Crack tip blunting by dislocation pair creation and separation

A smeared-out-dislocation-type analysis is used to determine conditions for the blunting of an atomically sharp crack that is opened intension. It is found that when the ratio of the theoretical tensile strength σmax to theoretical shear strength τmax is large, pairs of dislocations of opposite sign are created, as the stress is raised, on inclined slip planes whose intersection with the crack plane coincides with the crack tip. Dislocations of one sign move into the lattice away from the crack tip region and dislocations of the other sign pile-up against the crack tip. The pile-up barrier is the surface energy that is required to create new surface at the crack tip. If the stress is raised further, enough piled-up dislocations are created to enable the surface energy barrier to be penetrated. The piled-up dislocations enter the crack tip and in so doing blunt the tip. When σ max/τmax > 7 a crack tip blunts. The common f.c.c. metals satisfy this condition. The b.c.c. metals iron and tungsten have theoretical strength ratios somewhat below 7 and cracks in these metals may not blunt, at least at low temperatures.