Studies on the influence of through-the-thickness reinforcement on low-velocity and high strain rate response of woven S2-glass/vinyl ester composites

Due to their inherent weakness in the thickness direction, laminated fiber reinforced composites are susceptible to undergo large delamination damage as well as splitting when subjected to transverse loading and microbuckling under in-plane compressive loading. In the current work, an effort is made to improve the transverse strength by providing discrete 3D reinforcement in the form of pins and stitching the laminate in the thickness direction. The effectiveness of the 3D reinforcement is compared with 2D-Plain laminat es. The laminates were made of 15 layers of 2 x 2 twill weave S2-glass fabr ic and vinyl ester C-50 resin system. Specimens of size 75 x 100 mm were su bjected to low-velocity impact loading at energy levels of 20, 30 and 40 jo ules. The effectiveness of 3D-reinforcement to in-plane dynamic loading was studied using a Compression Split Hopkinson's Pressure Bar at three differ ent strain rates of 327/s, 436/s and 544/s using cubic samples of size 6.0 mm. The stitched samples were subjected to high strain rate loading in two configurations. In the first, the sample (referred to as 3D-Stitch1) had st itching along the loading direction at the center while in the second; the sample (referred to as 3D-Stitch2) had an additional stitch line along the width at the center. The results of the study indicate that, reinforcement using stitching confines the delamination growth under low-velocity impact loading. Reinforcement in the form of pins did not show considerable improv ement in the damage containment. However, under high strain rate compressiv e loading, 3D-Stitch1 and 3D-Pin reinforced laminates demonstrated higher c ompressive strengths at the strain rates of 327/s and 436/s. Stitch2 and 2D -Plain samples, in comparison, exhibited lower strength. Both 2D-Plain and 3D-Pin reinforced laminates exhibited increasing strength with the increase in the strain rate. Whereas, stitched laminates exhibited increase in stre ngth from the strain rate of 327/s to 436/s before dropping in magnitude at 544/s. This was attributed to the localized damage in the fabric due to pi ercing of the needle during stitching resulting in resin pooling.