Dislocation emission from sharp and blunt cracks in f.c.c. metals

We use computer simulations to study the behavior of sharp and blunted cracks in various f.c.c. metals. We find that in models representing platinum and gold a sharp crack is stable with respect to the emission of a dislocation from the crack tip, whereas for all other metals studied the sharp crack is unstable. This result cannot be explained by existing criteria for the intrinsic ductile/brittle behavior of crack tips. Blunting the crack makes dislocation emission easier in all the studied metals. The effect is relatively strong; the critical load for emission to occur is reduced by up to 20%. This behavior is caused by the surface stress in the surface at the end of the blunt crack. This can cause an order-of-magnitude increase in the fracture toughness of certain materials, in which a sharp crack would propagate by cleavage, but collisions with already existing dislocations blunt the crack, and thereby prevent further cleavage.