DYNAMIC SIMULATION OF DISLOCATION MICROSTRUCTURES IN MODE-III CRACKING

We have developed a new, self-consistent simulation method for modelin g crack growth with dislocation generation and motion in constant-load ing-rate, Mode-III fracture. The dislocations emitted from the crack i nitially self-organize and propagate in very sharply defined lines. Th ese lines undergo bifurcations, forming multiple new branches and shor tening the initial line. The growth and bifurcation of these lines occ urs repeatedly. Away from the crack, a highly structured plastic zone is formed that is approximately elliptical in shape with a dislocation free zone along its mid-plane. The rate of generation of new dislocat ions is limited by the rate at which previously generated dislocations move away from the crack tip. This rate is controlled by the crack lo ading rate (K) over dot(III) and the dislocation mobility. The size of the plastic zone scales as (K-III(2)/(K) over dot(III))(2/3). The cra ck tip stress intensity factor K-tip is very much smaller than the app lied stress intensity factor. K-tip increases sub-linearly with the lo ad and exhibits both jumps and serrations corresponding to instabiliti es in the dislocation microstructure. K-tip increases, however, with i ncreasing loading rate at fixed load and a transition is seen between brittle and ductile behavior with decreasing loading rate. Crack propa gation occurs when dislocations cannot be generated at the crack tip a t a rate sufficient to counterbalance the increasing loading. This gen eration rate increases with increasing dislocation mobility. Since dis location motion is thermally activated, this demonstrates that the bri ttle-to-ductile transition is ultimately controlled by dislocation mig ration. (C) 1997 Acta Metallurgica Inc.