Fracture modes of brittle junctions under shear

Abstract

Friction and wear of brittle materials during sliding contact are often associated with the fracture-induced failure of interlocking asperities under shear loading. Using systematic in-situ experiments we disclosed two dominating fracture failure modes associated with the aspect ratio of the junction: (I) small aspect ratio junctions fail antisymmetrically by stable propagation of two subsurface cracks, (i.e. hand-shaking mechanism), causing the roughness creation on both surfaces and formation of particles, (II) large aspect ratio junctions fail asymetrically by unstable propagation of a single crack. Numerical simulations revealed that the transition between these two failure modes is dictated by a competition between shear versus bending induced stress state. Combining the linear elastic fracture mechanics and Timoshenko beam theory, a simple theoretical model is developed, predicting the transition in the critical force associated with each mode as a function of junction’s aspect ratio and fracture properties. This understanding opens new venues towards developing a fracture model for surface asperities and physics-based predictive models for wear of brittle solids.