  Molecular-dynamics simulations of fracture in metallic glass-like systems are
observed to undergo embrittlement due to a small change in interatomic
potential. This change in fracture toughness, however, is not accompanied by a
corresponding change in flow stress. Theories of brittle fracture proposed by
Freund and Hutchinson indicate that strain rate sensitivity is the controling
physical parameter in these cases. A recent theory of viscoplasticity in this
class of solids by Falk and Langer further suggests that the change in strain
rate sensitivity corresponds to a change in the susceptibility of local shear
transformation zones to applied shear stresses. A simple model of these zones
is develped in order to quantify the dependence of this sensitivity on the
interparticle potential.
