Massively Parallel Molecular Dynamics Simulations of Two-dimensional Materials at High Strain Rates
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Massively Parallel Molecular Dynamics Simulations of Two-dimensional Materials at High Strain Rates Norman J. Wagner Department of Chemical Engineering University of Delaware Newark, DE 19716 and Brad Lee Holian Los Alamos National Laboratory Los Alamos, NM 87545 ABSTRACT Large scale molecular dynamics simulations on a massively parallel computer are performed to investigate the mechanical behavior of 2-dimensional materials. A model embedded atom manybody potential is examined, corresponding to "ductile" materials. A parallel MD algorithm is developed to exploit the architecture of the Connection Machine, enabling simulations of > 106 atoms. A model spallation experiment is performed on a 2-D triagonal crystal with a welldefined nanocrystalline defect on the spall plane. The process of spallation is modelled as a uniform adiabatic expansion. The spall strength is shown to be proportional to the logarithm of the applied strain rate and a dislocation dynamics model is used to explain the results. Good predictions for the onset of spallation in the computer experiments is found from the simple model. The nanocrystal defect affects the propagation of the shock front and failure is enhanced along the grain boundary. Introduction Recent attention has focused on understanding the molecular mechanisms that give rise to the macroscopic failure process [1]. Unlike many equilibrium material properties which can adequately be represented by suitable simulations of hundreds or thousands of atoms, fracture and failure involve both microscopic and macroscopic length scales. For example, crack propagation or stress concentration near a crack tip inherently depends on both the molecular properties of bonding and atomic arrangement, and the macroscopic properties of the crack length and crack tip geometry. Further, materials with multiple levels of structure, such as polycrystalline or nanocrystalline solids, require large ensembles of atoms to accurately represent these mesoscopic structures. Such considerations have motived the development of a molecular dynamics simulation tool on a massively parallel computer for investigating material behavior at the molecular level [2]. Through the use of molecular dynamics simulations we hope to elucidate the molecular mechanisms underlying material failure under spallation. This is motivated by two considerations: 1) detailed microscopic analyses are difficult to perform during spallation and 2) molecular dynamics is well suited to examine processes occurring on these short time scales. Simulations can also provide detailed information concerning microscopic mechanisms for spallation at high strain rates, about which little is known [3, 4]. In previous work, our simulation studies motivated the development of a dislocation dynamics model for the spallation process. The relevant mechanical process at the spall plane was suitably modeled as an adiabatic expansion. Consideration of nucleation and aggregation of defects lead to a stress-activated statistical model for the spallation process, wh
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