Modelling brittle and semi-brittle fracture processes

Recent advances in the microscopic modelling of fracture processes are summarised and discussed in this paper. Examples are taken from ab-initio simulations and empirical atomistic modelling of brittle fracture which show that the production of metastable fracture surfaces or directional cleavage anisotropy are readily anticipated consequences of the discrete nature of the bond breaking at the crack tip. Dislocation simulations are necessary to predict the dependence of fracture toughness on predeformation, temperature or loading rate in semi-brittle materials below the brittle-to-ductile transition. By comparison of fracture experiments on tungsten single crystals with simulations, it is shown that dislocation nucleation is the limiting factor at low temperatures, while the dependence on loading rate at intermediate temperatures can only be understood if dislocation mobility takes control. Furthermore, it is shown that the entire temperature and strain rate dependence of the fracture toughness can be scaled onto a master curve with one unique activation energy.