Analysis of crack migration in laminated composites using conventional and mesh-independent cohesive zone models

The use of carbon fibre-epoxy composites in aerospace structures can enable significant weight savings and crucial reductions in fuel consumption. However, the design of high-performance laminated structures still relies heavily on empirical rules, particularly where strength, damage tolerance and fatigue life are concerned. The main reason is the complex progressive damage behaviour of these materials, which are made of relatively brittle fibres and matrices with intricate layups and geometries. In order to predict the correct macroscopic behaviour of the structure in the presence of damage, it is necessary to consider the various mesoscopic damage mechanisms individually, i.e. delamination, matrix cracks and fibre failure, as well as the various interactions between them. One of the most challenging forms of interaction to model is the so-called ‘crack migration’ behaviour, where delaminations interact with matrix cracks allowing a crack to propagate through the thickness of a laminate, ‘jumping’ between adjacent ply interfaces. Crack migration and jumping are difficult to model using traditional Cohesive Zone Models (CZM) and the Virtual Crack Closure Technique (VCCT) because they involve large numbers of cracks with locations which cannot be know a priori. In the present work the crack migration behaviour of aerospace-grade carbon fibre-epoxy laminates has been investigated experimentally and numerically using both the traditional CZM formulation as well as a novel mesh-independent implementation based on the automatic introduction of additional degrees of freedom