Crack propagation in staggered structures of biological and biomimetic composites

Abstract

A phase field model is used to study crack propagation in staggered structures that are commonly found in several biological and biomimetic composites. The composite is modelled by creating an elastic mismatch between the two phases, ‘mineral’ and ‘organic’ which form into a staggered brick and mortar type micro-structure. The huge disparity in the stiffness of the two constituent phases gives rise to a non-uniform stress field near crack tips in these materials. Depending on the arrangement of the mineral platelets, different mechanisms of crack propagation may be observed. We find that cracks propagate straight when the aspect ratio of the mineral platelets is higher than a critical value. For lower values of aspect ratio, the cracks tend to exhibit a tortuous crack path in which fracture predominantly occurs in the soft organic phase. This critical aspect ratio is found to be a function of the mineral volume fraction as well as the elastic modulus mismatch. For some configurations, micro cracking in regions close to the crack tips is also observed. A simple theory is presented to analyse the observed crack paths in staggered composites.