13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- I/II Mixed Mode Fracture in Graphene Bin Zhang*, Lanjv Mei 1 State Key Laboratory of Mechanics and Control of Mechanical Structures, and College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China * Corresponding author: beenchang@nuaa.edu.cn Abstract Nanoscale fracture of pre-cracked graphene under coupled in-plane opening and shear mechanical loading in far-field is investigated by extensive molecular dynamics simulations. Under opening-dominant loading, zigzag edge cracks grow self-similarly. Otherwise, complex mechanical stresses concentrated in the vicinity of crack tip can manipulate the direction of crack initiation changing by 30°(or multiples of 30°) to the original crack line. Toughness determined from obtained critical stress intensity factors 2.63 ~ 3.38 nN Å-3/2 is relatively low, which demonstrates graphene is intrinsically brittle opposite to its exceptional high strength at room temperature. Graphene is easier to break along zigzag direction. Torn edges are in either zigzag or armchair manner, while zigzag edges are observed prevalently, and armchair edges are formed occasionally under particular loading conditions. Crack kinking is related to the proportion of opening and shear components of loading, and topological defects frequently appear at turning points. Our theoretical results indicate that cracking of graphene has a dependence on local mechanical stresses, edge energy and dynamic effects, which provide a possible way to regulate the edge structure of graphene. Keywords Graphene, Crack kinking, Stress intensity factor, Molecular dynamics 1. Introduction Graphene, as an atomic monolayer of graphite, is extensively studied after successful laboratory exfoliation [1], and it has attracted significant attention from the scientific community for its remarkable mechanical and electrical properties that are currently being explored for a number of applications including nanoelectromechanical systems, nano-electronics, etc. Recent mechanical experiments have shown that graphene is the strongest material measured hitherto with an elastic modulus of 1.0 TPa [2], which exceeds those of any previously existing materials. Rafiee et al. also reported that graphene as reinforcement has extraordinary effectiveness to resist fracture and fatigue in composites [3]. However, Hashimoto et al. [4] have provided a direct experimental evidence for the existence of defects in graphene layers. The extraordinary mechanical properties can be affected by the presence of defects that cause a more reduction of the strength. The existing works have treated defects in graphene as cracks that can initiate fracture. The research on fracture of graphene can date back to the simulations conducted by Omeltchenko et al. [5], in which a notched graphite sheet was loaded uniaxial tension and then underwent cleavage. However, that retention of the cutoff function of early version potential makes the quantitative aspects of results questionable. Recently, Belytschko et al. [6-8] carried out series of theoretical researches on the fracture of pre-cracked graphene under uniaxial tensile loading. The critical stress intensity factors under pure opening loading were obtained for zigzag and armchair cracks, while the propagation direction was manually specified. Lu et al. [9] also investigated fracture of graphene nano-ribbons (GNRs) under uniaxial tension. Furthermore, shear deformation plays an important role in the wrinkling and rippling behavior of graphene, which, in turn, controls charge carrier scattering and electron mobility [10]. It is even possible to modulate the graphene energy-gap from 0.0 to 0.9 eV by combining shear deformations with uniaxial strains [11]. In point
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