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

Abstract

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∘//−45∘ interface is investigated under remote mixed mode loading and compared to a 0∘//0∘ layout. Several experiments were conducted under three mode mixities φ=0.15, 0.35 and 0.65 as well as pure modes I (φ=0) and II (φ=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∘//0∘ layout, but increased significantly for the +45∘//−45∘ 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∘//0∘ layout for all values of φ. For the +45∘//−45∘ interface, fracture was dominated by delamination, ply splitting and crack migration with their extent dependent upon φ. 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.