  We use molecular dynamics to study the nucleation of cracks in a two
dimensional material without pre-existing cracks. We study models with zero and
non-zero shear modulus. In both situations the time required for crack
formation obeys an Arrhenius law, from which the energy barrier and pre-factor
are extracted for different system sizes. For large systems, the characteristic
time of rupture is found to decrease with system size, in agreement with
classical Weibull theory. In the case of zero shear modulus, the energy
opposing rupture is identified with the breakage of a single atomic layer. In
the case of non-zero shear modulus, thermally activated fracture can only be
studied within a reasonable time at very high strains. In this case the energy
barrier involves the stretching of bonds within several layers, accounting for
a much higher barrier compared to the zero shear modulus case. This barrier is
understood within adiabatic simulations.
