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 F1: Special Session on Isentropic Compression |
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Chair: Dana Dlott, University of Illinois at Urbana-Champaign Room: Fairmont Orchid Hotel Salon I/II |
Tuesday, June 26, 2007 8:00AM - 8:30AM |
F1.00001: Analyzing Isentropic Compression Wave Experiments Invited Speaker: Some common assumptions that are used to analyze shock wave experiments are invalid for analyzing ramp wave compression experiments. Analysis of a Doppler shift through a window usually assumes a steady wave in the window, a condition that is violated when a ramp compression wave steepens as it propagates, requiring separate consideration during the analysis (LiF to 20GPa). Introduction of a free or windowed interface produces large perturbations to the flow in the sample that must be reconciled to achieve required timing accuracy: when the specimen has a unique stress-strain compression relation, the equations of motion are hyperbolic so that stress-strain relation can be directly deduced from measurements on two samples (sapphire to 20GPa). If the sample is hysteretic like an elastic-plastic material, there is not a unique solution to the flow and a separate drive measurement is needed (W to 250GPa). Time-dependent plasticity (spall in aluminum or twinning in U6Nb) has parabolic equations and backward solutions are unstable. Analyses that compare experiment and simulation have very broad minima in the parameters used to model stress-strain. Unconstrained polynomial expansions can wander and converge to unreasonable results. Better convergence is achieved with constrained models like certain forms of the Mie-Gr\"{u}neisen EOS (Cu to 18GPa) but those poorly represent materials with large changes in compressibility with strain (HMX to 50GPa or $\beta$-$\gamma$ phase change in Sn at 5-8GPa). Maintaining small sample thickness to eliminate shock-up while maximizing thickness for accurate wave velocity measurement produces problems for designing high-stress experiments and leads to hybrid experimental designs. [Preview Abstract] |
Tuesday, June 26, 2007 8:30AM - 9:00AM |
F1.00002: Laser-driven Shockless Compression Invited Speaker: Laser produced x-ray drive was used to shocklessly compress solid Al, Ta, W, V and C targets to high peak longitudinal stresses over nanosecond timescales. Interface velocities versus time for multiple thicknesses were measured and converted to stress-density for near isentrope conditions using an iterative Lagrangian analysis. These are the most rapid shockless compression stress-density data ever reported. Stress-density are stiffer than expected from models that are benchmarked against both static and shock experiments, suggesting a larger than expected time dependent viscoelastic response. This time-dependent compression applied to Bi, Si and Fe samples results in multi-structural phase transformations. A time resolved velocity interferometer is used to measure the effects of new phases on a transmitted wave velocity profile yielding insights into the transformation kinetics. With different experimental techniques, it is now possible to vary the dynamic compression rise time applied to a given material by over 10 orders of magnitude. This capability of varying the ramp compression timescales enables the study of time-dependent material behavior associated with structural changes and deformation in solids subjected to extreme compressions. [Preview Abstract] |
Tuesday, June 26, 2007 9:00AM - 9:30AM |
F1.00003: New experimental capabilities and theoretical insights of high pressure compression waves Invited Speaker: While some high pressure, compression wave research seeks ever high pressures($>$10~Mbar), the exciting developments of high pressure research for gas-gun generated compression waves have spawned novel compression experiments as well as new theoretical insights into compression wave dissipation. The first half of the discussion covers the unique gradient density impactor (GDI) developed at LLNL, that has just matured into a viable tool to examine the material response along and significantly away from the principal paths of the Hugoniot and isentrope. This gives direct access to hot planetary isentropes or cyclic paths to understand hysteretic response at moderately high pressures ($<$5~Mbar). Recently, significant material design challenges pertaining to material control, planarity, parallel layers, and reproducibility have been overcome in the manufacturing of these impactors used to create (within 2$\mu$s) compression waves. These compression waves consist of the standard monotonic compression and of unique non-monotonic compression waves, which widens the field of research to include previously inaccessible parts of the thermodynamic phase diagram for a given material. These developments will be addressed in conjunction with hydrodynamic simulations discussing several interesting experiments that have taken place in the pursuit of understanding the high-pressure phase diagram of water and of understanding high-pressure strength. Closely connected to these compression experiments, in general, is the interpretation of the recorded particle velocity histories and the assumptions used to quantify those results, e.g. stress versus density. Therefore, a second theoretical discussion of solitary wave structure is given suggested by recent experimental observations. Dissipative and dispersive effects are expected to exist in general, however, these effects are not usually discussed within the context of the Korteweg-de Vries(KdV)-Burgers equation, thus, leading to a possible quantification of these effects. Specifically, observed ramped-pressure drives generate coherent structures consistent with solitons in the weakly dissipative limit, that evolve into a dissipative, localising kink structures coalescing into larger kinks. A simulation based on experiment evolves via the KdV equation these structures between two Lagrangian points. The aim being to quantify the dissipation and dispersion that develops in high-pressure compression waves. [Preview Abstract] |
Tuesday, June 26, 2007 9:30AM - 10:00AM |
F1.00004: PANEL DISCUSSION |
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