Experimentally measuring weak fracture toughness anisotropy in graphene

Graphene is known to display a number of attractive mechanical properties. Here, anisotropy in the fracture toughness of graphene is investigated by in-situ mechanical testing, revealing weak anisotropy between armchair and zigzag directions. 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, G(c(armchair))/G(c(zigzag)), of 0.94, which aligns with previous predictions from first-principles calculations and observed growth kinetics of graphene crystals in experiments.

Automated summary

This study investigates the anisotropy in fracture toughness of graphene and reveals weak anisotropy between armchair and zigzag directions using in-situ mechanical testing. In-situ tensile tests are carried out to observe the fracture process of graphene, and it is found that embryo cracks nucleated from the graphene edges deflect into major cracks with local kinking features. Extended finite element analysis with the maximum energy release rate criterion is used to model the fracture process, and a weak degree of anisotropy in the fracture toughness is determined.