A large-scale atomistic simulation study was made, of the emission process of multiple dislocations, by using embedded-atom method potentials that were based upon ab initio data. Molecular statics and dynamics techniques were used to study the configuration of the crack tip as the dislocation emission process evolved. In the configuration which was studied, the crack was oriented in a{111}-type plane, with a [110]-type crack front, and the dislocations were emitted into adjacent inclined {111}-type planes. The dislocations were Shockley partials, and formed a twinned region. The numbers of dislocations which were emitted increased with increasing applied stress intensity, and were limited if the dislocations were not permitted to reach their equilibrium positions. The shielding effect of the emitted dislocations decreased the total stress intensity factor at the crack tip, but also caused a net decrease in the mode-II stress intensity factor as projected onto the slip plane of the emitted dislocations. This lower stress intensity along the slip plane limited the emission of new dislocations and, after a number of dislocations had been emitted, the crack advanced by cleavage for several lattice periods. The process was then repeated and resulted in a combined dislocation-emission cum crack-propagation process. The results suggested a mechanism, for the brittle-ductile transition, that depended strongly upon the dislocation mobility and pinning behaviour.

Multiple-Dislocation Emission from the Crack Tip in the Ductile Fracture of Al. D.Farkas, M.Duranduru, W.A.Curtin, C.Ribbens: Philosophical Magazine A, 2001, 81[5], 1241-55