A theory which had attributed sharp brittle-ductile transitions to a cooperative Kosterlitz-Thouless instability, with respect to dislocation generation, was criticized. The results of simulations showed that, contrary to the claims made, the theory did not predict a strain-rate dependent temperature for the sharp transition. It instead predicted a strain-rate independent sharp transition or, in the quasi-brittle regime, a strain-rate dependent gradual transition. The theory in its original form therefore did not explain experimentally observed, strain-rate dependent, sharp transitions. Evidence gleaned from experiments and simulations suggested that such a transition was essentially due to the non-homogeneous emission of dislocations from the crack tip. Emission began at certain points along the crack tip and generated a strongly shielding plastic zone which traversed the whole length of the crack tip at the transition temperature, before the stress reached that for brittle fracture. For a given strain-rate, the transition temperature was therefore controlled by the dislocation velocity, and by a length which depended upon the original source distribution. This model correctly predicted the strain-rate dependence of the sharp transition, and explained the fact that it was structure-sensitive.

Comment on the Brittle-to-Ductile Transition - a Cooperative Dislocation Generation Instability; Dislocation Dynamics and the Strain-Rate Dependence of the Transition Temperature. P.B.Hirsch, S.G.Roberts: Acta Materialia, 1996, 44[6], 2361-71