The unstable stacking criteria for an ideal crystal under homogeneous shearing, and for a cracked crystal under pure mode-II loading, were analyzed. In the case of a crystal under homogeneous shearing, an unstable stacking energy was associated with shear (with no relaxation) in a direction that was normal to the slip plane. In the case of a relaxed shear configuration, the critical condition for unstable stacking did not correspond to the relative displacement, b/2, where b was the Burgers vector magnitude of the Shockley partial dislocation. It instead corresponded to the maximum shear stress. On the basis of this result, another unstable stacking energy was defined for the relaxed lattice. In a cracked crystal under pure mode-II loading, the dislocation configuration which corresponded to a relative displacement of b/2 was a stable state and no instability occurred during dislocation nucleation. The instability occurred at a relative displacement of about 3b/4. Yet another unstable stacking energy was defined which corresponded to the unstable stacking state at which dislocation emission took place. Molecular dynamics studies of an atomistic model confirmed this analysis.

Y.W.Zhang, T.C.Wang, Q.H.Tang: Philosophical Magazine A, 1995, 72[4], 881-9