Analysis of Mode I and Mode II Tests for Composites with Translaminar Reinforcements

Abstract

This paper presents finite element models to determine the Mode I and Mode II fracture toughness of stitched composites in conjunction with experiments. Two new test methods to measure the Mode I and Mode II fracture toughness of stitched composites are developed. From the experimental load- deflection diagram, the effective fracture toughness can be determined by the area method. However, in order to comprehensively understand the effect of stitches, two parameters, Gc-par (parent material’s fracture toughness) and Gc-eff (effective fracture toughness) are needed to represent the fracture behavior of stitched composites. Gc-par is the intrinsic property of the parent material and is independent of stitch behavior. Gc-eff is the effective critical strain energy release rate that includes the effects of stitching. The purpose of performing the finite element analysis is to identify the correct physical models that represent the stitch interactions with the parent laminate and stitch failure mechanisms. Gc-par and Gc-eff can be obtained through different paths of the J-integral in the FE models. The models help in understanding the effects of stitch stiffness and spacing on the fracture toughness without the need for fabricating and testing stitched composite specimens.