We use computer simulations to study the behavior of atomically sharp and blunted cracks in various f.c.c. metals. The simulations use effective medium potentials which contain many-body interactions. We find that when using potentials 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, but is probably caused by surface stresses. When the crack is no longer atomically sharp dislocation emission becomes easier in all the studied metals. The effect is relatively strong; the critical stress intensity factor for emission to occur is reduced by up to 20%. This behavior appears to be caused by the surface stress near the crack tip. The surface stress is a consequence of the many-body nature of the interatomic interactions. The enhanced dislocation emission can cause an order-of-magnitude increase in the fracture toughness of certain materials, in which a sharp crack would propagate by cleavage. Collisions with already existing dislocations will blunt the crack, if this prevents further propagation of the crack the toughness of the material is dramatically increased.