Graphene, a monolayer of carbon atoms packed together in a honeycomb lattice structure, has drawn remarkable attention due to its extraordinary fundamental properties that make it a suitable material for a wide range of technological applications. Very large-scale graphene films prepared by chemical vapor deposition (CVD) are polycrystalline sheets composed of single-crystalline pristine graphene grains of varying orientation joined by grain boundaries. Grain boundaries act as defects and alter the mechanical properties of graphene. It has been reported, that pristine graphene is the strongest material with Young’s modulus of 1.0 ± 0.1 TPa and strength of 130 GPa. However, inclusion of grain boundaries reduces the Young's modulus to 600 GPa. Reliable use of polycrystalline graphene sheet necessitates fundamental understanding of the effect of grain size and grain boundaries orientation on the mechanical and fracture properties of graphene in industrial application. The HR-TEM (high resulation transmission electron microscopy) data and molecular dynamics simulations indicate that grain boundaries (GBs) are composed of periodic and aperiodic sequence of dislocations and this dislocation are mainly composed of Stone-Wales defects.  So, a polycrstalline graphene is an assemble of several pristine graphene grain oriented with different direction and separated by GBs that are 1D line defects which is consisted with regular array of edge sharing pentagone/heptagone pairs. Simplest of such structure are bicrystalline graphene are studied in this paper, which are two crystalline grains linked by a GB.

Cohesive zone models (CZM) can be used to model intergranular crack initiation and propagation in polycrystalline graphene. The traction--separation laws of the cohesive zone model provide a representation of the fracture process zone at the crack tip and can be used to remove the stress singularity at the crack tip. Previous studies have used CZM with various functional form of the TSL (bilinear, trapezoidal, exponential and polynomial) to study the fracture of brittle and ductile materials.

 In this study, we use molecular dynamics (MD) simulations to investigate the fracture properties of bicrystalline graphene and to extract the cohesive zone laws of the grain boundaries. Using the MD results, a bilinear TSL is derived to characterise the properties of the grain boundary and the relation between the cohesive traction and the grain boundary properties. We conduct simulations using different misorientation angle to address the traction-separation laws (TSLs) of CZM at the crack tip. From the traction-separation curve, separation work or fracture energy is investigated and it is found that separation work has a similar relation with misorientation angle as grain boundary energies. We also investigate the traction-separation law at different temperature to understand the effect of temperature on the separation work at different misorientation angle.


Molecular Dynamics Simulation Based Cohesive Zone Representation of Intergranular Fracture Processes in Bicrystalline Graphene

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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