| Pristine graphene with perfectly hexagonal carbon rings exhibits extraordinary mechanical properties. However, large-scale monolayers of graphene grown by chemical vapor deposition usually contain grain boundaries, cracks and other topological defects and which might strongly influence the apparent properties of the 2D material. The direct experimental measurements on the mechanical properties of graphene are ususlly challenging; in contrast, molecular dynamics simulations provide an atomic-scale method to investigate the behaviors of graphene under various loading conditions, and have been widely used in studying the mechanical properties of both pristine and polycrystalline graphene.In the first two chapters of this thesis, we briefly introduce the two-dimensional material and the numerical method. The main research work of this thesis includes two parts, which are discussed in the third and fourth chapters, respectively.In the third chapter, we focus on the mechanical properties of graphene with pre-set crack aligned along the grain boundary direction. Uniaxial tension is applied perpendicular to the crack and grain boundary. Based on the stress-strain curves, we demonstrate the influence of the atomic structure at crack tip on the mechanical properties, with an emphasis on the length of crack and grain boundary in affecting the strength of graphene.In the fourth chapter, atomistic simulations of both nanoindentation and uniaxial tension are employed to explore the role of grain size in influencing the mechanical properties of polycrystalline graphene. In the simulations, the failure of polycrystalline graphene always occurs at grain boundary junctions, showing that the poly-graphene samples are weakened by the combination of grain boundary junctions, holes and topological defects, and the interpretations of Young’s modulus and breaking strength, from both nanoindentation and uniaxial tension, are strongly influenced by the grain size of poly-graphene. |
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