Mixed-mode fracture response of anti-symmetric laminates: Experiments and modelling

Multidirectional laminates are very often used in advanced structures. However, the existing data in the literature regarding their fracture response is not conclusive. In this work, the fracture response of a +45 degrees// -45 degrees interface is investigated under remote mixed mode loading and compared to a 0 degrees//0 degrees layout. Several experiments were conducted under three mode mixities phi = 0.15, 0.35 and 0.65 as well as pure modes I (phi = 0) and II (phi = 1.0) following the established standard procedures. Energy release rates were calculated using an appropriate reduced form of the J integral. Fracture toughness was independent of the interface at initiation but increased with mode mixity. Subsequent fracture resistance was negligible for the 0 degrees//0 degrees layout, but increased significantly for the +45 degrees/-45 degrees interface. Mechanistic investigations were carried out using X-ray computer tomography at 5 different load levels as well as transverse cross sections. The results showed lack of bridging in the case of 0 degrees//0 degrees layout for all values of phi. For the +45 degrees//-45 degrees interface, fracture was dominated by delamination, ply splitting and crack migration with their extent dependent upon phi. Numerical analyses based on the virtual crack closure technique demonstrated that the antisymmetric local fracture modes, interlaminar longitudinal shear vs. transverse shear, are responsible for delamination and crack migration, respectively. Based on the experimental observations, a mesoscale FE model was able to reproduce not only the failure mechanisms, i.e. delamination, transverse cracking and crack migration resulting in the zig-zag fracture pattern, but also give a reasonable approximation of the energy release rate obtained for each of the different mode mixities considered in this study.