  Atomistic simulations were utilized to obtain microscopic information of the
elongation process in graphene sheets consisting of various embedded symmetric
tilt grain boundaries (GBs). In contrast to pristine graphene, these GBs
fractured in an extraordinary pattern under transverse uniaxial elongation in
all but the largest misorientation angle case, which exhibited intermittent
crack propagation and formed many stringy residual connections after quasi
mechanical failure. The strings known as monoatomic carbon chains (MACCs),
whose importance was recently highlighted, gradually extended to a maximum of a
few nanometers as the elongation proceeded. These features, which critically
affect the tensile stress and the shape of stress-strain curve, were observed
in both armchair and zigzag-oriented symmetric tilt GBs. However, there exist
remarkable differences in the population density and the achievable length of
MACCs appearing after quasi mechanical failure which were higher in the
zigzag-oriented GBs. In addition, the maximum stress and ultimate strain for
armchair-oriented GBs were significantly greater than those of zigzag-oriented
GBs in case of the largest misorientation angle while they were slightly
smaller in other cases. The maximum stress was larger as the misorientation
angle increased for both armchair and zigzag-oriented GBs ranging between 32~80
GPa, and the ultimate strains were between 0.06~0.11, the lower limit of which
agrees very well with the experimental value of threshold strain beyond which
mechanical failure often occurred in polycrystalline graphene.
