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

The generalised Peierls-Nabarro model is used to study the transformation o f screw dislocation cores from the three-fold, non-planar state to the plan ar. 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, i s made easier by the crack opening when compared with the crack-free situat ion. The results suggest that there exists a critical stress intensity fact or K-mc of the order of similar to 0.12 mu root b (mu = 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 a nd mobile. K-mc is found to be lower than the critical stress intensity fac tor for nucleation from crack-tip. and so crack-tip emission is nucleation rather than mobility controlled. (C) 1999 Elsevier Science Ltd. All rights reserved.