Stitch fiber comparison for improvement of interlaminar fracture toughness in stitched composites

In this study, strain energy release rates are measured and compared for laminated composites reinforced by through-thickness stitching using different stitch fiber materials - Carbon, Kevlar, and Vectran. Strain energy release rates are evaluated experimentally using the DCB test and validated computationally using FEA. The FE model of the stitched composite incorporates a novel four-step stitch fracture process, namely: interfacial debonding, slack absorption, fiber breakage, and pull-out friction. The FE predictions of G(IC) show good agreement with the experimental results. It is revealed that the relationship between G(IC) and stitch density, or stitch thread volume fraction for all stitch materials follow a linear law. It is concluded that Vectran provides the toughest interlaminar reinforcement and is most suitable for stitch fiber application. The characterization and difference in fracture behavior between Vectran and Kevlar stitch fibers, by using a novel Interlaminar Tension Test (ITT), are presented and discussed.