Bulletin of the American Physical Society
20th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 62, Number 9
Sunday–Friday, July 9–14, 2017; St. Louis, Missouri
Session D5: First Principles and Molecular Dynamics II |
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Chair: John Belof, Lawrence Livermore National Laboratory Room: Regency Ballroom B |
Monday, July 10, 2017 2:00PM - 2:15PM |
D5.00001: Molecular Dynamics Simulations of Transverse Effects in Shock-Compressed Fibre-Textured Tantalum Polycrystals Patrick Heighway, Andrew Higginbotham, David McGonegle, Justin Wark Whilst uniaxially shock-compressed crystas have zero total strain transverse to the shock propagation direction, this is a global, rather than local constraint. For individual grains, expansion or contraction can occur via the Poisson effect, or via plasticity. Neighbouring grains in a polycrystal may therefore 'push' one another transverse to the shock, causing transverse strain anisotropy. Here we discuss the results of multi-million atom molecular dynamics simulations of elementary fibre-textured tantalum polycrystals shock-compressed along the [110] direction. Below the elastic limit, we observe transverse stress waves driven by the Poisson effect that cause bending of the grain boundaries. In our quadcrystal geometry, the average transverse strains were 15\% of the longitudinal strain, while the stress difference across the grain boundaries was of 2.5\% of the peak pressure, representing a small deviation from the Reuss limit. Transverse motion of the boundaries is also visible in the plastic regime, but analysis of the stress-strain state of the bulk material is complicated by twin and dislocation nucleation. Work is currently being undertaken to quantify the transverse strain anisotropy of plastically deformed polycrystals at pressures in excess of 40 GPa. [Preview Abstract] |
Monday, July 10, 2017 2:15PM - 2:30PM |
D5.00002: The plastic response of Tantalum in Quasi-Isentropic Compression Ramp and Release Alexander Moore, Justin Brown, Hojun Lim, J. Matthew D. Lane The mechanical response of various forms of tantalum under extreme pressures and strain rates is studied using dynamic quasi-isentropic compression loading conditions in atomistic simulations. Ramp compression in bcc metals under these conditions tend to show a significant strengthening effect with increasing pressure; however, due to limitations of experimental methods in such regimes, the underlying physics for this phenomenon is not well understood. Molecular dynamics simulations provide important information about the plasticity mechanisms and can be used to investigate this strengthening. MD simulations are performed on nanocrystalline Ta and single crystal defective Ta with dislocations and point defects to uncover how the material responds and the underlying plasticity mechanisms. The different systems of solid Ta are seen to plastically deform through different mechanisms. Fundamental understanding of tantalum plasticity in these high pressure and strain rate regimes is needed to model and fully understand experimental results. [Preview Abstract] |
Monday, July 10, 2017 2:30PM - 2:45PM |
D5.00003: Dynamic Evolution of Defects during Shock Loading and Spall Failure of Al Microstructures Garvit Agarwal, Avinash Dongare Large scale molecular dynamics (MD) simulations are carried out to investigate the links between the microstructure and evolution of dislocations during shock loading and spall failure of Al microstructures. The MD simulations suggest that the variations in the spall strengths for the [001], [110] and [111] loading orientations of single crystal Al are influenced by the evolution of the densities of Shockley partials and twinning dislocations in the microstructure. In addition, the evolution of dislocation densities are investigated for nanocrystalline Al (grain size of 40 nm) to investigate the effect of distribution of grain boundaries on the defect evolution and spall failure behavior. While MD simulations are able to provide critical insights in the evolution of defects, the capabilities are limited to small system sizes. The newly developed quasi-coarse-grained dynamics (QCGD) method is able to scale up the capabilities of the MD simulations to model the shock loading and the spall failure behavior as predicted by MD simulations. The QCGD simulations are able to implicitly capture the nucleation, evolution and interaction of dislocations as well as the nucleation, growth and coalescence of voids as predicted by MD simulations by solving the equations of motion for a reduced number of representative atoms and show significant promise to model the materials behavior at the mesoscales. The evolution of dislocation densities as predicted by the MD and the QCGD simulations and the role of microstructure will be presented. [Preview Abstract] |
Monday, July 10, 2017 2:45PM - 3:00PM |
D5.00004: The influence of grain boundary orientation on the strength and failure of tantalum bicrystals Eric Hahn, Saryu Fensin, Timothy Germann Non-equilibrium molecular dynamics simulations are used to investigate the dynamic tensile response of tantalum bicrystals. Specimens with grain boundaries aligned either perpendicular or parallel to the shock direction are generated and subjected to shock and release. Future investigations will evaluate grain boundaries inclined relative to the shock front. We find that perpendicular boundaries show a higher propensity to fail, however local plasticity and crystalline orientation conspire such that some grain boundaries fail preferentially to one another. When the spall plane lies precisely along the grain boundary we observe the rapid separation of the boundary as growing voids rapidly coalesce with one another. [Preview Abstract] |
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