Investigating the micromechanisms of mode II delamination in composite laminates

Abstract

The use of composite materials in primary aerospace structures is continually increasing due to their high stiffness and high strength to weight ratios. However, composite laminates are susceptible to delamination which can lead to the loss of global stiffness and potentially to catastrophic failure. Delamination is therefore a critical damage mechanism to account for when determining the durability and damage tolerance of composite materials. A large amount of research has already been carried out to investigate delamination and a number of failure criteria have been developed to predict delamination growth. However, the majority of these criteria are not based on the physical mechanisms that cause interlaminar fracture. Thus to be able to confidently predict delamination growth in real world applications a physically based failure criteria would be more appropriate. The key to developing physically based criteria is through fractographic observations to determine the dominant failure mechanisms of delamination. Thus the purpose of this research was to characterise the failure process of mode II shear fracture, and in particular to determine the formation of cusp features prevalent to mode II dominated fracture. A polyvinylchloride foam was used to macro-simulate mode II interlaminar failure of composite laminates. The in-situ fracture process was characterised successfully and the energy absorbed during cusp formation was determined. Results were compared with cusp formation in two carbon-fibre/epoxy laminates and the post fracture morphology of all the materials was similar. In addition the influence of parameters such as, material type, inter-fibre spacing, fibre diameter and ply orientation on the frequency, morphology and size of cusp formation were also investigated. Results indicated that cusp formation was via the initiation, saturation and propagation of tensile microcracks and that neither the cusp formation process nor the energy associated with crack formation was found to be influenced by the parameters studied.