Publications

The Hugoniot response of materials is centrally important in the field of high pressure science. Highly accurate Hugoniot measurements not only provide better material references but also allow for the detection of subtle material phenomena. A process has been developed utilizing the Sandia Z accelerator to measure Hugoniot response at multi-megabar pressure resulting in extremely high accuracy data. Key considerations are the use of large surface area flyer plates allowing measurement configurations with multiple targets and diagnostics. This allows for greatly reduced uncertainty in the data. The details of this process are given and each aspect is closely examined focusing on the individual contributions to the overall accuracy of the result. © 2010 IOP Publishing Ltd.

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Soda-lime glass (SLG) is a potential low-cost VISAR window for use at moderate shock pressures (up to 2430 GPa) where the material remains transparent. In order for SLG to be practical as a VISAR window, the correction factor, which describes the frequency correction related to the strain dependence of the refractive index, and hence the index of refraction itself, must be characterized as a function of pressure. Characterization data are reported in this paper and compared to previous results. The present data show good agreement with those of Dandekar [J. Appl. Phys. 84, 6614 (1998)] and separate study results by Gibbons and Ahrens [J. Geophys. Res. 76, 5489 (1971)] up to 7 GPa. However, at stresses over 7 GPa, marked discrepancies are evident between the present data and that of Gibbons and Ahrens. Differences in test methods may explain these discrepancies. © 2009 American Institute of Physics.

The behavior of a shocked tungsten carbide / epoxy mixture as it expands into a vacuum has been studied through a combination of experiments and simulations. X-ray radiography of the expanding material as well as the velocity measured for a stood-off witness late are used to understand the physics of the problem. The initial shock causes vaporization of the epoxy matrix, leading to a multi-phase flow situation as the epoxy expands rapidly at around 8 km/s followed by the WC particles moving around 3 km/s. There are also small amounts of WC moving at higher velocities, apparently due to jetting in the sample. These experiments provide important data about the multi-phase flow characteristics of this material.

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Silica based glasses are commonly used as window material in applications which are subject to high velocity impacts. Thorough understanding of the response to shock loading in these materials is crucial to the development of new designs. Despite the lack of long range order in amorphous glasses, the structure can be described statistically by the random network model. Changes to the network structure alter the response to shock loading. Results indicate that in fused silica, substitution of boron as a network former does not have a large effect on the shock loading properties while modifying the network with sodium and calcium changes the dynamic response. These initial results suggest the potential of a predictive capability to determine the effects of other network substitutions.

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Soda-lime glass (SLG) is a highly available low cost glass formulation commonly used in window applications. Over the past decade, there have been a number of studies which have examined the Hugoniot elastic limit (HEL) of this material resulting in a wide range of values from 3.1 to 6.0 GPa. The determination of the HEL is complicated by many factors including ramp loading due to the convex downward curvature of the Hugoniot at low pressures. Results of transmitted wave profile experiments up to 20 GPa are presented and analyzed to determine the loading and release characteristics of SLG near the HEL. Results indicate a response that is more complex than the elastic - plastic response typical of many materials, possibly explaining the wide range in initially reported HEL values. © 2007 American Institute of Physics.

Material heterogeneity appears to give rise to variability in the yield behavior of ceramics and metals under shock loading conditions. The line-imaging VISAR provides a way to measure this variability, which may then be quantified by Weibull statistics or other methods. Weibull methods assign a 2-parameter representation of failure phenomena and variability. We have conducted experiments with tantalum (25 and 40 μm grains) and silicon carbide (SiC-N with 5 μm grains). The tantalum HEL variability did not depend systematically on peak stress, grain size or sample thickness, although the previously observed precursor attenuation was present. SiC-N HEL variability within a single shot was approximately half that of single-point variability in a large family of shots; these results are more consistent with sample-to-sample variation than with variability due to changing shot parameters. © 2007 American Institute of Physics.

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Glass, in various formulations, may be useful as a transparent armor material. Fused quartz (SiO{sub 2}), modified with either B{sub 2}O{sub 3} (13 % wt.) or Na{sub 2}O (15 % wt.), was studied to determine the effect on the dynamic response of the material. Utilizing powder and two-stage light gas guns, plate impact experiments were conducted to determine the effect on strength properties, including the elastic limits and plastic deformation response. Further, the effect of glass modification on known transitions to higher density phases in fused quartz was evaluated. Results of these experiments will be presented and discussed.

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