Understanding the Strengthening Effect of Aramid Pulp as an Interlaminar Reinforcement for CFRP Composites Subjected to Low-Velocity Impact

Abstract

Composite materials are used widely in aerospace and naval sectors due to fatigue resistance, corrosive resistance, tailorability, lightweight, and high bending-tostiffness ratios. However, one of their major drawbacks is their low resistance to impact damage due to their poor through-thickness properties. Low-velocity impacts (LVI) produce barely visible impact damage (BVID) on the surface of the laminates but can carry the potential of significant internal damage such as delamination, fiber breakage, and matrix cracking. This is extremely dangerous as the structure can fail catastrophically without any previous warning. The region between the layers of fibers is the interlaminar region, and it is a resin-rich region very susceptible to damage. Therefore, different interlaminar reinforcements have been proposed to increase the through-thickness mechanical properties of laminated composites. Interlaminar reinforcements exist primarily in two forms, 3D and 2D. 3D reinforcements look to enhance the through-thickness properties by penetrating through the fiber layers, such as stitching and Z-pinning. 2D reinforcements only strengthen the interlaminar region, without any perforation through the layers, such as nanotubes or nanowires. While there have been great strides in research of these interlaminar reinforcements, previous studies have suggested significant drawbacks. For example, the 3D reinforcements tend to decrease the in-plane mechanical properties of the laminate by changing the fiber volume fraction or breaking the fibers. In the case of 2D reinforcements, they could damage the fibers when the reinforcements are synthesized on them. In order to increase the through-thickness properties of laminated composites while mitigating the potential of previously mentioned detriments, Aramid (Kevlar®) pulp has been proposed as a potential interlaminar reinforcement. The objective of this project is to investigate the optimum quantity of Kevlar® pulp needed in carbon fiber reinforced polymer (CFRP) composites subjected to LVI to mitigate the damage during a LVI event. Varying degrees of reinforcement were examined through the pulp to resin mixture ratios. Specimens were categorized as 1x, 2x, and 4x, where 1x is the recommended ratio of Kevlar® pulp to resin, 1:15 by volume, specified by the manufacturer (FibreGlast), 2x corresponds to the ratio 2:15, and 4x corresponds to 4:15. Drop weight impact tests were be performed using an Instron CEAST 9340, with an impact energy of 5J following ASTM D7136/D7136M-20 standards. The impact responses were evaluated based on recorded data including time, energy, contact force, and displacement. Experimental results illustrated a correlation between increased reinforcement ratios and decreased stiffness observed in the specimen. Specimens with greater ratios than 1x experienced higher degrees of damage, lower peak forces, longer impact durations, higher displacements, and reduced bending stiffness. The decrease in stiffness is attributed to an increase in air presence during manufacturing caused by air being trapped within the microfibers during the mechanical opening.