It was noted that, whereas many edge dislocations could be emitted from the tip of a mode-I crack, the distribution of these dislocations was found to depend upon the number of available slip systems as well as upon the order of emission. A study was made here of the situation in which the edge dislocations were emitted one at a time, along the slip plane on which the shear stress was greatest. The results showed that most of the dislocations were confined to 2 or 4 symmetrical planes; even though 34 slip planes were available. Dislocations were emitted along 2 symmetrical planes until they almost reached saturation, and emission then switched to another 2 nearby planes. This switching reduced the crack-tip shielding so that more dislocations were emitted in 4 planes than if the emission was limited to 2 planes. There was a dislocation-free zone, near to the crack tip, whose size increased with the critical stress intensity factor for dislocation emission.

C.F.Qian, J.C.M.Li: Mechanics of Materials, 1996, 24, 1-10