Atomistic Molecular Dynamics Simulation Based Failure Criterion of Polycrystalline Graphene Under Biaxial Loading

Graphene is a monoatomic thick layer of sp²-hybridized carbon atoms tightly packed in a honeycomb lattice structure. Since its discovery, it has drawn extensive attention to the science community for its unique 2D structure and studied for both basic science and commercial applications due to its extraordinary thermal, optical and mechanical properties. Large-scale graphene sheets required for industrial applications are fabricated by chemical vapor deposition (CVD). High resolution transmission electron microscopy (HR-TEM) data have shown that these sheets are typically polycrystalline — that is, composed of single-crystalline grains of varying orientation joined by grain boundaries (GBs). GBs are composed of periodic and aperiodic sequence of dislocations. The dislocations are 1D line defects consisting of regular array of edge sharing pentagone/heptagone pairs. GBs act as line defects and defective graphene show different mechanical properties from pristine graphene. It has been previously reported that defect free graphene is the strongest material with Young’s modulus of 1.0 ±0.1 TPa and strength of 130 GPa. However, inclusion of GBs reduces the Young's modulus to 600 GPa.

The effects of various factors such as system temperature, strain rate, surface defects, grain boundaries and chirality on the mechanical and fracture properties of graphene are studied in the past. Molecular dynamics (MD) has been extensively used to explore the mechanical properties of graphene and graphene like materials, due to the limitation of labratory experiment or first principle calculations. Most of these MD studies have focused on the mechanical and fracture properties under uniaxial loading for polycrystalline graphene. The studies on the fracture properties of polycrystalline graphene under biaxial loading is rare. In this study we use MD simulations to investigate fracture properties of polycrystalline graphene under uniaxial and biaxial loading. The molecular dynamics (MD) simulations are performed using screened environment-dependent potential of Pastewka et al., known as REBO2+S. A series of MD simulations are conducted on infinite polycristalline graphene sheets with various grain sizes in order to understand the failure criterion for polycristalline graphene in biaxial loading. We also have studied the effect of temperature and strain rate on mechanical properties of polycrystalline graphene for both uniaxial and biaxial loading conditions. Stresses along the Cartesian axis of polycrystalline graphene are normalized to the corresponding uniaxial failure strength values and combinations of this normalized strength are used to define failure envelopes in biaxial loading at different temperatures and strain rate. Our results show that a bilinear failure envelope can predict the tensile strength of polycrystalline graphene under biaxial loading at different temperature.