EFFECTS OF CRACK-TIP GEOMETRY ON DISLOCATION EMISSION AND CLEAVAGE - A POSSIBLE PATH TO ENHANCED DUCTILITY

We present a systematic study of the effect of crack blunting on subse quent crack propagation and dislocation emission. We show that the str ess intensity factor required to propagate the crack is increased as t he crack is blunted by up to thirteen atomic layers, but only by a rel atively modest amount for a crack with a sharp 60 degrees corner. The effect of the blunting is far less than would be expected from a smoot hly blunted crack; the sharp corners preserve the stress concentration , reducing the effect of the blunting. However, for some material para meters blunting changes the preferred deformation mode from brittle cl eavage to dislocation emission. In such materials, the absorption of p reexisting dislocations by the crack tip can cause the crack tip to be locally arrested, causing a significant increase in the microscopic t oughness of the crack tip. Continuum plasticity models have shown that even a moderate increase in the microscopic toughness can lead to an increase in the macroscopic fracture toughness of the material by seve ral orders of magnitude. We thus propose an atomic-scale mechanism at the crack tip, that ultimately may lead to a high fracture toughness i n some materials where a sharp crack would seem to be able to propagat e in a brittle manner. When the crack is loaded in mode II, the load r equired to emit a dislocation is affected to a much higher degree by t he blunting, in agreement with the estimates from continuum elasticity . In mode II the emission process is aided by a reduction of the free surface area during the emission process. This leads to emission at cr ack loadings which are lower than predicted from the continuum analysi s of Rice.