  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.
