Role of crack blunting in ductile versus brittle response of crystalline materials

Continuum concepts are used to evaluate the competition between crack advan ce and dislocation nucleation at the tip of a crack having a finite tip cur vature. The analysis reveals that the favorability of crack propagation ver sus dislocation emission depends upon the bluntness of the crack tip. One i mplication is that a crack may initially emit dislocations only to reinitia te cleavage upon reaching a sufficiently blunted tip geometry. The present framework is used to classify crystals as intrinsically ductile or brittle in terms of the unstable stacking energy, the surface energy, and the peak cohesive stresses achieved during opening and shear of the atomic planes. [ S0031-9007(99)09235-2].