Bulletin of the American Physical Society
17th Biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 56, Number 6
Sunday–Friday, June 26–July 1 2011; Chicago, Illinois
Session J3: First-Principles and Molecular Dynamics Calculations VI: Metals II |
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Chair: S. N. Luo, Los Alamos National Laboratory Room: Renaissance Ballroom AB |
Tuesday, June 28, 2011 11:00AM - 11:30AM |
J3.00001: Steady two-zone elastic-plastic shock waves in solids Invited Speaker: By decoupling time and length scales in moving window molecular dynamics simulations of steady shock waves, a new regime of shock wave propagation was observed. It is characterized by a steady two-zone elastic-plastic structure of a single shock wave, and consists of a leading low-pressure elastic zone followed by a high-pressure plastic zone. Both elastic and plastic shock fronts move with the same speed and have a fixed net separation that can extend to many microns. The material in the elastic zone is in a metastable state having a pressure that substantially exceeds the critical shock strength characteristic of the onset of the well-known split-elastic-plastic, two-wave propagation. The two-zone elastic-plastic single wave is a quite general phenomenon observed in simulations of a broad class of crystalline materials. It is the existence of the two-zone, elastic-plastic regime that allows for a consistent explanation of the anomalously high elastic wave amplitudes observed in recent experiments. [Preview Abstract] |
Tuesday, June 28, 2011 11:30AM - 11:45AM |
J3.00002: Finite Size Effects at High Speed Frictional Interfaces J.E. Hammerberg, R. Ravelo, T.C. Germann, B.L. Holian Non-Equilibrium Molecular Dynamics simulations have exhibited characteristic velocity weakening for the tangential frictional force at smooth single crystal interfaces for velocities greater than a critical velocity, v$_c$. This behavior has been seen in a number of mate-rial pairs including Cu-Ag, Ta-Al and Al-Al. Expressions for v$_c$ that characterize this behavior depend on system size. We discuss the size dependence for Al-Al single crystal interfaces for two cases: an Al(111)/Al(001) interface sliding along [1-10],N=1.5$\times10^6$, and an Al(110)[001]/Al(110)[1-10] interface sliding along [001], N=7.5$\times10^6$ corresponding to a three-fold increase in system size normal to the sliding direction. We find agreement with an inverse size scaling for v$_c$. We discuss the similarities in behavior for a highly defective plastically deformed sample with Al(110)[001]/Al(110)[1-10] orientation having the same normal dimension and N= 16.0$\times10^6$. [Preview Abstract] |
Tuesday, June 28, 2011 11:45AM - 12:00PM |
J3.00003: Elastic response of shocked aluminum single crystals: a continuum analysis of molecular dynamics simulations J.A. Zimmerman, J.M. Winey, Y.M. Gupta Molecular dynamics (MD) simulations were used to examine elastic shock wave propagation in aluminum single crystals along [100], [110] and [111] directions using four different embedded-atom method potentials. Continuum variables extracted from MD results show that stresses, densities, and temperatures for [100] shock propagation are significantly different for the various potentials, while the results for [110] and [111] propagation are similar for three of the four potentials. Overall, the recent potential by Winey, Kubota and Gupta [{\it MSMSE} {\bf 17}, 055004 (2009)] provides the best agreement with nonlinear elastic calculations that include elastic constants up to fourth order. Our MD-continuum approach provides a key step in establishing the applicability of classical MD potentials for dynamic compression. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, June 28, 2011 12:00PM - 12:15PM |
J3.00004: Ultrashort elastic and plastic shock waves in Aluminum Nail Inogamov, Vasily Zhakhovsky, Carter White, Ivan Oleynik, Vladimir Fortov Ultrashort shock waves in aluminum films generated by femtosecond laser pulses were studied using two-temperature hydrodynamics and molecular dynamics methods. We observed double wave breaking characterized by an independent formation of leading elastic and trailing plastic shock waves. Both the amplitude and speed of the plastic shock decrease quickly in time due to hydrodynamic attenuation, while the elastic shock front slowly decays during propagation. When the pressure in the plastic front becomes equal to the pressure in the elastic zone, the plastic wave disappears. Therefore, the distance between elastic and plastic fronts first decreases, then increases, with time. The elastic shock uniaxially compresses the crystal to very high pressures and extreme shear stresses comparable in magnitude to the aluminum shear modulus. For a short time, the crystal within the elastic zone remains in a metastable state, that lies on an extension of the elastic branch of the Hugoniot beyond the Hugoniot elastic limit. Our theoretical results explain the seemingly puzzling experimental findings, where high-pressure elastic shock waves were observed with pressures up to 12 GPa. [Preview Abstract] |
Tuesday, June 28, 2011 12:15PM - 12:30PM |
J3.00005: High strain rate uniaxial compression of single and nano-crystalline copper Virginie Dupont, Timothy C. Germann There is a great interest in the shock community to increase the reliability of models of the strength of metals under high strain-rate loading. This regime is difficult to access experimentally, and one way of getting more understanding in the mechanisms is to use molecular dynamics (MD) simulations. Moreover, MD simulations allow for a precise control of the strain rate, temperature and grain size. We are using MD simulations to study the influence of these parameters on the yield strength of Copper, for strain rates ranging from 10\textsuperscript{8} to 10\textsuperscript{11} s\textsuperscript{-1}. Single crystalline simulations with different orientations will be presented, followed by polycrystalline studies with different grain sizes. We show that for single crystals, the orientation of the metal along the compression axis is critical for controlling the yield stress. For the polycrystalline simulations, the yield stress varies a lot more with strain rate than for the single crystal, and we show that as a result, smaller is not always stronger. [Preview Abstract] |
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