Mixed mode crack propagation in staggered biocomposites using phase field modelling.

Exceptional fracture resistance and specific strengths observed in several natural biocomposites have inspired many researchers to discern the underlying mechanisms responsible for their mechanical behavior. Staggering of stiff mineral platelets in the layers of organic phase akin to the brick and mortar configuration is understood to be one of the key factors contributing to their high elastic modulus and toughness. The elastic heterogeneties in these configurations are shown to cause crack branching and kinking, leading to the increased resistance to fracture. Most of the fracture mechanisms discussed in the literature intrinsically assume mode I fracture. The presence of mixed modes of deformation in staggered composites may give rise to new interesting fracture mechanisms. In this paper we study crack propagation in staggered composites under mixed mode conditions using a phase field method. We find four different crack trajectories which will depend on the elastic modulus mismatch, microstructure geometry and the mode mixity. For very high elastic moduli mismatch of organic matrix and the mineral, we find that the crack trajectories are nearly independent of the mode mixity and the cracks propagate without kinking. For moderate elastic modulus mismatch and high mode mixity ratio (K/K) we find that the cracks divert into the interface leading to interface delamination. The mechanism that controls the crack trajectories is analyzed in terms of maximum tangential stress σ and strain energy density criteria at the crack tip.