Additional Comparisons of Interlocked Fabric and Laminated Fabric Kevlar 49/Epoxy Composites

Abstract

Compression, gas gun planar impact, and post-impact short beam shear test results are presented for a novel interlocked fabric reinforced Kevlar 49/epoxy composite. This material has roughly 4% fiber volume fraction in the through-the-thickness direction. For the purpose of comparison, similar results are also presented for a laminated Kevlar 49/epoxy composite. Both materials have a compressive stress-strain curve that can be approximated as bilinear with a secondary modulus of on the order of 10% of the initial modulus. Although the materials have comparable yield strengths (∼ 10% difference), their respective initial and secondary moduli are not in as close agreement (up to 50% difference). Gas gun planar impact spall test data indicate that relatively low levels of impact stress (∼1 kbar, ∼100 MPa) generate matrix cracking in both materials. The interlocked fabric's binder yarns, however, show no visible sign of damage even at an impact stress level of 5.1 kbar (510 MPa). Post-impact short beam shear test data for both materials show that once a threshold impact level is exceeded (∼1 kbar, ∼100MPa), specimen stiffness and load carrying capacity decrease as the impact level is increased. This is presumably because of mid-plane matrix cracks of increasing size and density. Even though the properties of the interlocked fabric reinforced composite are substantially reduced by impact at the 5.1-kbar (510-MPa) level, the binder yarns give the material a degree of integrity that would not otherwise exist. The interlocked fabric reinforced Kevlar 49/epoxy composite has a higher residual load carrying capacity than the laminated Kevlar 49/epoxy at all impact levels.