Atomistic simulation of dislocation nucleation and motion from a crack tip

A recently developed fully atomistic technique for fracture simulations is applied to the study of dislocation emission from a crack tip in an elastically anisotropic f.c.c. crystal. The detailed atomicscale mechanisms of dislocation nucleation and motion are investigated as a function of the external load. Analysis of the atomic configurations around the crack tip demonstrates an intimate coupling of the nucleating dislocation with a step formed at the crack surface. Displacement and stress fields around both nucleating and moving dislocations are compared to the predictions of the Peierls-Nabarro continuum-elastic model by Rice. The size of a nucleating (‘incipient”) dislocation is found to be larger than that of a fully-formed dislocation. Also, we elucidate the reasons why the value of the unstable-stacking energy estimated by means of the rigid-block sliding concept, a feature common to several continuum-elastic models, overestimates the activation energy for dislocation nucleation. We conclude that the concept of unstable-stacking energy should be replaced by the true energy barrier for dislocation nucleation, incorporating the full inhomogeneity of the displacement field.