Computer simulations were used to study the behaviour of atomically sharp or blunted cracks in various face-centered cubic metals. The simulations involved effective-medium potentials and many-body interactions. It was found that, when using potentials which represented Pt or Au, a sharp crack was stable with respect to the emission of a dislocation from the crack tip. The sharp crack was unstable for all of the other studied metals. This result could not be explained by existing criteria for the intrinsic ductile brittle behaviour of crack tips, but was thought to be caused by surface stresses. When the crack was no longer atomically sharp, dislocation emission became easier in all of the studied metals. This effect was relatively strong, and the critical stress-intensity factor for emission was reduced by up to 20%. The behaviour appeared to be caused by surface stresses near to the crack tip. The surface stress was a consequence of the many-body nature of the interatomic interactions. Enhanced dislocation emission could cause an order-of-magnitude increase in the fracture toughness of certain materials in which a sharp crack propagated by cleavage. Collisions with existing dislocations would blunt the crack. If this prevented further propagation of the crack, the toughness of the material was sharply increased.

The Influence of Surface Stress on Dislocation Emission from Sharp and Blunt Cracks in FCC Metals. J.Schiøtz, A.E.Carlsson: Philosophical Magazine A, 2000, 80[1], 69-82