Effect of unstable crack growth on mode II interlaminar fracture toughness of a thermoplastic PEEK composite

An end notched flexure (ENF) test was used to study the effect of growth stability on mode II interlaminar fracture toughness (G(II)(C)) of a carbon/PEEK composite. The instability of the ENF test was used to induce both stable and unstable crack growth with a static loading. Two techniques were used to evaluate mode II fracture toughness: the traditional compliance method and the infra-red thermography technique. The compliance technique has the advantage of being simple and it has already been confirmed for standard tests. Nevertheless its results may be inaccurate for unstable crack growth propagation. The infra-red thermography technique enables fracture toughness to be accurately measured for non-standard tests in which the crack growth may be unstable. Both methods are complementary and this study came to the conclusion that G(II)(C) is sensitive to crack growth velocity. Fracture toughness showed a low value for crack growth start and unstable propagation, and a high value for stable crack growth. The transition between a ductile fracture mode (for low crack growth velocity) and a brittle fracture mode (for high crack growth velocity) explained the important variability in G(IIC) values.