low-cost additive manufacturing (AM) process which uses thermoplastic materials to fabricate prototype parts from a computer aided design (CAD) model. Melt flow characteristics in FDM liquefier plays an important role in bead spreading architecture and contact angle, which eventually influences inter-bead voids and interfacial bead bonding. This paper focuses in studying the effects of nozzle exit geometries, on melt flow behavior inside the FDM liquefier chamber, void contents, and performance of carbon nanofiber (CNF) reinforced thermoplastic nanocomposites. A computational model for non-Newtonian flow is developed for the melt flow simulation of CNF/Polylactic Acid (PLA) nanocomposites to understand the behavior of FDM process parameters. The rheological properties of CNF/PLA nanocomposites with different weight percentage of CNF (0%, 0.5%, and 1%) have been determined through a standard rheology test. An existing FDM extrusion channel with characteristic dimensions have been modeled in a three-dimensional domain. The melt flow behavior in terms of the pressure drop, nozzle exit velocity profile, and axial temperature profile is studied by varying material property and nozzle geometry. A conceptual star shaped liquefier nozzle has been considered for improved bead architecture with minimum void contents. The results of the numerical simulations are compared with those for the published existing models which shows reasonable agreement. Tensile dogbone specimens have been printed with different liquefier nozzle diameters for mechanical testing. The optical microscopic examination show the presence of triangular and diamond shape void contents in printed CNF/PLA nanocomposites. The FDM printed nanocomposites show improved modulus and yield strength in comparison to neat PLA resin.
Thermal Conductivity of Carbon Nanotubes Based on Reverse Non-Equilibrium Molecular Dynamics-Size Effect
The thermal conductivity of carbon nanotubes (CNTs) estimated from molecular dynamics (MD) simulations has predicted a wide range of conductivity values. These variations are either attributed to different lengths, and chiralities of the nanotubes. To understand and develop efficient composite systems with the desired thermal characteristics; it is necessary to estimate the accurate thermal transport properties by considering the size parameters of the nanoparticles. It is seen that there is a high degree of variance of the dimensions of the nanotubes in nature and thus the accurate estimation of the nanotube size parameters needs to be carried out. In this work, the thermal conductivities of the nanotubes are determined using LAMMPS with AIREBO potential using the Reverse Non-Equilibrium Molecular Dynamics (RNEMD) approach. In RNEMD a constant amount of heat is added to and removed from the hot and cold junctions and the resulting temperature gradient is measured. The preliminary results indicate the variation of thermal conductivities of the nanotube with the length of the nanotube. To address the variation in the length and chirality, a probabilistic modeling of the CNT has been explored to obtain a better estimate of the variation in the thermal conductivities.
Failure Behavior and Scaling of Graphene Nanocomposites
This work proposes an investigation on the structural scaling of polymer/graphene nanocomposites. To this end, fracture tests on geometrically scaled Single Edge Notch Bending (SENB) specimens with varying contents of graphene nanoparticles were conducted to study the effects of nanomodification on the scaling. It is shown that, while the strength of the pristine polymer scales according to Linear Elastic Fracture Mechanics (LEFM), this is not the case for nanocomposites, even for very low graphene contents. In fact, small specimens exhibited a more pronounced ductility with limited scaling and a significant deviation from LEFM whereas larger specimens behaved in a more brittle way, with scaling of nominal strength closer to the one predicted by LEFM. This behavior, due to the significant size of the Fracture Process Zone (FPZ) compared to the specimen size, needs to be taken into serious consideration. In fact, it is shown that, for the specimen sizes investigated in this work, neglecting the non-linear effects of the FPZ can lead to an underestimation of the fracture energy as high as 113%, this error decreasing for increasing specimen sizes. A study on a large bulk of literature data confirmed that this is not a salient feature of polymer/graphene nanocomposites only but also of several other nanocomposites. It is shown that most of the specimen sizes investigated in the literature belong to the transitional region between ductile and brittle behavior where LEFM cannot characterize the fracturing behavior of these nanocomposites. In such cases, neglecting the non-linear effects of the FPZ can lead to an underestimation of the fracture energy as high as 156.8%, the underestimation being more significant for smaller specimen sizes and higher weight contents of nanofiller.
Atomic Resolution Imaging and Analysis of Microstructures and Interface of Aligned Carbon Nanotube Composites
Higher concentrations of long carbon nanotubes (CNTs) that are aligned and functionalized can improve the mechanical properties of CNT composites. For this purpose, pristine/random CNT sheets were mechanically stretched at different strains of 40 %, 60 % and up to 80 %. The higher degree of CNT alignment was confirmed by scanning electron microscope (SEM) images and the degree of alignment was quantified by X-ray scattering and polarized Raman spectroscopy. The highest degree of alignment of 0.93 was achieved by using this method. Aligned CNT sheets were functionalized and CNT/bismaleimide (BMI) composites were fabricated. The mechanical properties of the composites were improved with a higher degree of alignment achieving a modulus of 252 GPa and tensile strength of over 1.4 GPa for an 80% stretched CNT/BMI composite. To understand the internal CNT packing and the CNT/resin interface, thin composite cross sections that are perpendicular to the alignment direction were prepared using focused ion beam (FIB) with a lift-out process. Using a high-resolution transmission electron microscopy (HRTEM), relatively large diameter CNTs (> 5 nm) cross sections and double-walled CNT bundles were observed. An increased number of densely packed and collapsed CNT structures were also observed with higher stretch ratio, which confirmed the CNT alignment and dense packing along the stretch direction. The CNT/resin interface was also analyzed by electron energy loss spectroscopy (EELS). Carbon K-edge peak was used to identify the functionalization of CNT, and a ratio of 1s-* to 1s-* transition was mapped by a scanning TEM. A decreased */* transition ratio was observed at the edge of CNT packing and could be related to the reduced sp2 bonding after functionalization. This leads efficient load transfer between CNT and resin, thus higher mechanical properties.
Interactions between Different Nanoparticles Used as Binary Reinforcements in Polymer Nanocomposites
Interaction between commonly used nanoparticles as reinforcement in epoxy and their subsequent influence on properties of diglycidyl ether of bisphenol A epoxy SC- 780 resin system was studied. Montmorillonite nanoclay NanomerÂ® I.30E (MMT) and hexagonal boron nitride (hBN) were combined at different concentrations with amino functionalized graphene nanoplatelets (GNP) into two binary systems; (a) 5 wt. % hBN and 0.1 wt. % GNP; and (b) 3 wt. % MMT and 0.1 wt. % GNP. Effect of each binary system on mechanical, viscoelastic and thermal stability properties of SC-780 epoxy composite was investigated and results compared to that of unmodified and composites with identical loading of individual nanoparticles. Mechanical, viscoelastic and thermal properties were characterized using three-point flexure test, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) respectively. Binary nanoparticles infusion resulted in nominal increase in flexural strength, however, significant increase in flexural modulus was realized. Storage modulus however increased up to 9% for hBN/GNP samples and remained statistically unchanged in MMT/GNP samples. There was slight reduction in glass transition temperatures in all nanocomposites compared to unmodified system. All nanocomposites showed enhanced dimensional stability at elevated temperatures compared to unmodified epoxy composite.
Investigation of Stretch Ratio and Pre-Stress Condition on Electrospun Nanofibers for Composite Reinforcement
In this study, we investigate stretching and pre-stress conditions during stabilization of electrospun nanofibers on mechanical properties of yarns. The nanofibers are produced from 10% polyacrylonitrile (PAN) in N, N-dimethylformamide (DMF) solution using an environmentally controlled electrospinning set-up. As-spun nanofibers are mechanically stretched at different stretch ratio (Î» = 1-3) and subsequently stabilized at 260Â°C in air for 180 min. During stabilization the nanofibers were subjected to pre-stress in the range between 0-5 MPa. Fiber diameter distributions are investigated by scanning electron microscope (SEM) while tensile properties are measured by dynamic mechanical analyzer (DMA). It has been found that mechanical stretching significantly improved the tensile strength of both as-spun and stabilized nanofiber while decreasing average fiber diameter. Application of pre-stress during stabilization has significant effect on improving the mechanical properties of the nanofibrous yarns.
Nanofiller Geometry Effects on Electrical Properties of Composites
Nanofillers with highly anisotropic shapes, such as carbon nanotubes, graphene nanoplatelets, carbon black, and metallic nanowires are used as inclusions in polymer matrix materials to generate nanocomposites with superior electrical, mechanical, and thermal properties. In this paper, we report on our recent and ongoing studies focusing on the enhanced effective electrical conductivity of such composites. First, we report on Monte Carlo simulations of systems of polydisperse prolate and oblate ellipsoids using the critical path-based tunneling-percolation mode. For polydisperse prolate ellipsoids, the critical percolation volume fraction, c ï¦ , is shown to have a quasi-universal dependence on the weight-averaged aspect ratio. For polydisperse oblate ellipsoids, c ï¦ is shown to have a quasi-universal dependence on the apparent aspect ratio, which is a function of up to fourth moment of the size distribution, as given by percolation theory. In both cases, the function approaches the theoretical predictions for higher volume fractions and higher aspect ratios. The model predictions are then compared with experimental data to estimate the tunneling length scale which is found to be within the expected range. Next, we examine the effect of filler alignment on percolation behavior of nanocomposites using Monte Carlo simulations of monodisperse prolate and oblate hard-core soft-shell ellipsoids representing carbon nanotubes and graphene nanoplatelets, respectively. As expected, the percolation threshold is observed to increase with increasing extent of alignment. For a highly aligned system of rod-like fillers, the simulation results are shown to be in good agreement with the second virial approximation-based predictions. However, for a highly aligned system of disk-like fillers, the second virial approximation-based results are observed to significantly deviate from the simulations, even for higher aspect ratios. The anisotropy in percolation threshold is found to vanish with increasing system size even for highly aligned systems of fillers.
Temperature Influence on the Shear-induced Alignment of Carbon Nanotubes in a Polymer Composite via Real-time Polarized Raman Spectroscopy
The temperature influence on carbon nanotube (CNT) rotation in a shear alignment experiment is studied through real-time polarized Raman spectroscopy. The CNT/epoxy suspensions were aligned at shear rates of 0.01-0.1 s-1 at a temperature range of -10 to 21ÂºC. Increasing the temperature was found to reduce the degree of alignment but has no impact on the speed of CNT rotation. The results were compared with the observations of alignment in an electric field and revealed different controlling mechanisms between the two alignment methods.
Improving the Understanding of Tufted Energy Absorbing Sandwich Structures
Tufting has been shown to be an effective method for improving the performance of sandwich structures against edgewise impact loading. However, the mechanisms that take place during failure, and the role the tufts play within this are not well characterised. Experimental testing has therefore been carried out to improve the understanding of the failure behaviour of these structures, with the particular focus of this study being on investigating the behaviour of the tufts themselves. Firstly quasistatic edgewise crushing tests were carried out to visually identify the failure behaviour of the tufts, indicating that the tufts â€˜driftâ€™ and collide as they fail. Follow up testing was then carried out on the interaction between the tufts and the surrounding foam core. Whilst it was observed that single tufts only have a very small contribution to the overall energy absorption of a tufted panel, this contribution can increase significantly with the addition of multiple tufts, and allowing interactions to occur between them. This behaviour could therefore be designed into the structure to help maximise the energy absorption during an impact event.
Effects of Foam Density and Rise Ratio on 3D Mechanical Properties of PVC Foam under Different Loading Modes and Directions
Mechanical properties of rigid PVC foam are affected by material physical and structural parameters, such as foam density and cell microstructure. The effect of foam density on its mechanical properties has been investigated by many investigators; however, the effect of microstructure remains to be clarified. The authors previously examined out-of-plane compressive modulus and strength affected by density, cell rise ratio and their interaction in Divinycell H100 PVC foam. In this paper recently developed accurate foam test methods were used to investigate the important effects of foam density and cell rise-ratio on stiffness properties of PVC foams under different loading modes and along principal material directions. From an as-received large PVC foam panel with measurable density variation, foam compression, tension and shear test specimens with different densities and along different directions were prepared. For compression tests, straight-side specimens were used. In tensile tests, dumbbell shape specimens were fabricated by adhesively- bonded prismatic foam pieces. Shear tests were conducted with a scale-down ASTM standard test method. Foam density was determined for each test sample. The average foam cell aspect ratio (i.e., rise ratio) was determined with optical microscopy. Relationships of cell rise ratio to foam density were found to be different for the out-of-plane and the in-plane test specimens. The experimental results showed that the dependence of foam compressive, tensile and shear stiffnesses on density and cell rise ratio is different along in-plane and out-of-plane directions. The results reveal the important combined effect of foam density and cell rise ratio on stiffness properties in different loading modes and directions. Previously developed foam stiffness models based on composite micromechanics were used to correlate and predict the experimentally determined stiffness under different loading modes and directions.
Analysis of In-Plane Mechanical Response of Hexagonal Honeycomb Core: Effect of Node Bond Adhesive and Proposed Analytical Method
In this study, the development of an analytical model to predict the homogenized in-plane stress-strain behavior of hexagonal honeycomb core under large deformations is presented. Unlike in traditional models, the detailed geometric features of a commercial hexagonal honeycomb core cell such as cell wall curvatures, node bond adhesive layer, and node bond fillets are included in the analysis. In the analytical model, the honeycomb core is modeled to be of unit thickness under plane-strain conditions, and classical beam theory is applied to model the behavior of the cell walls. The homogenized stress-strain curves predicted by the analytical model for uniaxial tension loading along the transverse and ribbon directions are compared with test data for a commercial hexagonal honeycomb core (Hexcel HRP-3/8-4.5), as well as the predictions from a finite element model. The predictions of the proposed analytical model are in good agreement with the test data and predictions of the finite element model.
Axial Fatigue Characterization of Core Joints in Sandwich Composite Structures
Composite sandwich structures are currently being employed in a variety of structural applications where high strength/stiffness to weight ratio is critical. Applications include space structures, bridge decks, marine, train and truck flooring, wind turbine blades etc. Core joints exist in all these composite large structures and special attention with regards to the effect of core joint on the fatigue performance of sandwich structure is of interest to composite engineers. When core joints in composite sandwich structures loaded in-plane, mismatched Poissonâ€™s ratio between the joint and the core generates out-of-plane stresses (interlaminar) at the core joint/facesheet interface, creating a stress riser at this location that will eventually affect the fatigue life of the structure. Quantifying this effect and its consequence on the service life and reliability of the composite structures are not well developed. This experimental study characterizes the effect of different core joint designs in composite sandwich structures. Static, tension-tension and tension-compression fatigue tests with constant load amplitudes at varied frequencies were applied to describe the fatigue life prediction of composite sandwich structures with core joints. Post-Fatigue tensile tests were also performed to predict the residual strength of such structures. Nondestructive Evaluation Techniques (DIC, SEM and AE) were used to locate the stress concentrations and justify the damage mechanism. The experimental results show that the H100 and H200 scarf core joint designs have higher fatigue strength than the H100 and H200 butt core joint designs at low and high cycle fatigue regimes. The suggested scarf core joint design increased the axial stiffness of H200 sandwich panels by 21%. The traditional butt core joint design showed rapid decrease in stiffness causing an instant failure to the H100 and H200 specimens. A logical blend of experimental and analytical prediction of the service life of composite sandwich structures is carried out. This research study will benefit composite engineers and joint designers in both academia and industry to better apprehend the influence of core joints and its consequence on the functionality of sandwich structures.
Diffusion Behavior in Sandwich Composites: Modeling and Experiment
This study examines the moisture sorption behaviors in neat epoxy, glass fiberreinforced/ epoxy composites, and polyurethane foam core immersed in deionized water at 50 Â°C. In addition, the moisture diffusion in sandwich composites comprising of the above constituents is also studied. Diffusivity coefficients are first calibrated from experimental data. The classical Fickian diffusion model was applied to simulate the moisture sorption of liquid in the polymer composites. For fiber-reinforced polymer composites, the Springer diffusion model was considered. A finite difference method is used to solve for the classical Fickian diffusion model and the Springer diffusion model. The Fickian model is used to predict the one-dimensional moisture diffusion in thin plates and the multi-axial three-dimensional moisture diffusion in foams. With the calibrated diffusivities for the above constituents, the diffusion behavior in sandwich composite is predicted.
Progressive Failure Analysis of Composite Countersunk Bolted Joints Tests Using an Energy Based Continuum Damage Model
Progressive failure analysis of composite bolted joint specimens with countersunk fasteners is presented. The progressive failure analysis uses an energy based continuum damage model for material stiffness degradation under three-dimensional states of stress. The material model is implemented as a user defined material UMAT subroutine for use with the ABAQUSâ„¢ finite element software. The developed methodology was validated using open-hole tension tests results, then used to predict progressive failure of bolted joints. Model predictions are compared with previously obtained experimental test results. The results indicate the 3-D CDM model can predict the load to failure in the countersunk bolted joints.
A Hybrid Approach to Composite Failure Combining Synergistic Damage Mechanics and Peridynamics
Synergistic damage mechanics (SDM) is combined in this work with peridynamics (PD) to create an efficient and powerful hybrid approach for modeling failure in composite materials. The damage descriptors generated in the SDM enter the PD approach for failure analysis. The evolution of damage in composite materials occurs in stages going from the early failure events that are precursors of ply cracks, to multiplication of the ply cracks, to delamination, and fiber failure. The early failure events are found to be sensitive to the manufacturing induced defects and are therefore analyzed in a statistical simulation framework that constructs representative volume elements (RVEs). Energy based criteria are employed to evaluate initiation of fibermatrix debonding within the RVE under thermal cooldown followed by mechanical loading. Then, the process of ply crack formation is analyzed by investigating the kink-out and linkage of the debond cracks. A new peridynamic model for composites utilizes the statistical information from the SDM approach to simulate the coalescence of damage and growth of macrocracks. Based on an idea first introduced to provide an intermediate step in the homogenization process of peridynamic models of porous materials, the model for composites can easily integrate manufacturing defects into the simulation of failure. Simulations of crack growth through full failure of a unidirectional laminate with various fiber orientations are shown and future steps are described.
1D-Patterned Nanocomposites Structured Using Oscillating Magnetic Fields
A capability to organize and tailor nanofillers in bulk is a key, but currently missing, technology for manufacturing of polymer nanocomposites (PNCs) with effective property enhancement. Polymer nanocomposites, particularly those with carbon nanotube (CNTs) in thermoset matrices, are of high interest in aerospace engineering applications due to mass-efficient multi-functional properties; improved fracture toughness, thermal stability and management, and current carrying capacity are critical for aircraft and satellite structures to operate in extreme environments. When nanofillers are poorly dispersed, nanocompositing can result in ineffective enhancement or even degradation of PNCsâ€™ properties. In addition to good dispersions, anisotropic nanofiller organization can be advantageous to tailor anisotropic properties. Thus in this work, 1D organization of nanofillers using oscillating magnetic fields is studied as a solution to scalable manufacturing of PNCs while maintaining the quality of nanofiller organization within the matrix, with the use of a simple, energy-efficient setup. This work consists of three objectives: 1) to understand nanofillers movement in matrix suspension under oscillating magnetic fields, 2) to develop a capability to control 1D patterning of nanoparticles (alignment line length, width, and separation), and 3) to fabricate and characterize polymer nanocomposites and interface effects using magnetic assembly. This work is supported on the BAA (#N000141612172) and DURIP (#N000141712023) funds from the ONR Sea-Based Aviation Airframe Structures and Materials Program. The research focus of this past period was on the second and the third objectives.
Modeling and Simulation of Environmental Impact Damage of Aerospace Composites and Its Detection Scheme
Advanced composite materials are being used extensively in many commercial and military aircraft and rotorcraft due to their superior stiffness and high strength-to-weight ratio. These composite materials are subjected to unexpected environmental impact loads at high speeds, such as bird strikes and hail stone impacts. The damages developed in such cases is primarily in the form of delaminations and matrix cracking, known as barely visible impact damage (BVID), since they are not easily observable by the naked eye. These environmental impact damages can severely compromise the overall strength of the composite structure thereby reducing the maximum load it can withstand. Prediction of such environmental impact damages, which can occur in the service life of the aircraft, are of paramount importance in the aerospace industry. This paper demonstrates the use of the smoothed particle hydrodynamics (SPH) modeling method to simulate the extreme deformation of the impactor (hailstone and bird) along with the composite plate. Since the physical testing of the aforementioned impact scenarios is extremely expensive and time consuming, the SPH modeling approach can be used as a cost-effective tool in the design of composite structures. This paper presents the results of simulated hailstone impact and bird impact occurring on an 8-layer unidirectional carbon fiber reinforced polymer (CFRP) plate. This paper also proposes a non-invasive microwave non-destructive testing (NDT) technique that could be employed as a detection methodology to detect the simulated impact damages. Radar signals operating in the X-band microwave frequency range (8â€“12 GHz) could be used to detect the delaminations occurring in between the layers of the unidirectional CFRP plate to obtain high-resolution 2D images of the impact damages.
Design and Performance of a Microwave Nondestructive Testing System for Damage Analysis of FRP Composites
Fiber-reinforced polymer (FRP) composites are known for their outstanding strength, stiffness, corrosion resistance, and light weight nature, which are used in such fields as aerospace, marine, automotive, and construction industries. Although, FRP composites have many advantages, as compared to metals, defects may occur during the production process or during the service life due of the laminated structure. Therefore, there is a growing demand for nondestructive testing (NDT) approaches that can pre-detect such defects before they become critical and cause failure. Radio frequency (RF) signals are able to penetrate dielectric materials with relatively low loss, which make microwave NDT an attractive solution among the other numerous inspection techniques. This paper presents the theory and development of an ultrawideband (UWB) microwave imaging system, covering 8.2 to 12.4 GHz frequency band, based synthetic aperture radar (SAR) algorithms designed to detect damages and defects within the FRP structures.
Detection of Kissing Bonds with Laser Speckle Imaging
Composite materials are widely used in aircraft to reduce manufacturing costs, improve structural performance, and boost fuel efficiency. However, safety concerns arise because of the susceptibility of these composites to inadequate adhesive bond quality, including so-called â€œkissing bondsâ€ which may occur because of initial fabrication or service-related issues. Such weakened bonds may have occurred due to contamination at the bondline surface or in the adhesive, imperfect adhesive thermal cure, or inadequate mixing of adhesive constituents. Because of the widespread and rapidly growing use of composite materials in military aircraft, there is a need for an easily used detection approach to routinely monitor the health of composite materials, both in the factory and in deployed aircraft. To address this need, we have developed a new optical/acoustic approach to test bond quality in composite materials. Lamb wave vibrations are induced in the sample with a high frequency transducer and are visualized using motion-contrast laser speckle imaging with a fast-framing commercial camera. In measurements conducted on test panel sandwiches prepared from SGP370-8H 8552 fiber/epoxy plates and epoxy adhesive, the processed video imagery reveals information on the presence and location of bond flaws in the adhesive layer.
Investigation of Delamination Initiation and Growth Behavior at Fastener Locations in Primary Composite Structures
Current maintenance practice dictates that aircraft are routinely inspected after performing a fixed number of flight hours to mitigate risks from damage that grows during operation. This research aims to support maintenance operations and fleet management, and also to help improve aircraft design practice, by investigating delamination initiation and propagation in the vicinity of fastener holes within fiberreinforced composite materials. Static and fatigue bearing tests were performed on modified countersunk double lap shear (DLS) and single lap shear (SLS) test configurations. DLS static test results presented damage onset that occurred at a lower bearing stress level but higher ultimate stress compared to the SLS static test. The fatigue bearing test data showed that when the bolted hole elongates, stiffness decreases and damage area growth becomes detectable through C-scan. From microscopy observations, matrix cracking and compression-induced fiber kinking were found to occur in the straight shank region of the hole. Complex damage morphology forms in this region, emanating from the loaded bearing face, creating large wedge-shaped regions that act to drive delamination propagation with additional loading cycles. Furthermore, these results indicate that the straight shank region of the countersink-head bolt hole carries most of the bearing loads. For the SLS test configurations with fullytorqued bolts, damage beyond the countersink conical region was not detected. Thus, bolt pre-load adds out-of-plane constraints that act to prevent damage from growing during fatigue tests.
Damage Interactions in CFRP Composites under High Cycle Fatigue
This study provides a computational investigation of the interaction of damage mechanisms in carbon fiber reinforced polymer (CFRP) laminated composites subjected to fatigue loading. The investigation particularly focuses on [60,0,-60]3S laminates, for which the behavior clearly demonstrates strong interactions between the relevant damage modes. The numerical studies are performed using a multiple spatiotemporal homogenization based multiscale life prediction framework. The model parameters pertaining to the material constituents are calibrated using a suite of experimental data from static and fatigue response of unidirectional laminates. The analysis results indicate a strong interactive effect of subcritical damage mechanisms on the fatigue survivability of laminated composites under high cycle fatigue loading.
A Stabilized Finite Element Method for Modeling Mixed-mode Delamination of Composites
Delamination of composite materials is commonly modeled using intrinsic cohesive zone models (CZMs), which are generally incorporated into the standard finite element (FE) method through a zero-thickness interface (cohesive) element; however, intrinsic CZMs exhibit numerical instabilities when the cohesive stiffness parameters is assumed to be large relative to the elastic stiffness of the composite material. To address this numerical instability issue, we propose a stabilized finite element method by combining the traditional penalty method with the Nitscheâ€™s method that is equally effective for any specified initial stiffness of the cohesive (traction-separation) law. The key advantage of the proposed method is that it generalizes the Nitscheâ€™s method to any tractionseparation law with arbitrary large values of initial stiffness and provides a unified way to treat cohesive fracture problems in a variationally consistent and stable manner. We implemented the stabilized method in the commercial finite element software Abaqus via the user element subroutine and simulated benchmark tests for mode I and mixed-mode delamination in isotropic materials to establish the viability of the approach. Ongoing work is aimed at extending the method to model delamination in transversely isotropic laminated composites.
Aspects of Prepreg Layer Friction on Defect Formation on Thick-Walled Composite Manufacturing
As composite manufacturing processes evolve and composite structures become ever more complex the role of geometry and built-in geometric features increase the role of ply movement during mold closure and compaction. With a greater amount of constraints and ply movement, the friction that develops between the prepreg plies and the tooling surfaces and/or between one ply to another become a significant force which is a factor in formation of defects such as ply wrinkles. As such a greater understanding of ply friction and its implementation into process modeling is justified. In this work Hexcel IM7/8552 8HS, is characterized for ply friction using a custom characterization rig. The rig compresses a layer of prepreg onto a tooling surface with a prescribed pressure and heats the prepreg layer in accordance to an autoclave heating cycle while pulling the tooling surface out in a testing frame capable of measuring the required force to do so. Current simulation tools for modeling the autoclave process in order to predict outcomes such as a structures spring-in due to manufacturing utilize only simple models for this frictional behavior. This work uses an approach with currently available tools to implement experimentally determined ply friction data for modeling the manufacturing process using user subroutines. Results of simulation predicts that compression along the fiber direction during the compaction stage of the autoclave cycle is concentrated in the radius region which is experimentally shown to cause ply wrinkling. In conclusion this work will present various aspects of ply friction characterization, coupon level experimentation with curved beam samples, and process modeling.
Multiscale Analysis of Environmental Degradation in Composite Materials
There are variety of factors affecting degradation of composite materials due to environmental effects. In the present manuscript, two sources of degradation are studied. We first consider an accumulation of damage in carbon-fiber/epoxy-resin material system subjected to cyclic load. A space-time multiscale approach is developed wherein carbon fiber is modeled by continuum damage model whereas damage accumulation in epoxy is modeled by a hybrid fatigue damage-plasticity model. We then consider degradation of glass-fiber/Nylon material system due to moisture accumulation. A multiphysics-multiscale approach is developed by which a coupled diffusion-reaction-mechanical process is simulated at multiple spatial scales.
Silsesquioxanes with Phenylethynyl Terminated Oligoimides
The incorporation of cage-like silsesquioxanes (SQ) to form polymers has demonstrated property enhancements in areas such as: thermal and mechanical characteristics, flame retardance, dielectric properties, and oxidative resistance. However, with most of hybrids investigated, the attached SQs are pendant with respect to the reaction site, thus mechanical performance did not take full advantage about the rigid nature of SQ core. A recently developed class of these nano-structured, cage-like silsesquioxanes, formally known as double decker silsesquioxanes (DDSQ), offers the opportunity to form hybrids with the SQ cage as the bridge between multiple reaction sites. This advancement provided a unique opportunity to form networks with Si-O core connecting those reaction junctions, which may positively affect the mechanical performance on thermosetting polymers. It was discovered during the capping reaction, these functionalized DDSQs generate cis and trans isomers with respect to the 3D Si-O core. In addition meta- and para- functional groups may be used to form hybrid reacting monomer mixture containing six different isomers. Depending on the relative ratio of isomers in the mixture, viscosity can be well controlled with no effect on the reaction mechanism and kinetics. Therefore, it is expect to improve the processing of high temperature oligoimide resins while improve thermoxidative stability (TOS) of organic networks. Synthesis details, different methods of incorporating DDSQ mixtures and their TOS performance were presented.
Quality-Driven Definition of Composite Materials and Structures
A general framework for the probabilistic definition of laminated composite materials and structures is proposed and justified according to quality-driven analysis of typical load-bearing thick aircraft composite components. The approach and its computational implementation are described. Specific variants of the developed probabilistic model are considered for typical laminated composite structures, including variants for 2D and 3D statements of analysis. Representative quantitative examples demonstrate simplicity of the numerical implementation and the interpretation of generated data. In addition, a direct link between the proposed probabilistic definition and structural response of composite materials is outlined. Its quantitative demonstration is illustrated on examples of typical laminated carbon-fiber reinforced composite structural elements under bending load.
An Integrated Numerical Model for Predicting Flow and Compaction of 2D Textile Composite in the VARTM Process
The vacuum assisted resin transfer molding (VARTM) technique is a low-cost alternative to the autoclave process suitable for manufacturing large-scale composite parts. During the VARTM process, carbon fiber sheets are covered by a vacuum bag, and a vacuum pump is utilized to assist the resin flow in the infiltration process. However, when dealing with carbon fiber fabrics with complicated microstructure such as 2D textile fabrics, the gaps between the fiber tows are sometimes not fully saturated by the resin flow, creating starvation areas that potentially compromise the mechanical performance of the composite part. In addition, during the resin infusion process, the cross-linking reaction is activated under the preheated condition, making the material properties of resin both temperature and degree of cure (DOC) dependent. Therefore, the key is to establish a multi-physics model that incorporate heat transfer analysis and cure kinetics into the flow compaction model. In this paper, an integrated flow compaction model is developed to predict the flow front advancement inside the textile fabrics during the VARTM process. The flow velocity and pressure distribution inside the resin flow are governed by the continuity equation and Darcyâ€™s law. Due to the fact that the behavior of the resin flow is influenced by the local permeability, which in return is a function of fluid pressure, the flow model and the compaction model are coupled, and they are solved simultaneously to determine the flow behavior of resin inside the fiber fabrics. In the meantime, the viscosity model of the resin is incorporated into the flow compaction model through the material properties of the resin to account for the effects of infusion parameters. Using this flow compaction model, the permeability of the carbon fabrics used in this study is determined by comparing the flow front predicted by the flow compaction model to that observed in the radial injection experiment. Moreover, once the fluid distribution is determined, the local fiber volume fraction and thickness variation of
Effect of Alignment and Sizing on Mechanical Properties of Discontinuous Recycled Carbon Fiber Composites
The exceptional combination of properties offered by carbon fiber composites has propelled their incorporation into high performance markets such as aerospace, wind energy, and luxury cars and sporting goods. However, the high price and energy cost associated with carbon fiber production acts as a significant barrier of entry into larger volume markets, such as the automotive industry. Recycled carbon fiber, reclaimed from carbon fiber composites, can potentially provide comparable mechanical properties to those of virgin carbon fibers at a fraction of the cost. However, the recycling process tends to alter the fiber surface and typically produces discontinuous fibers in loosely packed, fluffy clumps, which require further processing before composite remanufacturing is feasible. In this study, the effect of sizing and alignment on the mechanical properties of recycled carbon fiber composites was investigated. Composites containing Toray T800 carbon fibers were recycled using a wet chemical recycling method (by Adherent Technologies Inc.), and a polymer sizing was applied to a portion of the fibers. The surface of these fibers was characterized with scanning electron microscopy (SEM) and x-ray photoelectron microscopy (XPS) to determine the level of damage and functional groups resulting from the recycling and sizing processes. The mechanical properties of the recycled fibers were measured using single fiber tensile tests, and the interfacial shear strength was found using single fiber fragmentation tests. The fibers were then processed into aligned and randomly oriented mats that were manufactured into composites using either resin transfer molding (RTM) or wet layup. The effect of sizing, alignment, and manufacturing method are then assessed with respect to the composite tensile properties.
Computational Modeling and Comparison with Experiments on 3D Permeability in Vacuum Assisted Resin Infusion of Glass-Fabric Reinforced Composite Laminates
Vacuum-assisted resin infusion molding has been increasingly used in manufacturing large composite structures, such as MW-scale wind turbine blades, due to structural size, cost-effectiveness, and out-of-autoclave curing requirements. Among various material and processing parameters, 3D permeability of composites with thicksection laminate construction is critical in controlling resin flow in an efficient manufacturing process development by accurate numerical modeling and simulation. Reactive resin flow in a mold with complex geometry and material systems, such as a wind turbine blade with a foam core and multiple fabric orientations, is strongly affected by the 3D permeability of the fabric composite. An experimental method to measure the orientation and the magnitude of individual principal component of permeability tensor for thick-section fabric reinforcements has been addressed in a previous study. The objectives of this research are to: (1) use the 3D permeability measurements from the previous experimental study as inputs into ANSYS Fluentâ€™s volume-of-fluid model to simulate flow front progressions, (2) compare the simulated flow front progression with experimental results, and (3) identify the sources in the experimental and numerical methods that contribute to the discrepancies between experimental and numerical results. A brief review is provided on the 3D permeability measurement theory and the experimental method and results from the previous study. Numerical simulations of the flow front progression of a Newtonian fluid into an isotropic porous media are first compared to analytical results to select the computational models and discretization schemes used in this study. The choice of time-step and mesh discretization is discussed along with a convergence study. Insight into the limitations of the 3D permeability measurement theory and numerical methods is provided by comparing simulation results with circular and spherical inlets. Suggestions are made on methods to overcome incompatibilities of assumptions on inlet geometry between the in-plane and throughthickness permeability measurement theories.
Prepreg Platelet Morphology and Scale Effects on Molding Processability
Unidirectional, prepreg platelet-based molding systems have gained considerable attention for manufacturing parts with moderate performance and processability metrics. A numerical study of squeeze flow, using a recently developed technique for coupled flow simulation and fiber orientation analysis, has been performed to investigate the effects of the platelet morphology on the processing of prepreg platelet based composite molding systems. Two competing effects have been shown to drive the processability of such a material: (i) increasing extensional viscosity due to increasing platelet length and (ii) increasing apparent viscosity due to increasing uniformity of orientation distribution. By adjusting the platelet dimensions, the above phenomena have been isolated in the numerical simulations to demonstrate their relative importance on the processability of platelet based molding systems.
Verification and Validation of Residual Stresses in Simple Composite Structures
Process-induced residual stresses commonly occur in composite structures composed of dissimilar materials. These residual stresses form due to differences in the composite materialsâ€™ coefficients of thermal expansion and the shrinkage upon cure exhibited by polymer matrix materials. Depending upon the specific geometric details of the composite structure and the materialsâ€™ curing parameters, it is possible that these residual stresses could result in interlaminar delamination or fracture within the composite. Therefore, the consideration of potential residual stresses is important when designing composite parts and their manufacturing processes. However, the experimental determination of residual stresses in prototype parts can be time and cost prohibitive. As an alternative to physical measurement, it is possible for computational tools to be used to quantify potential residual stresses in composite prototype parts. Therefore, the objective of this study is the development of a simplistic method for simulating the residual stresses formed in polymer matrix composite structures. Specifically, a simplified approach accounting for both coefficient of thermal expansion mismatch and polymer shrinkage is implemented within the Sandia National Laboratoriesâ€™ developed SIERRA/SolidMechanics code Adagio. Concurrent with the model development, two simple, bi-material structures composed of a carbon fiber/epoxy composite and aluminum, a flat plate and a cylinder, are fabricated and the residual stresses are quantified through the measurement of deformation. Then, in the process of validating the developed modeling approach with the experimental residual stress data, manufacturing process simulations of the two simple structures are developed and undergo a formal verification and validation process, including a mesh convergence study, sensitivity analysis, and uncertainty quantification. The simulationsâ€™ final results show adequate agreement with the experimental measurements, indicating the validity of a simple modeling approach, as well as a necessity for the inclusion of material parameter uncertainty in the final residual stress predictions.
Numerical Analysis of Steel-concrete Composite Beams Strengthened with Pre-stressed CFRP Plates
Applying pre-stressed Carbon Fiber Reinforced Polymers (CFRP) plate to steelconcrete composite beams is considered an active strengthening technique that can create permanent internal stresses in the beam opposite to the internal straining actions due to the service loads. This study presents a numerical analysis of steelconcrete composite beams stiffened with pre-stressed CFRP plates. The pre-stressing effect on CFRP plate is enforced in the finite element (FE) model as an initial stress on the solid element that used to simulate the strengthening plate and the epoxy is considered as a thin layer between CFRP plate and the steel flange. The deterministic and stochastic analysis is performed to obtain the long-term random responses of the strengthened beams as well as plain beams due to the effect of shrinkage. In addition, the effect of CFRP plate rigidity and strengthening schemes are investigated. By using CFRP plates, strengthening can be achieved up to 25% for the system explored. During the post-elastic range, stiffness is improved when adding pre-stressed CFRP plates and deflection of strengthened beams is reduced by 20% in comparison to the reference beam at the ultimate capacity. Also, a slight drop in ductility occurs with the addition of CFRP plates.
Polynomial Chaos Characterization of Uncertainty in Multiscale Models and Behavior of Carbon Reinforced Composites
Design of non-crimp fabric composites (NCF) entails major challenges pertaining to (1) the complex fine-scale morphology of the constituents, (2) the manufacturingproduced inconsistency of this morphology spatially and across the layers, and thus (3) the ability to build reliable, robust, and efficient computational surrogate models to account for this complex nature. Traditional approaches to construct computational surrogate models have been to average over the fluctuations of the material properties up along the scale length. This fails to account for the fine-scale features and fluctuations in morphology, material properties of the constituents, as well as fine-scale phenomena such as damage and cracks. In addition, it fails to accurately predict the scatter in macroscopic properties, which is vital to the design process and behavior prediction. In this paper, we present an approach for addressing these challenges with minimum assumptions, by relying on polynomial chaos representations of both input parameters and material properties at different scales. Moreover, we emphasize the efficiency and robustness of integrating the polynomial chaos expansion with multiscale tools to perform multiscale assimilation, characterization, propagation, and prediction, all of which are necessary to construct the data-driven surrogate models required to design under the uncertainty of composites. These data-driven constructions provide an accurate map from parameters (and their uncertainties) at all scales and the system-level behavior relevant for design. While this perspective is quite general and applicable to all multiscale systems, NCF composites present a particular hierarchy of scales that permits the efficient implementation of these concepts.
Approach of Probabilistic Characterization of Progressive Damage Process in Laminated Composite Materials
This work is motivated by demanding reliability requirements exemplified by ultra-low failure probabilities for aircraft composite structures designed according to damage tolerance criteria. The main objective of this work is the development of an experimental probabilistic implementation to characterize progressive damage in laminated composite components. This is demonstrated as a stochastic process on examples of typical thick laminated composite beams under bending load conditions. The developed approach is primarily focused on laminated composite materials and structures with predominantly interlaminar damage modes. It considers both the random damage initiation and growth of multiple interlaminar damages. Demonstration of the approach is considered on an example of thick commerciallyavailable carbon-fiber IM7/8552 laminated composite beams under multi-point bending loads. Significant variability of damage patterns is illustrated. Finally, preliminary comparison of generated statistical test results with corresponding independent probabilistic modeling predictions is performed and discussed.
Electrical Resistance-Based Damage Sensing of Carbon Fiber-Reinforced Composites Utilizing the Concept of Effective Conducting Thickness
In this paper, finite element models were developed in ABAQUS for studying damage in carbon fiber-reinforced polymer matrix (CFRP) specimens based on the 4- probe electrical resistance method. The concept of effective conducting thickness was leveraged in the development of the electrical finite element models. The 4-probe electrical models were created assuming a linear current-voltage relationship and the damage response was recorded for CFRP materials with varying thickness and layup. Damage was modeled with prescribed shape and location in order to represent failure modes of fiber breakage, matrix cracking, and delamination. Fiber breakage and matrix cracking were identified using the top resistance measurement plane. The top resistance measurement was generally found to be more sensitive to fiber breakage compared to matrix cracking. It was demonstrated, however, that several factors affect the sensitivity of the response to damage including: (1) orientation of the fibers relative to the line of current probes, (2) location of the damage, and (3) type of damage. The material properties were then theoretically optimized such that Ohmic response was achieved throughout the full thickness of the CFRP specimen. With the use of optimal electrical conductivity, the model demonstrated improved sensitivity to fiber breakage near the top and bottom surfaces of the specimen.
On the Effect of Electrical Impedance Tomography Error and Regularization Norms for Damage Identification in Piezoresistive Composites
Electrical impedance tomography (EIT) is a promising tool for the structural health monitoring (SHM) of composites that have been modified to be piezoresistive by the addition of carbon-based nanofillers. However, most studies have formulated the EIT problem to minimize an error vector in the least-squares sense while simultaneously using a least-squares term for regularization. This approach has important limitations in the context of SHM such as being extremely sensitive to outlier data due to damaged or faulty electrodes. Utilizing a least-squares term for regularization also makes EIT unable to image discontinuous conductivity losses such as those induced by fracture events. More sophisticated techniques that surmount these limitations have been studied in medical and mathematical venues, but these methods have not been thoroughly explored for piezoresistive imaging in SHM. Therefore, this article explores the effect of different error minimization and regularization norms on the ability of EIT to image impact damage in a carbon black (CB)-modified glass fiber/epoxy laminate.
Piezoelectric Zinc Oxide Nanowires for Use in Acoustic Emission Testing
Acoustic emission testing has proven useful for real-time damage detection in a variety of situations, however the need for externally bonded sensors limits the feasible applications. This work investigates an alternative approach to traditional acoustic emission testing by using piezoelectric zinc oxide (ZnO) nanowires fully integrated into fiber-reinforced polymer composites. The ZnO nanowires are conformally grown on the surface of woven aramid fabrics via a hydrothermal growth process after which the fabric with ZnO nanowires is placed between layers of woven carbon fiber using vacuum assisted resin transfer molding forming multilayer composites. While the nanowires serve as embedded sensors in the samples, the carbon fiber layer serve as electrodes across which the voltage of the composite can be measured. The composite samples are subjected to a standard tensile test during which the sample voltage output is measured using wire leads attached to the carbon fiber electrodes. Damage to the composite sample during testing results in elastic waves which are converted to measurable electric field by the piezoelectric nanowires. The recorded sample voltage is used to detect instances of internal damage by evaluating the amplitude and number of acoustic emission hits during mechanical testing up to the point of failure. The measured voltage is used to detect internal damage to the composite at less than 50% of the maximum strain thus demonstrating the use of ZnO nanowires in real-time damage detection.
Optimum Sensing Configurations for Delamination Identification Using Electrical Resistance Tomography
Damage in Carbon Fiber Reinforced Polymer (CFRP) composites due to cracking modifies the internal electrical conductivity of the composite material. Electrical Resistance Tomography (ERT) is a non-destructive evaluation (NDE) technique that takes advantage of electrical conductivity changes to determine the extent of damage. Implementation of ERT for damage identification in CFRP composites requires the optimal selection of the sensing sites for accurate results. This selection depends on the measuring scheme used. The present work uses an effective independence (EI) measure for selecting the minimum set of measurements among all possible for ERT damage identification using three measuring schemes: two-probe, four-probe and multi-probe. The electrical potential field in two CFRP laminate layups with 14 electrodes is calculated using finite element analyses (FEA) for a set of specified delamination cases that span the space of possible damage states. The measuring schemes are used considering the cases of 14 electrodes distributed on both sides and 7 electrodes are on one side of the laminate, for each layup. The effectiveness of EI reduction is demonstrated by comparing the inverse identification results of delamination cases for the full and the reduced sets using the measuring schemes and electrode sets. This work shows that the EI measure optimally reduces electrode and electrode combinations in ERT based damage identification for different measuring schemes used.
On-line Detection of Composite Structural Faults Utilizing Piezoelectric Impedance Measurement
The timely and accurate detection and identification of structural faults during their operations can play a vitally important role in ensuring the safety and sustainability of various structural systems. This mission, however, is inherently difficult, because structural faults/degradations usually have extremely small characteristic lengths with infinitely many possible patterns/profiles. It becomes even more challenging for layered composites due to the lack of understanding of many failure modes peculiar to such structures. In this research, we integrate the piezoelectric transducer into layered composites to facilitate piezoelectric impedance-based damage detection. In order to enhance the measurement quality, several circuitry integration techniques are introduced.
Carbon Nanotube Yarn Sensors for Precise Monitoring of Damage Evolution in Laminated Composite Materials: Latest Experimental Results in In-Situ and Post-Testing Validation
The increasing demand for materials with high mechanical strength, stiffness, without the weight of traditional metals like steels in industrial applications have led to the need for an improved understanding of the lifecycle and failure mechanisms of composite materials. Composite materials and structures are known to be tailorable to specific industrial needs due to their ability to combine properties from different materials to achieve high strength-to-weight ratio, high stiffness, creep-, fatigue- and corrosion-resistance, without losing its weight advantage. However, laminated composites like carbon fiber- reinforced plastics (CFRPs) and glass fiber-reinforced plastics (GFRPs) with their excellent in-plane properties are weakened by the poor outof- plane properties, which may lead to early failure. The failure mechanisms in laminated composites are dissimilar to the well-understood failure in conventional materials like metals or ceramics. They are dominated by fiber breaking, delamination and matrix cracking. Of concern is that delamination and matrix cracking can occur at any time in the composite structure unlike fiber failure that typically occurs at the end of a lifecycle of a fiber-reinforced plastics (FRPs). They occur mostly within the composite structure often times undetected leading to catastrophic failure. The results of a laminated composite sample implemented with integrated carbon nanotube yarn sensors demonstrate the ability of the sensors to detect the damage and provide their locations as a function of the applied load and in real-time. The previous results obtained with the CNT yarn sensors were validated using two experimental techniques: integrated optical fibers monitored through time-domain reflectometry, and x-ray tomography of the entire laminated composite samples post-testing. High performance plastic multimode optical fibers were integrated in the laminated composite plates. Furthermore, a three-dimensional computerized tomography (CT) imaging technique was used to corroborate the damage in the laminated composite samples.
The Effect of Length on the Orientation Randomization of Aligned Carbon Nanotubes in a Polymer
The speed of carbon nanotube rotation was analyzed using a real-time, in situ polarized Raman spectroscopy technique. Nanotubes dispersed in an epoxy matrix are aligned with an electric field, and the orientation randomization upon removing the field is shown to be a function of the nanotube length. Through a study of nanotube batches with different length distributions, a characteristic time to become unaligned was found to be consistent with the rotational diffusion of rigid rods in a polymer melt.
Tearing Properties Investigation of a New Airship Envelope Fabric Based on Single Edge Notched Film Experiment
In the present work, a new phenomenal model is proposed for modeling and simulation of tearing mechanical properties on an airship envelope fabric. The greatest advantage of the phenomenal model is that such model is capable of calculating the tear strength of a plain weave fabric by simply uniaxial tearing tests rather than the expensive and complex pressurized cylinder tests. The geometric progression technique is employed to calculate the stress distribution on the crack tip. The stress transfer coefficient Q is derived to be 0.9656. Three types of SEN specimens are researched during the uniaxial tearing tensile tests. The effects of specimen width and the crack length ratio on the tear strength of the airship envelope fabric is studied thoroughly. A good agreement between such model and experimental data are obtained, and the deviation is lower than 5%. Meanwhile, compared with the new phenomenal model and the Thieleâ€™s formula, this new phenomenal model is verified to be more accurate while calculating the tear strength of a plain weaving airship envelope fabric in the uniaxial tearing tensile load.
Strength and Failure Characterization of a Glass Fiber SMC-R Composite Under Biaxial Loads
Sheet molding compound composites containing randomly oriented short fibers (SMC-R) are among the most commonly used composites in the automotive and many non-aerospace applications. Many studies have reported the static and fatigue properties of SMC-R composites under uniaxial stress conditions. However, there are many applications in which they may be subjected to biaxial stress conditions and their biaxial properties have not been reported in the literature. The current study considers the mechanical characterization of a glass fiber SMC-R under biaxial loading conditions that generate various combinations of normal and shear stresses. It also includes a study on the damage development and failure prediction under biaxial loads.
Testing of Nanoparticle-Toughened Carbon/Epoxy Composites Using the Short Beam Strength Method
ASTM D2344, referred to as the short beam strength (SBS) method, is a commonly used test method for assessing the interlaminar shear strength of composite materials. The method utilizes a short unidirectionally reinforced composite beam specimen loaded in three-point bending. Due to the three-point loading configuration, the failure mode can be influenced by a combination of interlaminar shear at the midplane of the specimen, transverse contact stress under the central loading roller, and fiber-direction compression stress under the central roller. The objective of the current investigation is to explore two approaches for improving the interlaminar shear strength of carbon/epoxy compositesâ€”nanosilica or and/or core-shell-rubber (CSR) particles added to the epoxy matrixâ€”and to explore the use of short beam testing a means of measuring interlaminar shear strength in carbon fiber composites made with these matrix materials. Matrix properties such as mass density, Youngâ€™s modulus and Mode I fracture toughness are provided along with the constituent contents and SBS of the fiber composites. It is shown that the use of different nanoparticles significantly affects the density, Youngâ€™s modulus, and Mode I fracture toughness of the matrix. Proper interlaminar shear failure modes were observed in the baseline epoxy and NS toughened composite systems, but not in any of the CSR toughened composites including the CSR/NS hybrids. The reduction of matrix modulus caused by CSR appears to promote an indentation/flexure failure before the interlaminar stresses are sufficiently high to fail the specimen by shear.
A Methodology for Characterization of Material Constants for Strain-locking Materials
Several natural materials and engineered composites exhibit a strain limiting behavior. The effective tangent modulus increases by several orders of magnitude when the strain reaches a critical range. This stiffening is produced when constituents within the composite with a higher relative modulus abruptly take on a larger percentage of the applied load after specific amount of deformation. The tangent modulus increases by an order of magnitude or more, so that the bulk composite material exhibits an effective â€œstrain lockingâ€ behavior. A typical structure of such a material is one with a flexible matrix with wavy reinforcements. The strain locking occurs when the wavy reinforcements straighten out. Recently, implicit nonlinear constitutive models have been proposed to describe such materials with strain limits. This paper presents and validates a methodology for systematically characterizing the parameters of a strain limiting implicit constitutive model from experimentally obtained stress-strain data. The proposed methodology uses an intermediate function space to find the best fit parameters of the implicit constitutive model. The proposed characterization method employs two distinct phases. The first phase entails matching the best fit coefficients of intermediate functions that represent the experimental data and material constitutive model. This provides a good initial point for refining the coefficients of the constitutive law with a direct minimization of the error between the experimental and modeled stress-strain data. The proposed method is demonstrated using an experimental data set obtained from the literature that exhibits a highly abrupt tangent modulus shift and resembles a bilinear elastic material. The resulting parameter values are shown to be at a true minimum within the parameter space.
Thermal Treatment and Particle Size Effects on Coalescence in Extrusion-based Additive Manufacturing
Transverse and out-of-plane mechanical properties of objects fabricated by fused filament fabrication, an additive manufacturing technique, are typically 20-50% lower than bulk properties. A major contributor to this knockdown is poor raster-toraster and interlayer bond strength driven by incomplete polymer coalescence. The two major steps in the coalescence process responsible for bond strength development are neck growth and interfacial diffusion. The goal of this work is to explore the effects of temperature, time, and particle size on coalescence of ABS polymer in a controlled environment to identify appropriate thermal history to target during the printing process for enhanced raster-to-raster and interlayer bond strength. Particle size was found to have a significant impact on neck growth where doubling the size reduced the normalized neck growth by 58%. Diffusion was qualitatively assessed by tracking absorbance of red pigment from a red colored ABS particle into the neighboring white ABS particle at 550 nm. Smaller particle size held at elevated temperature for longer time had pigment diffusion across longer distances from the neck region. Results suggest that smaller diameter rasters promote faster and more complete coalescence.
Elevated-Temperature Wear and Friction of PTFE/PEEK Composite
A study was conducted on elevated-temperature tribological behavior of a PEEKmatrix composite with 15% (by volume) PTFE particulates. Neat PEEK and PTFE were also investigated at the elevated temperature to provide reference information. Tribological experiments on neat PEEK, neat PTFE and the PEEK/PTFE composite (C15) were carried out with a pin-on-disk tribometer with its contact surfaces heated at 200C. Friction and wear mechanisms of the composite were studied based on SEM and optical microscopic observations of test samples and steel counterfaces. At 200C, friction coefficient of C15 composite decreased by about 30% from that at 45 C. The specific wear rate of the composite at elevated temperature (200C) was lower by 500% than that at low temperature (45C). The C15 composite exhibited lower friction and wear than those of neat PEEK and neat PTFE at both low and elevated temperatures. The synergistic effect of solid-state lubrication of PTFE in C15 composite was also found at high temperature. Dominating friction and wear mechanisms in the composite at the elevated temperature were identified to include PTFE adhesion, viscoplastic flow, and most importantly, formation of transfer films under combined compression and surface traction.
Chopped Carbon Fiber Reinforced Thermoplastic Composites
This paper explores the development of a long (greater than 10 mm in length) chopped carbon fiber reinforced polyphenylene sulfide (PPS) composite. A novel manufacturing approach that uses compression molding to infuse thin PPS resin films sandwiching non-woven mats of chopped carbon fiber is developed for obtaining a composite organo sheet. The relationship between processing, microstructure and the resulting thermo-mechanical properties of the composite is evaluated. Laminated composite samples are prepared using a compression molding process using custom aluminum molds with simple design for scalability in size. The consolidated panels are subsequently annealed at various temperatures to induce cold crystallization to evaluate its role on mechanical properties. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) are used to explore the thermo-mechanical properties of the composite. Annealing increased the thermal resilience of the composite specimens through a crystallization mechanism. Optical microscopy is used to evaluate the microstructure of the material. A tensile modulus of over 25 GPa and a failure strength of over 200 MPa are routinely obtained, showing significant promise for manufacturing lower cost and high throughput discontinuous carbon fiber composites with the potential of utilizing recycled carbon fiber for reinforcement.
Influences of Matrix Morphology and Graphite Fillers on Tribological Behavior of Carbon-Fiber Reinforced PTFE/PPS Composites
A study was conducted on tribological behavior of carbon-fiber reinforced PTFE/PPS composites with different matrix morphologies and graphite flakes. The PPS matrix morphology was controlled by sintering parameters. The morphology of the composite was characterized X-Ray diffractometry (XRD), optical macroscopy and DSC. The composite mechanical properties were determined by hardness, compressive tests and DMA. The tribological behavior was studied with a precision pin-on-disk tester. The results show that the PPS matrix can be modified to a semicrystalline or a fully amorphous PPS matrix. The matrix morphology had a significant influence on mechanical properties of the composite, consequently its friction and wear behavior. The graphite flakes reduced the composite coefficient of friction but had a substantial, negative impact on the composite compressive failure strain. The amorphous PPS matrix, however, had a low friction coefficient and also reduced the specific wear rate of the composite.
Behavior of Continuous Steel-Concrete Composite Beams Reinforced with CFRP sheets
The improvement in the flexural behavior of continuous steel-concrete composite beams is evaluated after adding carbon-fiber-reinforced polymers (CFRP) laminates to the concrete surface at the hogging moment regions. In this paper, an experimental program consists of three specimens tested in an inverted position under four-point loading is carried out to simulate a part of continuous steel-concrete composite beams at the hogging moment region. The experimental findings are used to develop and validate a reliable finite element (FE) model to simulate the nonlinear flexural performance of strengthened beams as well as plain beams. However, CFRP laminates enhance the beam capacity, a reduction in the beam ductility occurs and causes a limitation in the moment redistribution at the hogging moment regions Moreover, adding more layers of CFRP laminates leads to higher values of the beam stiffness and capacity but more reductions in ductility will occur. As the longitudinal steel rebars ratio increases, the contribution of CFRP laminates to enhance the beam capacity is diminished. Also, applying CFRP laminates through the full length of the hogging moment region helps to reduce the creation of fatigue cracks in the steel rebars.
Effect of Interfacial Adhesion of Polycarbonate Film on Mode I and II Fracture Toughness of Interleaved Epoxy Composites
Polycarbonate (PC) films were used to interleave bi-directional glass fiber (GF) weave/epoxy composites to study the effect on mode I and mode II interlaminar fracture toughness. Films of PC with smooth and textured surface finish and oxygen plasma treatment were used to examine the effect of interfacial adhesion between the epoxy and PC film on composite toughening. The standard double cantilever beam (DCB) and end notched flexure (ENF) tests were used for mode I and II interlaminar fracture toughness characterization respectively, considering both neat and interleaved laminates. Results indicate an increase up to 59% in mode I interlaminar fracture toughness for interleaved GF/epoxy composites with textured surface PC films. Morphological observations of fractured DCB surfaces indicate that the improved toughness in the case of textured PC film interleaves is related to surface roughness, promoting cohesive-adhesive failure of the film due to better interfacial adhesion of PC film and epoxy matrix. Furthermore, the results of ENF tests indicate that the PC films can also enhance mode II interlaminar fracture toughness of GF/epoxy composites by 42% in the case of oxygen plasma treated PC films. Additionally, dynamic mechanical analysis of PC interleaved composites showed that high operational temperature can be preserved in the interleaved composites.
Numerical and Experimental Design of a Cruciform Specimen for a Novel Cryobiaxial Experiment
This study presents a combined numerical and experimental attempt to develop a new cruciform geometry for cryobiaxial loading. Two candidate cruciform specimens with crossply stacking sequences are evaluated at room temperature based on their ability to initiate and evolve intralaminar damage in the gage region. The first design, developed by the Utah Composites Lab (UCL), was meant to ensure damage would occur at the center of the specimen. The second design utilizes sharp corner transitions to arrest unwanted damage from evolving outside of the gage region of the specimen. Numerical simulations were done using AFRLâ€™s BSAM code to optimize each geometry and determine load intervals for progressive damage tests. Three specimens were then tested for each geometry using a custom table-top biaxial load frame. Experimental results revealed that the UCL design successfully initiated damage at the desired location, but extensive damage accumulation occurred in the chamfer region. Ultimately, a through-thickness crack network was not formed before unstable delamination growth extended across the gage region of the entire specimen. The Hallett specimen initiated damage in the corners during the first load interval, but at increased load steps numerous intralaminar cracks evolved in each of the loading arms. Once again a through-thickness crack network was not formed. Despite observing several issues with the two designs proposed in this study, a wealth of numerical and experimental data was collected that will be used to improve each cruciform design. Numerical results indicated that BSAM is an effective tool for designing and optimizing tape-laminate cruciform specimens. It is expected that an improved cruciform design for the investigation of intralaminar failure under cryobiaxial load will be achieved as a result of this work.
Experimental Determination of Strength Distribution of S2-glass Fibers Across a Wide Range of Gage Lengths
In a unidirectional polymer-matrix composite under axial tensile loading, the failure process is governed by the dynamic localization and clustering of multiple fiber breaks. Experimental evidence shows that it takes only a cluster of 15-30 fiber breaks to cause the composite to fail catastrophically. Thus it is extremely important to develop a better understanding of this micromechanical damage localization mechanism. The brittle fracture and clustering of fiber breaks is a locally dynamic process which is governed by the size (which, in turn, determines the strength) and spatial distribution of defects within the fiber. Single fiber tensile tests are performed on specimens in the gage length on the order of 4-30 mm and fitted to a Weibull distribution to describe the statistical and gage-length dependent nature of strength in these fibers. However, since the micromechanical damage localization zone is on the order of a few fiber diameters (10 um diameter for S-glass fiber), Weibull paramaters were also determined using the single fiber fragmentation test, which correspond to a gage length of 365 micrometers. The Weibull modulus obtained from the 2 methods are significantly different, indicating that a different flaw population dominates the fiber failure behavior at these smaller length scales. The results indicate that a bi-modal strength distribution similar to the ones developed for Carbon fibers might be more appropriate to describe the statistical variation in the strength of S2-glass fibers. This paper ends with a description of a new experimental technique that is being developed where these fibers are continuously subjected to bending over small radii of curvature on the order of 10-100 microns, triggering the critical surface defects on the tension side and causing the fibers to fragment, at characteristic distances that are significantly smaller than the saturation length in the fragmentation experiments.
Static Performance Assessment of Recyclable Bio-Based Resin for Wind Turbine Blades Using Sub-Component Testing
Bio-based resin systems are attractive for use in wind turbine blade manufacturing because they offer opportunities for recycling at the end-of-life of the blade and an environmentally friendly alternative to the currently used epoxy-resin systems. However, before any bio-based resin can be used in blades, its structural and fatigue performance must be determined to be as good as or better than the currently used epoxy resins. Certification of new material systems for utility-scale wind turbine blades starts with coupon testing of the materials and culminates with the full-scale testing of the blade. Coupon testing is not necessarily fully representative of the structural performance of the blade and full-scale testing is costly, time consuming and labor-intensive. Subcomponent testing has the potential to bridge the gap between the coupon and the full-scale testing and to facilitate and expedite the introduction of new material systems into the blade manufacturing industry. In this paper, two sets of scaled-down composite I-beams representing the spar cap and the shear web geometry of a utility-scale blade are designed and manufactured. One set is made from a conventional epoxyresin system, and the other set is made using a recently developed bio-based resin system. The beams are loaded in a quasi-static four-point bending test configuration, and the strain fields of their flanges are measured using digital image correlation. The strain fields of the beams made of the conventional resin system are compared to their counterpart bio-based beams. The results show a comparable flexural stiffness for the bio-based resin beams to the conventional-resin beams.
Development of an Input Suite for an Orthotropic Composite Material Model
An orthotropic three-dimensional material model suitable for use in modeling impact tests has been developed that has three major components â€“ elastic and inelastic deformations, damage and failure. The material model has been implemented as MAT213 into a special version of LS-DYNA and uses tabulated data obtained from experiments. The prominent features of the constitutive model are illustrated using a widely-used aerospace composite â€“ the T800S/3900-2B[P2352W-19] BMS8-276 Rev-H-Unitape fiber/resin unidirectional composite. The input for the deformation model consists of experimental data from 12 distinct experiments at a known temperature and strain rate: tension and compression along all three principal directions, shear in all three principal planes, and off axis tension or compression tests in all three principal planes, along with other material constants. There are additional input associated with the damage and failure models. The steps in using this model are illustrated â€“ composite characterization tests, verification tests and a validation test. The results show that the developed and implemented model is stable and yields acceptably accurate results.
Influent Fabric Parameters on the Energy Absorption During Dynamic Tensile Loadings
Based on numerous research works [1] [2] [3] [4] [5] [6], fabric parameters such as: the yarnâ€™s properties, the yarn-to-yarn friction coefficient and the distribution of end and pick densities, have been revealed as influent on the energy absorption mechanism during ballistic impact. However, most of these parameters are measured at quasi-static velocity while the ballistic impact involves high strain rate. Knowing the dynamic behavior of a yarn inside the woven structure is essential to determine the ballistic behavior of fabrics during an impact. This work aims at studying the mechanical behavior of a para-aramid yarn inserted in a fabric structure during a dynamic and longitudinal load. To that end, we have designed a new experimental device to achieve these tests with the use of a gas gun: the Tensile Impact Test for Yarn (TITY). Many test campaigns have been performed to identify the influence of weave diagram, and consequently the number of crossover points between warp and weft yarns for the same pick density and for each fabric, and the energy absorption during dynamic loading of the yarn inserted in the woven structure up to its rupture. This study reveals the influence of three main structural parameters such as: the total inter-yarns contact surface, the number of binding points and the transverse and lateral inter-yarns pressures. These influent parameters have an impact on different yarn break criteria at different scales of the fibrous material, such as: the structural defects and hysteresis phenomena inside the filament, stress concentrations and frictional forces concerning the interactions between the tested yarn and the rest of the fabric.
Fusion Bonding Simulations of Semi-Crystalline Polymer Composites in the Extrusion Deposition Additive Manufacturing Process
Extrusion Deposition Additive Manufacturing (EDAM) is a technology where 3-D digital objects are manufactured extruding beads of molten material in a layer-by-layer basis. This technology offers the flexibility for processing pelletized material, and thus a semi-crystalline polymer highly-filled with carbon fiber has been used for printing. Significant technology improvements in the EDAM technology have been made recently, however, the design of the printing strategy is still mostly driven by empirical development. Hence, there is a need for simulation tools that capture the phenomena involved in the EDAM process to drive the development and optimization of this technology. Currently, one limitation of printed parts in terms of mechanical properties is the strength of the interface developed between printed layers. Therefore, the focus of this work is to couple the phenomena involved in the bonding process of adjacent layers to predict the degree of bonding, such as the temperature history, the melting and crystallization of the semi-crystalline polymer and the diffusion of polymer chains. The models utilized to describe these phenomena and their couplings were implemented in a UMATHT user subroutine in Abaqus to predict the evolution of the degree of bonding during the EDAM process. The effects of the couplings implemented in this approach are demonstrated by predicting the evolution of the degree of bonding throughout the simulation of the printing process of a geometry with sections that undergo different cooling and crystallization rates.
Thermoelastic Stress Analysis and Modelling for Mechanical Property Assessment of Composite Materials
Thermoelastic Stress Analysis (TSA) is a full-field stress measurement technique that has proven effective for experimentation with metallic alloys. In addition, its recently improved portability has facilitated application to built-up aircraft structure as well. This has led to an interest in applying the method to composite airframes. Since the thermoelastic response is more complex in layered orthotropic materials, it is not possible to relate measured temperature to stress without a model to facilitate the conversion. The current study comprised experimentation and thermomechanical (multi-physics) analysis of tension loaded open-hole notch specimens using aluminium and composite laminates typical of use on fifth generation fighter aircraft. The results confirmed the accuracy of TSA for measuring stress in isotropic metallic materials. The results for the composite materials show that the thermoelastic response is sensitive to a range of conditions, including natural volume fraction variation across the surface ply. The predictions, however, compared reasonably well with the experimental results. The work provides a new tool for realizing the goal of using TSA for in-situ measurement of stress on composite airframes. It also provides a basis for understanding current limitations as pertaining to its use on composites and areas where development needs to be focused.
New Approaches to Quantifying Tensile Strength Variability and Size Effects in Unidirectional Composites
Two methods were used to investigate the variability of tensile failure strain of unidirectional high strength carbon/epoxy. Scaled tests on glass/carbon hybrid specimens showed a significant size effect and reduction of failure strain with stressed volume consistent with a Weibull modulus of 25. Ply fragmentation tests were also carried out, producing multiple carbon fractures in a single specimen. Strains from these tests also fitted a Weibull distribution, and gave a similar Weibull modulus to the scaled tests.1
Effect of Sizing on the Interfacial Properties of Carbon Fiber Composites
Any heterogeneous structural material has interfaces that transfer load from one phase to another through the interface thereby making it a critical constituent that affects overall performance of the material. Fiber-reinforced polymer (FRP) composite materials, such as carbon fiber epoxy composites, are excellent examples of these heterogeneous systems. The interface between carbon fiber and epoxy matrix is a key element of the carbon fiber composites that transfers load from matrix to the fibers and thus affects the performance of composites. One of the key characteristics of FRP composites is to have good interlaminar shear strength (ILSS) which is essentially a measure of the interfacial bonding between fiber and matrix. Various sizing chemistries were considered to coat the unsized carbon fiber and ILSS values were determined using short beam shear (SBS) experiments. In addition to changing the sizing formulations, a broad range of laminate thicknesses were considered for the performance evaluation of the composite laminates. There were two key findings in this work. Firstly, interlaminar shear strength measured using short beam shear has a strong dependence on the laminate thickness. Secondly, interfacial properties of composites can be significantly altered by modifying the fiber surface characteristics by means of sizing chemistries.
Prediction of Damage Initiation and Simulation of Damage Propagation in 3D Woven Composites During Processing
Some configurations of 3D woven composites are known to be susceptible to processing induced damage in the form of microcracks that develop in the polymer matrix during curing. The microcracking is believed to originate from high residual stresses that develop due to a significant mismatch in the coefficients of thermal expansion between the constituent materials. In this paper, we investigate the applicability of several commonly used stress-based failure criteria for glassy polymers â€“ the von Mises, the Bauwens (Drucker-Prager), the parabolic stress, and the dilatational strain energy density. We study the microcracking phenomenon on the example of the one-to-one orthogonal configuration of the epoxy matrix/carbon fiber 3D woven composites. This configuration is characterized by the high level of the throughthickness reinforcement which appears to exacerbate the matrix damage. The investigation is based on a high-fidelity mesoscale finite element model of an orthogonally reinforced 3D woven composite. We simulate the materialâ€™s response to the uniform temperature drop from the curing to room temperature and compare the results of the simulation with the X-ray computed microtomography. We conclude that the curing induced matrix failure is well predicted by the parabolic stress criterion with a proper choice of the material constants. Initiation and propagation of this failure are simulated via sequential deactivation of the elements exceeding the allowable equivalent stress.
A Fabric Architecture Based Constitutive Model for Fiber Reinforced Rubbers
The focus of this paper is to develop a fabric architecture-based constitutive model for a rubber hose reinforced by knitted fibers. The composite hose is modeled using a representative unit cell (RUC) consisting of a fiber layer embedded in two rubber layers. The rubber is considered as a Neo-Hookean material, while the knitted fibers are modeled by cords with negligible stiffness in bending. The effect of reinforcement comes from the conservation of the total length of the fiber cords. This approach enables the determination of the constitutive behavior of the composite sheet in closed form based on the constituent rubber and fiber properties and geometry.
Toward Enhanced Forming Simulation of Woven Fabrics Using a Coupled Non-Orthogonal Hypoelastic Constitutive Model, Integrated with a New Wrinkling Onset Criterion
One of the advantages of woven fabrics over unidirectional prepregs is their superior formability thanks to the large shear deformation capability. There exists, however, a limit on the shear deformation of woven fabrics, namely the wrinkling. Applying tension to delay wrinkling during forming processes, a consequence of the inherent coupling in woven fabrics, is widely known to the industry. Yet, inherent coupling â€“ change in the effective material properties of a given direction of the fabric due to the applied deformation in other directions - has not been fully understood and implemented in the forming simulations of fabric reinforcements. Coupling should be incorporated in numerical optimization routines to accurately predict the deformation of the material under complex forming set-ups, and more importantly to predict a realistic yarn tension level that can suppress wrinkles. Towards this goal, the present study proposes and implements a new coupled non-orthogonal model which predicts not only the stress-strain path, but also the critical point (shear wrinkling) of the woven fabrics, under combined loading conditions similar to the draping processes. Furthermore, the study reveals that the concept of inherent coupling raises a new issue in fabric forming simulations; the load history dependency of the fabric. Accordingly, the constitutive model has been enhanced to a hypoelastic form to capture the load path dependency of the forming material. Finally, the constitutive model has been integrated with a newly developed analytical fabric instability criterion by authors to account for the occurrence of wrinkling, based on the fabric properties and the level of tension applied during forming. To show its application, the model has been implemented in ABAQUS via a UMAT code to predict the stress and strain fields as well as the onset of wrinkling under large shear deformations.
Prediction of Tow Architecture and Stress Distributions for a 3D Woven Composite
Finite element analysis was used to investigate a 3D orthogonally woven composite. The tow architecture was generated using VTMS, a tool created by AFRL for generating textile composite models. Because of the complexity of the geometry, the study included an evaluation of the cross-sectional area variation. A nominal cross-sectional area was defined by dividing the volume of the tow by the length of the tow path. The cross-sectional area of the tows varied between -13% and 18% of the nominal value, which would result in up to a 26% range in the fiber volume fraction. The average stress for the tow cross-sections varied along the tow path for all tows. The locations of the peak average stresses were sensitive to both the tow undulation and the surrounding material. The locations of highest stress concentrations did not always match the locations of peak average stress.
Spectral Stiffness Microplane Modeling of Fracture and Damage of 3D Woven Composites
A constitutive model is proposed to simulate the orthotropic stiffness, pre-peak nonlinearity, failure envelopes, and the post-peak softening and fracture of 3D woven composites. Following the microplane model framework, the constitutive laws are formulated in terms of stress and strain vectors acting on planes of several orientations within the material meso-structure. The model exploits the spectral decomposition theorem to define orthogonal strain modes at the microplane level. These are associated to the various types of deformation. Strain-dependent constitutive equations are formulated for each mode, corresponding to different damage mechanisms, to relate the microplane eigenstresses and eigenstrains. Model calibration and verification were performed against the experimental data. The capability of the model capturing the intra-laminar size effect of composite structures was also verified.
In-Situ Observations of Longitudinal Compression Damage in Carbon-Epoxy Cross Ply Laminates Using Fast Synchrotron Radiation Computed Tomography
The role of longitudinal compressive failure mechanisms in notched cross-ply laminates is studied experimentally with in-situ synchrotron radiation based computed tomography. Carbon/epoxy specimens loaded monotonically in uniaxial compression exhibited a quasi-stable failure process, which was captured with computed tomography scans recorded continuously with a temporal resolutions of 2.4 s and a spatial resolution of 1.1 Î¼m/voxel. A detailed chronology of the initiation and propagation of longitudinal matrix splitting cracks, in-plane and out-of-plane kink bands, shear-driven fiber failure, delamination, and transverse matrix cracks is provided with a focus on kink bands as the dominant failure mechanism. An automatic segmentation procedure is developed to identify the boundary surfaces of a kink band. The segmentation procedure enables 3-dimensional visualization of the kink band and conveys the orientation, inclination, and spatial variation of the kink band. The kink band inclination and length are examined using the segmented data revealing tunneling and spatial variations not apparent from studying the 2-dimensional section data.
Assessment of Intralaminar Progressive Damage and Failure Analysis Methods Using an Efficient Evaluation Framework
Reducing the timeline for development and certification for composite structures has been a long standing objective of the aerospace industry. This timeline can be further exacerbated when attempting to integrate new fiber-reinforced composite materials due to the large number of testing required at every level of design. Computational progressive damage and failure analysis (PDFA) attempts to mitigate this effect; however, new PDFA methods have been slow to be adopted in industry since material model evaluation techniques have not been fully defined. This study presents an efficient evaluation framework which uses a piecewise verification and validation (V&V) approach for PDFA methods. Specifically, the framework is applied to evaluate PDFA research codes within the context of intralaminar damage. Methods are incrementally taken through various V&V exercises specifically tailored to study PDFA intralaminar damage modeling capability. Finally, methods are evaluated against a defined set of success criteria to highlight successes and limitations.
Verification and Validation Process for Progressive Damage and Failure Analysis Methods in the NASA Advanced Composites Consortium
The Advanced Composites Consortium is a US Government/Industry partnership supporting technologies to enable timeline and cost reduction in the development of certified composite aerospace structures. A key component of the consortiumâ€™s approach is the development and validation of improved progressive damage and failure analysis methods for composite structures. These methods will enable increased use of simulations in design trade studies and detailed design development, and thereby enable more targeted physical test programs to validate designs. To accomplish this goal with confidence, a rigorous verification and validation process was developed. The process was used to evaluate analysis methods and associated implementation requirements to ensure calculation accuracy and to gage predictability for composite failure modes of interest. This paper introduces the verification and validation process developed by the consortium during the Phase I effort of the Advanced Composites Project. Specific structural failure modes of interest are first identified, and a subset of standard composite test articles are proposed to interrogate a progressive damage analysis methodâ€™s ability to predict each failure mode of interest. Test articles are designed to capture the underlying composite material constitutive response as well as the interaction of failure modes representing typical failure patterns observed in aerospace structures.
Material Characterization for the Analysis of Skin/Stiffener Separation
Test results show that separation failure in co-cured skin/stiffener interfaces is characterized by dense networks of interacting cracks and crack path migrations that are not present in standard characterization tests for delamination. These crack networks result in measurable large-scale and sub-ply-scale R curve toughening mechanisms, such as fiber bridging, crack migration, and crack delving. Consequently, a number of unknown issues exist regarding the level of analysis detail that is required for sufficient predictive fidelity. The objective of the present paper is to examine some of the difficulties associated with modeling separation failure in stiffened composite structures. A procedure to characterize the interfacial material properties is proposed and the use of simplified models based on empirical interface properties is evaluated.
Verification and Validation of Progressive Damage Analysis Methods for Explicit Analysis of Progressive Delamination
The NASA Advanced Composite Consortium (ACC), which includes as its members Lockheed Martin Aeronautics, Boeing, GE, UTC, Aurora, Orbital ATK, NASA, and FAA has embarked on an ambitious mission to reduce aircraft structural certification timeline by 30% using a combination of technologies, one of which is high fidelity progressive damage computational methods. A highly structured process is adopted to accomplish this mission where the progressive damage methods are first verified and validated before it is applied to structures of increasing complexity akin to a typical building-block approach currently adopted for structural certification of aircrafts. This study focuses on the verification of two methods for studying progressive interlaminar damage; the cohesive zone method embedded in Abaqus native cohesive element and a beta version of the Discrete Cohesive Zone (DCZM) model, acquired through the University of Michigan, implemented as a user defined element within Abaqus. The methods are verified by predicting delamination initiation and growth in the Double Cantilever Beam (DCB), the End Notched Flexure (ENF), and the Mixed Mode Bending (MMB) and comparing the predictions to high fidelity numerical solution obtained using Virtual Crack Closure Technique (VCCT). The lessons learned in the verification exercise are then applied to a validation exercise of a three-point bend doubler problem. For the doubler, in addition to modeling the interlaminar progressive damage, the intralaminar progressive damage is modeled using two continuum damage models; NASAâ€™s CompDam and a beta version of the Enhanced Schapery Theory (EST), acquired through the University of Michigan. Analysis predictions for the doubler using the different progressive damage techniques are compared.