Several activated configurations of embryonic dislocations that nucleated from the tip of a cleavage crack were analyzed. The activation configurations included nucleation on inclined planes, on oblique planes and on cleavage ledges, and were treated within the classical Peierls framework. A variational boundary integral method, with a previously developed inter-planar tension shear potential, was used to deduce the saddle-point configurations of embryonic dislocation loops, and their associated energies. On the basis of the assumption that the brittle-to-ductile transition in cleavage fracture was a nucleation-controlled process (especially in metals such as -Fe), the results of the calculations were used to estimate the brittle-to-ductile transition temperature. It was concluded that dislocation nucleation on cleavage ledges alone led to realistic values for the transition temperature. The homogeneous nucleation of dislocations on inclined or oblique planes required transition temperatures that were well above the melting point. This suggested that the nucleation of dislocations from a crack tip in intrinsically brittle crystals was possible only at local crack-front heterogeneities such as cleavage ledges, and that homogeneous dislocation nucleation from a straight crack front was not possible. This suggestion was supported by the experimental observation that dislocation nucleation from a crack tip was a rare event, which occurred preferentially at heterogeneities.

Critical Configurations for Dislocation Nucleation from Crack Tips. G.Xu, A.S.Argon, M.Ortiz: Philosophical Magazine A, 1997, 75[2], 341-67