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Fracture size effects in nanoscale materials: the case of graphene

Nanoscale materials display enhanced strength and toughness but also larger fluctuations and more pronounced size effects with respect to their macroscopic counterparts. Here we study the system size-dependence of the failure strength distribution of a monolayer graphene sheet with a small concentration of vacancies by molecular dynamics simulations. We simulate sheets of varying size encompassing more than three decades and systematically study their deformation as a function of disorder, temperature and loading rate. We generalize the weakest-link theory of fracture size effects to rate and temperature dependent failure and find quantitative agreement with the simulations. Our numerical and theoretical results explain the crossover of the fracture strength distribution between a thermal and rate-dependent regime and a disorder-dominated regime described by extreme value theory.