Publications

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Unidirectional carbon fiber reinforced epoxy composite samples were tested to determine the response to one dimensional shock loading. The material tested had high fiber content (68% by volume) and low porosity. Wave speeds for shocks traveling along the carbon fibers are significantly higher than for those traveling transverse to the fibers or through the bulk epoxy. As a result, the dynamic material response is dependent on the relative shock - fiber orientation. Shocks traveling along the fiber direction in uniaxial samples travel faster and exhibit both elastic and plastic characteristics over the stress range tested; up to 15 GPa. Results detail the anisotropic material response which is governed by different mechanisms along each of the two principle directions in the composite.

Magnetically-Applied Pressure-Shear (MAPS) is a new experimental technique to measure material shear strength at high pressures and has been developed for use on MHD driven pulsed power platforms [1]. By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse velocity interferometry system (VISAR) from which the sample strength is determined [2]. This paper presents and analyzes the 2D MHD simulations used to design the MAPS experiments. © 2011 IEEE.

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Pressure-shear experiments were performed on granular tungsten carbide and sand using a newly-refurbished slotted barrel gun. The sample is a thin layer of the granular material sandwiched between driver and anvil plates that remain elastic. Because of the obliquity, impact generates both a longitudinal wave, which compresses the sample, and a shear wave that probes the strength of the sample. Laser velocity interferometry is employed to measure the velocity history of the free surface of the anvil. Since the driver and anvil remain elastic, analysis of the results is, in principal, straightforward. Experiments were performed at pressures up to nearly 2 GPa using titanium plates and at higher pressure using zirconium plates. Those done with the titanium plates produced values of shear stress of 0.1-0.2 GPa, with the value increasing with pressure. On the other hand, those experiments conducted with zirconia anvils display results that may be related to slipping at an interface and shear stresses mostly at 0.1 GPa or less. Recovered samples display much greater particle fracture than is observed in planar loading, suggesting that shearing is a very effective mechanism for comminution of the grains.

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A new experimental technique to measure material shear strength at high pressures has been developed for use on magnetohydrodynamic (MHD) drive pulsed power platforms. The technique is referred to as Magnetically-Applied Pressure-Shear (MAPS). By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse velocity interferometry system (VISAR) from which the sample strength is determined. The strength of materials is defined as the ability of a material to sustain deviatoric (shear) stresses. Strength is an important aspect of the response of materials subjected to compression to high pressure. Beyond the elastic response, material strength will govern at what pressure and to what extent a material will plastically deform. The MAPS technique cleverly exploits the property that, for a von Mises yield criterion at a given longitudinal stress, the maximum amplitude shear wave that can be transmitted is limited by the strength at that stress level. Successful fielding of MAPS experiments to measure shear stresses relies upon correct numerical simulation of the experiment. Complex wave interactions among forward and reflected longitudinal and shear waves, as well as the advancing magnetic diffusion front of the MHD drive, can make the design of the experiment complicated. Careful consideration must be given to driver, sample, and anvil materials; to the thicknesses of the driver, sample and anvil layers; as well as to the timing of the interacting waves. This paper will present and analyze the 2D MHD simulations used to design the MAPS experiments. The MAPS experiments are modeled using Sandia's ALEGRA-MHD simulation code. ALEGRA-MHD is an operator-split, multi-physics, multi-material, arbitrary lagrangian-eulerian code developed to model magnetic implosion, ceramic fracture, and electromagnetic launch. We will detail the numerical investigations into MHD shear generation, longitudinal and shear stress coupling, timing of wave interactions, and transmission of shear at material interfaces.

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A new experimental technique to measure material shear strength at high pressures has been developed for use on magneto-hydrodynamic (MHD) drive pulsed power platforms. By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse VISAR system from which the sample strength is determined.

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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.

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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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