A simple continuum-based model was developed which assumed the existence of a dislocation-free core of size, R, and a crack tip which did not emit dislocations. The core size was chosen in a self-consistent manner by equating a maximum equivalent stress in the plastic zone to one that was predicted by a phenomenological hardening law which made the flow stress proportional to 1/R. When the inner elastic stress field was matched to the approximate stress field within the plastic zone, it was found that the applied energy release rate that was required in order to initiate crack extension was several orders of magnitude greater than the ideal work of fracture. This apparent shielding of the crack tip was found to depend sensitively upon the ideal work of fracture.

D.M.Lipkin, G.E.Beltz: Acta Materialia, 1996, 44[4], 1287-91