Nucleation of cracks in a brittle sheet.

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 nonzero shear moduli. In both situations, the time required for crack formation obeys an Arrhenius law, from which the energy barrier and prefactor 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 bond. In the case of nonzero shear modulus, thermally activated fracture can only be studied within a reasonable time at very high strains. In this case, the energy barrier is much higher compared to the zero shear modulus case. This barrier is understood within adiabatic simulations.