  We study Mode I crack propagation in amorphous material via a molecular
dynamics simulation of a binary alloy with pairwise central-force interactions.
We find that when the system is subjected to constant displacement after
introduction of a seed crack, the crack does not propagate. This was found to
be true both for the modified Lennard-Jones potential of Falk and a
shorter-range potential of our own devising. Crystalline samples prepared with
these potentials exhibit normal brittle fracture, with running cracks. Only
when subjected to a constantly increasing strain did the crack in the glasses
lengthen. Even here, once the strain stopped increasing, the crack tip stopped
moving. We examined the stress-strain curve for our model materials, and found
that they did not exhibit the saturated stress plateau characteristic of
ductile materials. We attribute the failure of crack propagation in this system
to the availability of many meta-stable configurations available to the crack
tip which serve to soak up the energy which would otherwise drive the crack. We
conclude that this class of models is inadequate to describe crack propagation
in brittle glasses.
