It was noted that dislocations were observed to be emitted, as Shockley partials, from crack tips during atomistic simulations of face-centered cubic metals. The crystallographic anisotropy that was associated with emission as partials, rather than perfect dislocations, was considered. The stress intensity which was required in order to emit a dislocation was derived in terms of both anisotropic and isotropic versions of a Peierls-like criterion. The latter had been proposed for dislocation emission, and involved a critical emission force. By systematically changing the orientation of the crystal relative to the crack coordinates, the behavior of atomic models (embedded-atom method) was observed to change from emission to crack extension in a way that was quantitatively consistent with predictions. Upon using the value of the critical emission force that best represented monocrystal results, the predictions were also found to agree with the behavior of a double-ended crack on a grain boundary; where one end emitted dislocations while the other end extended in a brittle manner. The emission of trailing dislocations was also considered, and it was found that the second and third dislocations required slightly higher stress intensities for their emission. However, because the critical emission force required to extend the crack was also increased, the emission of the first dislocation remained the critical step. It was suggested that, although the critical emission force was often associated with the unstable stacking-fault energy, it might not always be adequately defined merely in terms of the unstable stacking-fault energy.

Crystallographic Aspects of Dislocation Emission from a Crack Tip in an FCC Metal R.G.Hoagland: Philosophical Magazine A, 1997, 76[3], 543-63