Effect Of Atomic Defects On Fracture Toughness Of Monocrystalline/polycrystalline Graphene

Graphene has attracted extensive attention due to unique optical,electrical and mechanical properties.However,various defects(such as cracks and holes)observed in graphene nanoribbons,which inevitably affects various properties of graphene.In this paper,finite element(FE)analysis and molecular dynamics(MD)simulation are employed to systematically study effects of atomic defects on mechanical properties of single-crystalline and polycrystalline graphene.In addition,effects of the interaction between defects and cracks on mechanical properties of single-crystalline graphene have been further discussed.The main contents of this study are presented as follows:(1)Based on the Reactive Empirical Bond-Order interatomic potential of second generation function(REBO),a nonlinear Timoshenko beam model of C-C bonds in grahpene is established using molecular structural mechanics.First,comparisons between some available experimental results and predictions of MD simulations are also performed to validate the accuracy of FE simulations.Second,the interaction between cracks and defects in single-crystalline graphene with different chiral angles under the type I loading condition has been systematically studied.Results of MD and FE simulations exhibit that tilt angles ? of defects,distances r between defects and the crack tip and the chiral angle of grapheme determine the shielding effect of stress fields at the crack tip.In addition,MD and FE models with defects with different pore sizes and pentagon-heptagon defects with different densities have been established.The results show that the strength of single-crystalline graphene decreases significantly with increasing of pore sizes and defect densities.(2)Mechanical properties of polycrystalline graphene have been further addressed by MD and FE simulations.It was found that with increasing grain size,fracture strengthes of polycrystalline graphenes increased while variation of fracture strains are not ovbious.At the same time,combined with Griffith crack propagation theory,MD and FE are employed to disucuss effects of round hole defects on mechanical properties of polycrystalline graphene.As same to the results in single-crystalline graphene,the strength of polycrystalline graphene decreases significantly with the increase of defect sizes.Moreover,models of bicrystalline graphene with different crystal angles and polycrystalline graphene with multiple grain boundaries have been established.The simulations found that the strengthes of polycrystalline graphene along grain boundary are significantly higher than those of along vertical grain boundary.At last,for polycrystalline graphenes with pre-branched cracks,cracks in graphene can propagate along a certain path by setting different grain boundaries.This study may provide theoretical and numerical modeling support for the reliability and stability of the application of two-dimensional material paper-cutting structures in miacro-electromechanical systems,and has important theoretical guiding for low-dimensional materials to prevent material failure.