Elastic criterion for dislocation nucleation

The notion of theoretical strength, the critical stress at which a perfect crystal under uniform loading becomes structurally unstable, is extended to non-uniform loading. A position-dependent defect nucleation criterion has been derived and applied in molecular dynamics (MD) and finite-element simulations of dislocation emission in single-crystal nanoindentation. The resulting measure has the physical meaning of a local stiffness; it provides a rigorous basis for modeling the incipient plasticity in a thin-film material. Furthermore, a close connection has been shown to exist between the initial unstable elastic wave and the final atomistic defect.