Molecular dynamics methods were used to analyze the propagation of an atomistic crack tip. The simulations revealed that the crack propagated at a relatively constant overall velocity which was much lower than the Rayleigh-wave velocity. However, the local propagation velocity oscillated wildly, and was limited by the longitudinal wave velocity. The oscillation in crack velocity was attributed to repeated crack-tip blunting and sharpening. When the crack-tip opening displacement exceeded a critical value, lattice instability occurred and resulted in dislocation emission from the crack tip. On the basis of this observation, a criterion was proposed for dislocation emission from a moving crack tip. The simulation also indicated that the emitted dislocation was a source of micro-crack nucleation.

Y.W.Zhang, T.C.Wang: Journal of Applied Physics, 1996, 80[8], 4332-6