Effects of topological point reconstructions on the fracture strength and deformation mechanisms of graphene

Recent experimental studies have identified a number of topological reconstructions of point defects in graphene, however, their influence on fracture strength has not been studied. From a weakest-link perspective, each of these defects are potentially strength-limiting features, necessitating investigation of their effects on mechanical properties. In the present study, molecular dynamics tensile simulations are performed to quantify the strengths of single and divacancy topological reconstructions with consideration of temperature and strain rate effects. Fracture strengths in the range of 92-101 GPa are obtained for the topological reconstructions at 300 K and a strain rate of 10(9)/s. This range shifts to 81-95 GPa when the loading rate is decreased to 5 x 10(6)/s, highlighting the significant influence of kinetic factors on fracture strength. Similarly, an increase in temperature causes appreciable strength reductions, resulting in fracture strengths of 87-97 GPa at 450 K and a 10(9)/s strain rate. In order to provide a meaningful comparison to the limited experimental data available, the energy barriers for fracture are determined from thermal activation theory. Analytical calculations predict fracture strengths in the range of 50-79 GPa at 300 K and 10(0)/s, which agrees well with experimental reports. Surprisingly, the topological point reconstructions with under-coordinated atoms and highest potential energies are found to be the strongest defects. Physically, under-coordinated atoms are observed to undergo bond rotations, enabling a deformation accommodation mechanism that suppress brittle fracture and leads to improved flaw tolerance. This finding is supported by Quantized Fracture Mechanics calculations. (C) 2014 Elsevier B. V. All rights reserved.