A new self-consistent simulation method was developed for modelling crack growth, involving dislocation generation and motion, during constant loading-rate mode-III fracture. The dislocations which were emitted by the crack initially self-organized and propagated in very sharply defined lines. The latter underwent bifurcation to form multiple new branches and shorten the initial line. The growth and bifurcation of the lines occurred repeatedly. A highly structured plastic zone formed, away from the crack, that was approximately elliptical in shape and had a dislocation-free zone along its mid-plane. The generation-rate of new dislocations was limited by the rate at which previously generated dislocations moved away from the crack tip. This rate was controlled by the crack loading-rate and the dislocation mobility. The crack-tip stress-intensity factor was much smaller than the applied stress-intensity factor. The former increased sub-linearly with load, and exhibited jumps and serrations which corresponded to instabilities in the dislocation microstructure. However, the crack-tip stress-intensity factor increased with increasing loading rate at a fixed load, and a transition between brittle and ductile behavior occurred with decreasing loading-rate. Crack propagation occurred when dislocations could not be generated at the crack tip at a rate which was sufficiently high to counterbalance the increasing loading. This generation rate increased with increasing dislocation mobility. Since dislocation motion was thermally activated, this demonstrated that the brittle-ductile transition was ultimately controlled by dislocation migration.

Dynamic Simulation of Dislocation Microstructures in Mode III Cracking. N.Zacharopoulos, D.J.Srolovitz, R.Lesar: Acta Materialia, 1997, 45[9], 3745-63