A systematic study was made of the effect of crack blunting upon subsequent crack propagation and dislocation emission. It was shown that the stress intensity factor that was required in order to propagate the crack increased as the crack was blunted by up to thirteen atomic layers. The increase was relatively modest for a crack with a sharp 60° corner. The effect of blunting was far less than was expected, for a smoothly blunted crack, because the sharp corners preserved the stress concentration and reduced the effect of blunting. For some material parameters, blunting changed the preferred deformation mode from brittle cleavage to dislocation emission. In such materials, the absorption of pre-existing dislocations by the crack tip could cause the crack tip to be locally arrested; thus causing a significant increase in the microscopic toughness of the crack tip. Continuum plasticity models showed that even a moderate increase in the microscopic toughness could lead to an increase, in the macroscopic fracture toughness, of several orders of magnitude. An atomic-scale mechanism at the crack tip was proposed that could lead to a high fracture toughness in materials where a sharp crack was expected to be able to propagate in a brittle manner. When the crack was loaded in mode-II, the load that was required in order to emit a dislocation was affected to a much greater degree by the blunting; in agreement with the predictions of continuum elasticity. In mode-II, the emission process was aided by a reduction in the free surface area during emission. This led to emission at crack loadings which were lower than those which were predicted by the Rice continuum analysis.

Effects of Crack-Tip Geometry upon Dislocation Emission and Cleavage. J.Schiøtz, L.M.Canel, A.E.Carlsson: Physical Review B, 1997, 55[10], 6211-21