Molecular dynamics simulation of nanoindentation of single-layer graphene sheet

The nanoindentation of single-layer graphene sheets is simulated using molecular dynamics simulation. The nano-mechanical properties and failure mechanisms of single-layer graphene sheets are investigated at atomic scale. A typical load-displacement curve is obtained and the influences of the radii of indenter and graphene, temperature and different boundary conditions on the simulation results are analyzed. Observation of the deformation process shows that, when the indenter displacement reaches a critical depth, hc, the single-layer graphene sheet undergoes plastic ripping damage due to the atomic bond breaking beneath the indenter. Further analysis of the stress distribution on the graphene around the destruction area reveals that, when a gap is generated on the graphene, the maximum stress decreases rapidly and a homogenization distribution occurs. In addition, repeated loading-unloading processes are performed on the graphene sheet. When the penetration depth is less than the critical value, hc, the sheet undergoes an entire elastic deformation. However, when penetration depth is larger than hc, the deformed hexagon beneath the indenter can not be fully restored to the original state. Easier deformation of graphene may initiate from these bonds, which in turn, results in significant decrease in strength and deformation of the graphene. In addition, the elastic modulus and deformation mechanism of the graphene sheet are strongly dependent on the temperature. The rise of temperature leads to the decrease in the elastic modulus and the failure limit to some extent. Different radii of the single-layer graphene sheets affect the critical penetration depth on nanoindentation. This is due to the homogenization distribution of deformation in a wide range on graphene sheet. But it has a little effect on the elastic modulus and failure stress.