PSI - Issue 28

Kotrechko Sergiy et al. / Procedia Structural Integrity 28 (2020) 116–123 Author name / Structural Integrity Procedia 00 (2019) 000–000

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2. MD-simulation technique Molecular dynamics simulation was performed using LAMMPS software [Plimpton (1995)] with the included library of interatomic potentials Atomistica [https://github.com/Atomistica/atomistica]. For modelling, the REBO2 potential [Pastewka et al. (2008)] was used in a modified form. The modification was to change the dynamic cut-off function parameters to describe correctly atomic interaction at large deviations of atoms from the equilibrium position (anharmonicity region). The original version of potential REBO2 in this region has two maxima on the atomic bond strain diagram [Pastewka et al. (2008)]. To address this, the following parameters were selected: 1 90A 1 . ,  сс in R , 2 10A 2 . ,  cc in R , 2 20A 1 . ,  cc ar R , 4 00A 2 . ,  cc ar R , 1 86 A 1 . ,  cc bo R , 4 00A 2 . ,  cc bo R . A nanoelement containing 400 atoms and consisting of two graphene sheets connected by a 10-atom carbyne chain was simulated. The force was applied along the zig-zag orientation. Periodic conditions were set along the X axis (Fig. 1). This was necessary to avoid the edge effects at the graphene boundary.

Fig. 1 (colour online) Carbyne-graphene nanoelement for simulation

Simulation was performed at a constant value of the applied force, which values were updated every 10 iterations. Five force values were selected within the range   un F   0 72 0 87 . . , for each of which 60 numerical “experiments” were performed, as well as 72 “experiments” with a force value un F  0 68 . . The positions of atoms and velocities in time were obtained by integration on the Nose-Hoover style non-Hamiltonian equations of motion [Nose (1984)], which enables to obtain a canonical ensemble of particles. The temperature of the chain was 750 K. Tensile diagrams of the carbyne-graphene nanoelement obtained by MD-simulation using the modified REBO2 potential with dynamic cut-off function differ from that built according to the DFT-calculations [Kotrechko et al. (2019)]. This shows the limited use of both this and similar Brenner potentials for modelling the deformation and fracture of the objects under consideration. Despite attempts made by Los et al. (2005) and Perriot et al. (2013), this task remains unsolved today. Therefore, the CGNs’ lifetime calculations via the suggested fluctuation model were performed on the basis of atomic interaction data derived from modified REBO2 potential with dynamic cut-off function. This enables to compare correctly the MD-simulation findings with predictions of the fluctuation model. 3. The atomic mechanism of contact bond breaking Thermal fluctuations cause short-term bond instability. To break the interatomic bond, it is necessary for this fluctuation to be “picked up” by the applied force (Fig. 2). This means that the critical value of the fluctuation induced displacement, c  , is not a constant. Its magnitude is pre-determined by the force field level. In addition, it depends on the rate of change in the force of atomic interaction on the descending branch of the strain curve. As shown by Kotrechko et al. (2017), the presence of regions of instability (IZ - instability zone) is a characteristic feature of the falling branch of strain curve for two-dimensional graphene-carbyne elements (Fig. 3). Furthermore, the presence of IZ was also observed by Timoshevskii et al. (2019) for 3D carbyne-graphene nanoelements containing five-atom carbyne chains, i.e. the existence of IZ is a fundamental feature of nanoelements, consisting of a combination of 1D, 2D, and 3D-nanostructures. An explanation of the physical nature of IZ is the subject of a separate publication. From the point of view of the micromechanics of nanoelement failure, it is important, that transition from the point “A” to the point “B” is accompanied by a significant change in interatomic distances. The