Characterization of mode-II delamination fracture energy via computational homogenization

Due to the complex underlying microscopic damage processes, the failure behavior of composite materials is challenging to understand and predict. Fracture mechanics approaches [1] and cohesive zone models [2] that simulate delamination generally consider the fracture energy as a material characteristic that exclusively depends on the fracture mode. However, it is not properly understood whether this mode-dependence completely characterizes the fracture energy or even what are the underlying mechanisms of the fracture energy variation. Computational homogenization [3] is a promising approach to provide more insight into the dissipative processes that contribute to the delamination fracture energy. Processes such as matrix plasticity and fibre/matrix interface debonding are best described on the microscale. Through computational homogenization, the microscale energy dissipation can be upscaled to the mesoscale where cohesive tractions are computed on-the-fly without prior knowledge of fracture energy. At the same time the effect of material geometry and lay-up on the microscopic mechanisms can be captured by the model. In this work, mode-II delamination of unidirectional laminated composites is simulated by a computational homogenization model which is designed to tackle fracture on both scales. On the mesoscale XFEM is adopted while on the microscale matrix plasticity and cohesive cracking are considered. The model is first applied to a material with simplified micro-structure, in comparison to a layered embedded cell model (DNS) to prove its validity. After that, a micromodel representing composites with random fibre distribution is considered. The contribution of different processes to the fracture energy is analysed by separately tracking energy dissipation due to plastic deformation, fibre/matrix debonding and matrix cracking. The effect of geometry and boundary conditions on the delamination fracture energy is investigated