Defects in solid commonly limit mechanical performance of the material. However, recent measurements reported that the extraordinarily high strength of graphene is almost retained with the presence of grain boundaries. We clarify in this work that lattice defects in the grain boundaries and distorted geometry thus induced define the mechanical properties characterized under specific loading conditions. Atomistic simulations and theoretical analysis show that tensile tests measure in-plane strength that is governed by defect-induced stress buildup, while nanoindentation probes local strength under the indenter tip and bears additional geometrical effects from warping. These findings elucidate the failure mechanisms of graphene under realistic loading conditions and assess the feasibility of abovementioned techniques in quantifying the strength of graphene, and suggest that mechanical properties of low-dimensional materials could be tuned by implanting defects and geometrical distortion they leads to.
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