Elementary mechanisms of brittle and semi-brittle fracture

Advances in our understanding of the mechanisms of brittle and semi-brittle fracture processes are often made by direct comparsion of microscopic modeling and carefully designed experiments. In this paper, examples are taken from ab-initio simulations and empirical atomistic modeling 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. Both phenomena have been reported from fracture experiments on silicon single crystals. Dislocation simulations are helpful in analyzing the dependence of fracture toughness on predeformation, temperature or loading rate in semi-brittle materials below the brittle-to-ductile transition. By comp arison 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 requires that dislocation mobility takes control. Furthermore it is shown that the intermediate temperature regime up to the brittle-to-ductile transition temperature can be scaled onto a master curve with one unique activation energy.