Bulletin of the American Physical Society
15th APS Topical Conference on Shock Compression of Condensed Matter
Volume 52, Number 8
Sunday–Friday, June 24–29, 2007; Kohala Coast, Hawaii
Session Q2: First Principles and Molecular Dynamics Calculations IV |
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Chair: Thomas Mattsson, Sandia National Laboratories Room: Fairmont Orchid Hotel Amphitheater |
Thursday, June 28, 2007 1:45PM - 2:00PM |
Q2.00001: Dynamics of Plastic Deformation in Atomistic Shock Compression Simulations Wilhelm Wolfer, Alison Kubota Dynamics simulations of shock compression of aluminum are evaluated in terms of a continuum mechanics descriptions to obtain detailed stress and strain distributions behind shock fronts. The equivalent or von Mises stress reaches steady values that coincide with the dynamic yield strength obtained by Huang and Asay from the analysis of particle velocity profiles of release and reshock experiments. This agreement is all the more significant as the experimental results are indirect, while the atomistic simulation results extract the dynamic strength directly. Additional information is obtained when the evolution of the von Mises stress is viewed in the Lagrangian frame. Different material elements experience nearly the same stress evolution: a rapid rise, as the elastic wave front passes the material element, followed by an exponential decline of the von Mises stress. This evolution implies a constitutive law for plastic deformation that can be used as is in finite element codes, and/or it can be further interpreted by the dislocation mechanisms that the atomistic simulations so vividly reveal and display. Temperature can also be extracted directly from the atomistic simulations and be separated into adiabatic and plastic heating. [Preview Abstract] |
Thursday, June 28, 2007 2:00PM - 2:15PM |
Q2.00002: Frictional interactions at compressed Al interfaces J.E. Hammerberg, R. Ravelo, T.C. Germann, B.L. Holian We discuss the velocity and temperature dependence of the frictional force at sliding Al-Al interfaces. A series of 3-D 1.5 million atom Non Equilibrium Molecular Dynamics (NEMD) simulations for single crystal Al incommensurate interfaces have been carried out for a range of imposed boundary temperatures and sliding velocities at an imposed boundary pressure of 15 GPa in the solid state. Velocities in the range 50-2000 m/s are considered for temperatures of 232, 464, and 696 K. We discuss the regimes of interfacial stability, from stable, anharmonic phonon dominated at low velocities, through plasticity dominated interfacial instability at intermediate velocities, to high velocity Couette flow, and present a scaled model for the frictional force in the intermediate to high velocity regimes. Connection will be made to recent dynamic friction experiments carried out at the DOE ATLAS pulsed power facility. [Preview Abstract] |
Thursday, June 28, 2007 2:15PM - 2:45PM |
Q2.00003: Single Crystal Plasticity in Ramp- and Cyclically-Loaded Aluminum Invited Speaker: In recent years, there has been a great deal of interest in using large-scale atomistic simulations to model wave propagation in order to get a qualitative picture for inelastic deformation in materials. Although these simulations have provided a great deal of qualitative insight into the phenomena of high strain rate deformation in materials, there is an increasing need to be able to obtain quantitative continuum descriptions for the atomistic simulations in terms of stresses and strains so one can construct constitutive laws for plastic deformation. This presentation will be focused on recent work to develop this seamless approach from atoms to continuum and apply it shock compression of single-crystal Aluminum using very large-scale atomistic wave-propagation simulations. In the first part of this presentation, we discuss the performance of the available and newly developed interatomic potentials in terms of their ability to reproduce thermoelastic and plastic properties such as elastic moduli, specific heat, thermal expansion coefficient, thermal conductivity and gamma surface. We then describe recent large-scale simulations of ramp- and cyclically-loaded single-crystal Aluminum with varying initial defect concentrations. Using the method of characteristics, we impose a time-varying longitudinal force to generate ramp waves with specified shock-up locations in the material. Our calculations demonstrate that (1) the initial defect densities have a strong effect on the time-dependence and the maximum achieved equivalent stress, (2) shock-up consistently occurs earlier than expected due to the inherent inability for materials to sustain large gradients in the shock front at these micron length-scales, and that (3) the observed flow stress followed in the coarse-grained Lagrangian elements before shock-up are consistently higher than those after shock-up and those from shock-loading simulations. We discuss these differences in terms of continuum phenomenological models for plastic deformation constructed from coarse-grained analyses of these simulations, and consider future possible developments in large-scale atomistic simulations. [Preview Abstract] |
Thursday, June 28, 2007 2:45PM - 3:00PM |
Q2.00004: Liquid Metal Free Energies from Ab Initio Potential Surfaces Carl Greeff, Raquel Lizarraga, Eric Chisolm For prediction of high pressure melting curves and liquid thermodynamic properties, it is desirable to evaluate liquid free energies. Direct free energy calculations from {\em ab initio} potential surfaces are very computationally intensive, especially for transition metals. Here we investigate Monte Carlo methods that involve sampling on the surface defined by a reference system. In principle, this allows for large gains in efficiency because the random walk is carried out on the (much faster) reference potential, and the {\em ab initio} potential is only evaluated on a small subset of uncorrelated configurations. We investigate the feasibility of these methods, and the role of the reference system. Results will be presented for liquid Mg and Ta. [Preview Abstract] |
Thursday, June 28, 2007 3:00PM - 3:15PM |
Q2.00005: Quantum Molecular Dynamics Simulations of Optical Reflectivity of Shock-Compressed Tin Joel Kress, Lee Collins, Stephane Mazervet Shock-compression experiments have measured the optical reflectivity of tin to detect: (1) a solid-solid phase transition ($\beta$ to BCT); (2) melting on the Hugoniot curve, and; (3) melting during the release of the strongly shocked material. Recent quantum molecular dynamics (QMD) simulations have been successful at determining the optical properties of warm, dense materials such as shock-compressed deuterium, exploding wires made of aluminum and copper, and laser-heated thin films of gold. In this work, we present QMD calculations of the optical conductivity and reflectivity of solid (cold) $\beta$ tin, representative of shock-compressed and shock-released states. Calculated differences in the optical reflectivity between the cold and warm states will be compared with the measurements from shock-compressed experiments. [Preview Abstract] |
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