Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M26: Focus Session: Materials in Extremes: High-Strain-Rate Phenomena II |
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Sponsoring Units: GSCCM DCOMP DMP Chair: Bruce Remington, Lawrence Livermore National Laboratory Room: 502 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M26.00001: Integrating Simulation and Data for Materials in Extreme Environments Invited Speaker: Timothy Germann We are using large-scale molecular dynamics (MD) simulations to study the response of nanocrystalline metals such as tantalum to uniaxial (e.g., shock) compression. With modern petascale-class platforms, we are able to model sample sizes with edge lengths over one micrometer, which match the length and time scales experimentally accessible at Argonne's Advanced Photon Source (APS) and SLAC's Linac Coherent Light Source (LCLS). I will describe our simulation predictions and their recent verification at LCLS, as well as outstanding challenges in modeling the response of materials to extreme mechanical and radiation environments, and our efforts to tackle these as part of the multi-institutional, multi-disciplinary Exascale Co-design Center for Materials in Extreme Environments (ExMatEx). ExMatEx has initiated an early and deep collaboration between domain (computational materials) scientists, applied mathematicians, computer scientists, and hardware architects, in order to establish the relationships between algorithms, software stacks, and architectures needed to enable exascale-ready materials science application codes within the next decade. We anticipate that we will be able to exploit hierarchical, heterogeneous architectures to achieve more realistic large-scale simulations with adaptive physics refinement, and are using tractable application scale-bridging proxy application testbeds to assess new approaches and requirements. The current scale-bridging strategies accumulate (or recompute) a distributed response database from fine-scale calculations, in a top-down rather than bottom-up multiscale approach. I will demonstrate this approach and our initial assessments, using the newly emerging capabilities at new 4$^{\mathrm{th}}$ generation synchrotron light sources as an experimental driver. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M26.00002: Bridging simulations and experiment in shock and ramp induced phenomena Dawn Flicker The high pressure materials physics program at Sandia's Z facility includes strong collaboration between theory, simulations and experiments. This multi-disciplinary approach has led to new insights in many cases. Several examples will be discussed to illustrate the benefits of bridging simulations and experiments. Results will be chosen from recent work on the xenon equation of state, phase change in MgO, shock induced chemistry in CO2 and tantalum strength. Sandia National Laboratories is a multi-program laboratory 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] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M26.00003: High-speed velocimetry inside imploding cylindrical liners Daniel Dolan, Ray Lemke, Devon Dalton, Eric Harding, Ryan McBride, Matthew Martin, Brent Blue, Scott Walker Dynamic planar compression is conceptually simple but difficult to maintain at extreme pressure ($>$5~Mbar). Higher pressures are attainable with imploding cylindrical liners, using Photonic Doppler velocimetry (PDV) to track the liner interior. PDV measures Doppler shift directly---1~GHz of beat frequency for every 1~km/s of velocity---requiring a special ``leapfrog'' approach for liners traveling in excess of 20~km/s. Single-point and multi-point PDV measurements have been performed in aluminum, beryllium, and tantalum liners under ramp compression, and the technique can readily applied to other implosion experiments. Combined with electrical current diagnostics, these measurements test thermodynamic equations of state at pressures up to 10~MBar and beyond. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M26.00004: Surface evolution effects observed in velocimetry of materials at high strain rates Erik Moro, Matthew Briggs, Lawrence Hull According to the accepted model for photon Doppler velocimetry (PDV), a particular probe measures the bulk (or average) motion of a surface moving along its beam axis. Utilizing this model, a surface's velocity vector may be reconstructed via a number of probes, at distinct angles of incidence, all of which view the same region on the surface. However, this approach does not account for localized effects of surface evolution, which may interact with PDV's interferometer in ways that are not yet fully appreciated. Consider, for example, that the material flow of a straining surface occurs tangent to the surface and may project along the beam axes of non-normal probes. We present a recent series of explosive tests, whose results suggest that non-normal PDV probes measure the effects of surface evolution as it projects along their beam axes. We believe that these effects have not been observed before. The implication is that PDV probes are capable of measuring the bulk motion of a surface, as well as measuring discrete events associated with surface evolution and failure. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 1:03PM |
M26.00005: Progress on Understanding Structure and Compression of Laser Ramp-Compressed Matter into the Terapascal Pressure Regime Invited Speaker: Amy Lazicki Recent results from x-ray diffraction experiments at the Omega and NIF laser facilities on dynamically-compressed Sn, Ta, MgO, Pb and diamond will be presented. In order to relate laser ramp-compression results to the equilibrium phase diagram, it is necessary to understand the affects of the rapid compression on the phase transitions, microstructure and temperature of a material. To accomplish these measurements, a number of technical challenges must be overcome. The stress profile and history and the temperature of the sample need to be adequately controlled to make a measurement in a highly-compressed solid state, requiring precise laser pulse shaping and timing. Accurate understanding of the spectral emission from a plasma created during the laser ablation process is required in order to filter this emission and produce high contrast diffraction images. Progress towards understanding and resolving these scientific and technical issues will be discussed along with the experimental diffraction results. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M26.00006: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M26.00007: Simulating laser speckle dynamics that result from surface evolution Will Warren, Erik Moro, Matthew Briggs In Photon Doppler velocimetry (PDV), motion along a laser beam's axis is quantified via frequency shifts in the backscattered light. The local intensity of the backscattered field varies spatially due to interference between coherent reflections. The randomly arranged bright and dark regions that result from this interference are commonly referred to as laser speckles. As a consequence of surface evolution, new microfacets become illuminated and the speckle pattern changes. While strain-induced speckle dynamics have been experimentally observed, little work has been done towards understanding the direct relationships between surface evolution and quantifiable speckle properties. We present a computational model that simulates the scattering of electromagnetic waves off of a rough surface and that simulates conditions inherent to PDV experiments. The results contribute to our understanding of how surface evolution relates to the speckle dynamics measured in PDV. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M26.00008: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M26.00009: Failure of brittle heterogeneous materials: Intermittency, Crackling and Seismicity Jonathan Bar\'es, Lamine Hattali, Davy Dalmas, Daniel Bonamy The problem of the solid fracture is classically addressed within the framework of continuum mechanics. Still, stress enhancement at crack tips makes the failure behavior observed at the continuum-level scale extremely dependent on the presence of microstructural inhomogeneities. This yields statistical aspects which, by essence, cannot be addressed using the conventional engineering continuum approaches. We designed an experimental setup that allows growing well-controlled tensile cracks in brittle heterogeneous solids of tunable microstructure, over a wide range of loading speed. The crack dynamics and the evolution of stored and released mechanical energy are monitored in real time. In parallel, the acoustic emission is recorded via a series of acoustic transducers, and analyzed in a way similar to that develop by geophysicists to process seismic signals. This experiments allowed us to characterize quantitatively the crackling dynamics of cracks, also to evidence intriguing statistical similarities between the seismicity associated with this simple situation (single crack under tension) and the much more complex situation of multicracking in compressive fracture and in earthquakes. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M26.00010: Strong plastic deformation and softening of fast colliding Lennard-Jones nanoparticles by Molecular Dynamics simulations Yoichi Takato, Surajit Sen, Jeremy Lechman We present a Molecular Dynamics study of the coefficient of restitution $e$ for colliding two equal sized nanoparticles. Nanoparticles often show distinctly different mechanical and dynamical properties than bulk materials. We investigate the collision velocity $v_{\mathrm{coll}}$ and the nanoparticle size dependence of coefficient of restitution. We find that the size dependent yield velocity $v_{Y}$, a sharp crossover point between elastic collision and plastic collision, appears to approach the theoretical constant value for macroscopic spheres as the nanoparticle size grows. We also find that above $v_{Y}$, the coefficient of restitution $e \propto v_{\mathrm{coll}}^{-\alpha}$, where $\alpha \sim 1$, which is distinct from the inelastic macroscopic sphere collision case, $\alpha = 1/4$. It indicates that nanoparticles colliding at high collision velocity are softened. We discuss possible insights of the size dependent yield velocity and the soft nanoparticles above $v_{Y}$. [Preview Abstract] |
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