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 T3: High Pressure Strength IV |
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Chair: Stewart McWilliams, Carnegie Institute of Washington Room: Renaissance Ballroom AB |
Thursday, June 30, 2011 11:00AM - 11:30AM |
T3.00001: Flow Strength of Shocked Aluminum in the Solid-Liquid Mixed Phase Region Invited Speaker: Shock waves have been used to determine material properties under high shock stresses and very-high loading rates. The determination of mechanical properties such as compressive strength under shock compression has proven to be difficult and estimates of strength have been limited to approximately 100 GPa or less in aluminum. The term ``strength'' has been used in different ways. For a Von-Mises solid, the yield strength is equal to twice the shear strength of the material and represents the maximum shear stress that can be supported before yield. Many of these concepts have been applied to materials that undergo high strain-rate dynamic deformation, as in uni-axial strain shock experiments. In shock experiments, it has been observed that the shear stress in the shocked state is not equal to the shear strength, as evidenced by elastic recompressions in reshock experiments. This has led to an assumption that there is a yield surface with maximum (loading)and minimum (unloading), shear strength yet the actual shear stress lies somewhere between these values. This work provides the first simultaneous measurements of unloading velocity and flow strength for transition of solid aluminum to the liquid phase. The investigation describes the flow strength observed in 1100 (pure), 6061-T6, and 2024 aluminum in the solid-liquid mixed phase region. Reloading and unloading techniques were utilized to provide independent data on the two unknowns ($\tau _{c}$ and $\tau _{o})$, so that the actual critical shear strength and the shear stress at the shock state could be estimated. Three different observations indicate a change in material response for stresses of 100 to 160 GPa; 1) release wave speed (reloading where applicable) measurements, 2) yield strength measurements, and 3) estimates of Poisson's ratio, all of which provide information on the melt process including internal consistency and/or non-equilibrium and rate-dependent melt behavior. The study investigates the strength properties in the solid region and as the material transverses the solid-mixed- liquid regime. Differences observed appear to be the product of alloying and/or microstructural composition of the aluminum. [Preview Abstract] |
Thursday, June 30, 2011 11:30AM - 11:45AM |
T3.00002: Shearing Resistance of Aluminum at High Strain Rates and at Temperatures Approaching Melt Stephen Grunschel, Rodney Clifton, Tong Jiao High-temperature, pressure-shear plate impact experiments have been conducted to investigate rate-controlling mechanisms for plastic deformation of high-purity aluminum at high strain rates (10$^{6}$s$^{-1})$ and at temperatures approaching melt. The objective of these experiments was to look for a possible change in the rate-controlling mechanism of dislocation motion from thermally activated motion of dislocations past obstacles to phonon drag as the temperatures becomes high enough that thermal activation becomes relatively unimportant. The experimental results show an upturn in shearing resistance with increasing temperature at high temperatures, suggestive of a change in rate-controlling mechanism. However, the upturn is too steep to be described by a usual phonon drag model with a drag coefficient that is proportional to temperature. Simulated results show that the modeling of strain rate hardening based on a phonon drag model leads to too strong an increase in flow stress with increasing strain rate in the drag-dominated regime. [Preview Abstract] |
Thursday, June 30, 2011 11:45AM - 12:00PM |
T3.00003: Evolution of Plastic and Elastic Shock Waves on the Ultrafast Time Scale in a Face Centered Cubic Metal Jonathan Crowhurst, Michael Armstrong, Kimberly Knight, Joseph Zaug, Elaine Behymer We characterize the deformation of pure aluminum in the first few hundred picoseconds subsequent to a dynamic load, at peak stresses up to 44 GPa and strain rates of in excess of 10$^{10}$ s$^{-1}$. For strong shocks we obtain stresses, strain rates and plastic rise times. At peak stresses below 30 GPa, prior to the onset of plasticity we observe elastic stresses that are nearly ten times larger than those observed in traditional (longer time scale) shock compression experiments. We show that in the strong shock regime plastic rise times are very short ($< \quad \sim $ 30 ps) and spatial rises are less than $\sim $ 300 nm. We show that our data are consistent with a simple power law dependence of strain rate on shock stress established at much lower strain rates; thus verifying this dependence over an additional 3 orders of magnitude in the strain rate. Together with the consistency of our strong shock data with the known shock adiabat of aluminum we conclude that the dynamic behavior of even very thin films ($\sim $ 1 $\mu $m) is indistinguishable from the bulk material, but below the strong shock threshold, time and hence length scale become critically important. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
Thursday, June 30, 2011 12:00PM - 12:15PM |
T3.00004: Dynamic Properties of $\alpha$-Cerium Brian Jensen, James Asay, Tariq Aslam Understanding the multiphase properties of metals remains a significant scientific challenge. Dynamic experiments are needed to examine the relevant pure phases, to measure transition kinetics, locate phase boundaries, and obtain information on other material properties such as strength. Cerium is an ideal material for such work because of the large body of static data available that describes a complex multi phase diagram at relatively moderate pressures and temperatures. The main objective of the current experiments was to obtain information on the mechanical response of cerium at shock stresses that span the $\alpha$-phase and approach the melt transition. Plate impact experiments were performed to generate shock-release and re-shock loading conditions in cerium to obtain Hugoniot data, sound speed data, and estimates of strength in the shocked state. Details of the experimental methods and results will be presented. [Preview Abstract] |
Thursday, June 30, 2011 12:15PM - 12:30PM |
T3.00005: Dynamic Strength of Molybdenum Approaching Melt Conditions Geremy Kleiser, Lalit Chhabildas, William Reinhart, William Anderson The purpose of this study is to investigate the dynamic strength of molybdenum at high pressures as it approaches melt. Symmetric impact experiments were conducted using a two-stage gas gun and VISAR diagnostic system to examine the molybdenum behavior up to pressures of 3.5 Mbar. The approach required compensating for the wave interaction occurring due to the low impedance LiF window, but provided detailed information regarding the release behavior from the Hugoniot state. This paper describes the strategy, experimental method, and corresponding results which are used to draw conclusions about the dynamic behavior of molybdenum at high pressure. [Preview Abstract] |
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