  The deformation and fracture behaviour of one-atom-thick mechanically
exfoliated graphene has been studied in detail. Monolayer graphene flakes with
different lengths, widths and shapes were successfully prepared by mechanical
exfoliation and deposited onto poly(methyl methacrylate) (PMMA) beams. The
fracture behaviour of the monolayer graphene was followed by deforming the PMMA
beams. Through in-situ Raman mapping at different strain levels, the
distributions of strain over the graphene flakes were determined from the shift
of the graphene Raman 2D band. The failure mechanisms of the exfoliated
graphene were either by flake fracture or failure of the graphene/polymer
interface. The fracture of the flakes was observed from the formation of cracks
identified from the appearance of lines of zero strain in the strain contour
maps. It was found that the strength of the monolayer graphene flakes decreased
with increasing flake width. The strength dropped to less than 10 GPa for large
flakes, much lower than the reported value of 130 GPa for monolayer graphene,
thought to be due to the presence of defects. It is shown that a pair of
topological defects in monolayer graphene will form a pseudo crack and the
effect of such defects upon the strength of monolayer graphene has been
modelled using molecular mechanical simulations.
