It was recalled that stress relaxation at a crack tip relied upon the material’s ability to generate dislocations. However, in spite of the extensive literature devoted to crack–dislocation interactions, it had not yet been explained how dislocations appeared and multiplied in order to build a fully plastic zone. It was shown here how a simple event, such as the intersection of a unique incoming dislocation with a crack front, induced the generation of new dislocations via so-called stimulated emission. When presented to the applied crack stress field, these dislocations could repeat the stimulation process step-by-step all along the crack front via a cross-slip mechanism. Such a rapidly increasing rate of dislocation nucleation led to a sudden growth of the plastic zone (avalanche). The resultant avalanche dislocation multiplication was explained by cross-slipping. This mechanism was more or less efficient, depending upon the material. It was expected that, the easier this process (large number of slip-planes, high stacking-fault energy), the faster was the relaxation process at the crack tip, and the tougher the material. Since the dislocation multiplication process was dynamic, it influenced the brittle-to-ductile transition which resulted from a dynamic competition between the applied loading rate and the shielding rate. According to the previous observations, the behaviour of materials having a Frank network (engineering materials) should be more ductile than materials having zero grown-in dislocations.

Interaction of a Dislocation with a Crack Tip: From Stimulated Emission to Avalanche Generation. G.Michot: Acta Materialia, 2011, 59[10], 3864-7