Mobility of non-planar screw dislocations ahead of a mode III crack tip

Abstract

The generalized Peierls-Nabarro model is used to study the transformation of screw dislocation cores from the three-fold, non-planar state to the planar, glissile state ahead of a mode III crack tip. The image effects of the crack result in extended dissociation on slip planes that are inclined with respect to the crack plane. The conversion to the planar state, however, is made easier by the crack opening when compared with the crack-free situation. The results suggest that there exists a critical stress intensity factor Kmc of the order of approximately 0.12 μ√b (μ = shear modulus, b = Burgers vector), such that if the applied stress intensity factor is larger than this value, all dislocations ahead of the crack tip would be planar and mobile. Kmc is found to be lower than the critical stress intensity factor for nucleation from crack-tip, and so crack-tip emission is nucleation rather than mobility controlled.