  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.
