Experimentally measuring weak fracture toughness anisotropy in graphene

Abstract

The two-dimensional nature of graphene offers a number of interesting mechanical properties. Amongst these, fracture toughness has received substantial interest, yet computational works have not reached a consensus regarding anisotropy in its fracture energy when graphene is loaded in armchair or zigzag directions. Here, we resolve the steps involved during fracture of graphene by carrying out in situ tensile tests. Embryo cracks nucleated from the graphene edges are observed to deflect into major cracks with local kinking features, as explained by an evolving stress intensity factor during crack advance. Extended finite element analysis with the maximum energy release rate criterion is used to model the fracture process. We determine a weak degree of anisotropy in the fracture toughness, Gc(armchair)/Gc(zigzag), of 0.94, which aligns with previous predictions from first-principles calculations and observed growth kinetics of graphene crystals in experiments.