Atomistic Simulations of Strain Rate Dependent Deformation Behavior at Continuum Timescales
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0976-EE01-05
Atomistic Simulations of Strain Rate Dependent Deformation Behavior at Continuum Timescales Vikas Tomar Arospace and Mechanical Engineering, University of Notre Dame, 376 Fitzpatrick Hall, Notre Dame, IN, 46556 ABSTRACT A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at a timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (~108 s-1) rarely observed during experiments. In this investigation, a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain rate dependent) atomistic mechanical deformation of nanostructures at continuum timescales is established. In this method there is no restriction on the size of MD timestep except that it must be such that to ensure a reasonable acceptance rate between consecutive Monte-Carlo (MC) time-steps. For the purpose of comparison HMC analyses of a Cu nanowire deformation at two different strain rates (108 s-1 and 109 s-1) (each with three different timesteps 2 fs, 4fs, and 8 fs) are compared with the analyses based on MD simulations at the corresponding strain rates with the MD timestep of 2 fs. As expected, the defect formation in the Cu nanowire is found to be strain rate dependent. In addition, HMC with a timestep of 8 fs correctly reproduced defect formation and stress-strain response observed in the case of MD with a timestep of 2 fs (for the interatomic potential used, 2 fs is the maximum permissible MD timestep). Simulation time analyses show that by using HMC an order of 4 savings in computational time can be achieved bringing the atomistic analyses closer to the continuum timescales. INTRODUCTION A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Except in a very limited number of cases, such as experimental verification of the MD results of [1] on deformation twinning in nanocrystalline Al, results of MD simulations have not been directly verified using experiments. The primary reason for this discrepancy has been the limitation on timescale (of the order of nanoseconds) of MD simulations. Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (~108 s-1) rarely observed in experiments. Solutions to this problem are methods that allow the use of larger time-steps in MD simulations with simultaneous resolution of atomic level vibrations, such as MD time-acceleration methods, see [2], and hybrid-Monte Carlo (HMC) method (cf. e.g. [3]). In this investigation, a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain r
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