Crack blunting effects on dislocation emission from cracks

A series of atomistic calculations is performed in order to establish criteria for dislocation emission from cracks in a model hexagonal lattice. We find that except for special orientations, additional tensile broken-bond effects must be added to existing theories based on shear effects, in order to correctly estimate the critical stress intensity ${\mathit{K}}_{\mathit{e}}$ required for dislocation emission. From this, a new ductility criterion for materials is proposed which does not depend on the intrinsic surface energy, but contains only the unstable stacking fault parameter.