The generalized Peierls-Nabarro model was used to study the transformation of screw dislocation cores, from the threefold non-planar state to the planar glissile state, ahead of a mode-III crack tip. The image effects of the crack resulted in extended dissociation on slip planes that were inclined with respect to the crack plane. Conversion to the planar state was made easier by crack opening, as compared with the crack-free situation. The results suggested that there existed a critical stress intensity factor which was of the order of 0.12μ√b; such that, if the applied stress intensity factor was larger than this value, all of the dislocations ahead of the crack tip would be planar and mobile. The above stress intensity factor was lower than the critical stress intensity factor for nucleation from the crack tip, so crack-tip emission was nucleation-controlled rather than mobility-controlled.

Mobility of Non-Planar Screw Dislocations ahead of a Mode III Crack Tip. A.H.W.Ngan, H.F.Zhang: Mechanics of Materials, 1999, 31, 367-79