Modelling of dislocation mobility controlled brittle-to-ductile transition

The phenomenon of brittle-to-ductile transition (BDT) is known to be controlled by the competition between cleavage fracture and dislocation activity at crack tips. But the transition could be determined by one of the two successive processes, dislocation nucleation, or dislocation motion. The model material is assumed to undergo elastic-rate dependent plastic deformation with the plastic strain rate scaled with dislocation velocity. An isotropic plasticity theory is used. The BDT is assumed to occur when the crack tip is shielded by the surrounding plastic zone such that it never reaches the critical stress intensity for cleavage fracture. The crack tip shielding due to plastic deformation is evaluated using the finite element method. The dimensionless groups which affect the BDT are identified, and a parametric study is performed to reveal the effects of various dimensionless paramters. Numerical results using the specific material properties of Si single crystals are compared with experimental data. Good agreements are obtained. Some interesting features of the BDT behavior are predicted by our computer simulations which require confirmation by experimental studies