Publications

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Explosive initiation and energy release have been studied in two sample geometries designed to minimize stochastic behavior in shock-loading experiments. These sample concepts include a design with explosive material occupying the hole locations of a close-packed bed of inert spheres and a design that utilizes infiltration of a liquid explosive into a well-defined inert matrix. Wave profiles transmitted by these samples in gas-gun impact experiments have been characterized by both velocity interferometry diagnostics and three-dimensional numerical simulations. Highly organized wave structures associated with the characteristic length scales of the deterministic samples have been observed. Initiation and reaction growth in an inert matrix filled with sensitized nitromethane (a homogeneous explosive material) result in wave profiles similar to those observed with heterogeneous explosives. Comparison of experimental and numerical results indicates that energetic material studies in deterministic sample geometries can provide an important new tool for validation of models of energy release in numerical simulations of explosive initiation and performance.

A previously-developed experimental facility has been used to determine gas-surface thermal accommodation coefficients from the pressure dependence of the heat flux between parallel plates of similar material but different surface finish. Heat flux between the plates is inferred from measurements of temperature drop between the plate surface and an adjacent temperature-controlled water bath. Thermal accommodation measurements were determined from the pressure dependence of the heat flux for a fixed plate separation. Measurements of argon and nitrogen in contact with standard machined (lathed) or polished 304 stainless steel plates are indistinguishable within experimental uncertainty. Thus, the accommodation coefficient of 304 stainless steel with nitrogen and argon is estimated to be 0.80 {+-} 0.02 and 0.87 {+-} 0.02, respectively, independent of the surface roughness within the range likely to be encountered in engineering practice. Measurements of the accommodation of helium showed a slight variation with 304 stainless steel surface roughness: 0.36 {+-} 0.02 for a standard machine finish and 0.40 {+-} 0.02 for a polished finish. Planned tests with carbon-nanotube-coated plates will be performed when 304 stainless-steel blanks have been successfully coated.

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A suite of impact experiments was conducted to assess spatial and shot-to-shot variability in dynamic properties of tantalum. Samples had a uniform refined {approx}20 micron grain structure with a strong axisymmetric [111] crystallographic texture. Two experiments performed with sapphire windows (stresses of approximately 7 and 12 GPa) clearly showed elastic-plastic loading and slightly hysteretic unloading behavior. An HEL amplitude of 2.8 GPa (corresponding to Y 1.5 GPa) was observed. Free-surface spall experiments showed clear wave attenuation and spallation phenomena. Here, loading stresses were {approx} 12.5 GPa and various ratios of impactor to target thicknesses were used. Spatial and shot-to-shot variability of the spall strength was {+-} 20%, and of the HEL, {+-} 10%. Experiments conducted with smaller diameter flyer plates clearly showed edge effects in the line and point VISAR records, indicating lateral release speeds of roughly 5 km/s.

A combined experimental/modeling study was conducted to better understand the critical role of gas-surface interactions in rarefied gas flows. An experimental chamber and supporting diagnostics were designed and assembled to allow simultaneous measurements of gas heat flux and inter-plate gas density profiles in an axisymmetric, parallel-plate geometry. Measurements of gas density profiles and heat flux are made under identical conditions, eliminating an important limitation of earlier studies. The use of in situ, electron-beam fluorescence is demonstrated as a means to measure gas density profiles although additional work is required to improve the accuracy of this technique. Heat flux is inferred from temperature-drop measurements using precision thermistors. The system can be operated with a variety of gases (monatomic, diatomic, polyatomic, mixtures) and carefully controlled, well-characterized surfaces of different types (metals, ceramics) and conditions (smooth, rough). The measurements reported here are for 304 stainless steel plates with a standard machined surface coupled with argon, helium, and nitrogen. The resulting heat-flux and gas-density-profile data are analyzed using analytic and computational models to show that a simple Maxwell gas-surface interaction model is adequate to represent all of the observations. Based on this analysis, thermal accommodation coefficients for 304 stainless steel coupled with argon, nitrogen, and helium are determined to be 0.88, 0.80, and 0.38, respectively, with an estimated uncertainty of {+-}0.02.

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Of special promise for providing dynamic mesoscale response data is the line-imaging VISAR, an instrument for providing spatially resolved velocity histories in dynamic experiments. We have prepared two line-imaging VISAR systems capable of spatial resolution in the 10-20 micron range, at the Z and STAR facilities. We have applied this instrument to selected experiments on a compressed gas gun, chosen to provide initial data for several problems of interest, including: (1) pore-collapse in copper (two variations: 70 micron diameter hole in single-crystal copper) and (2) response of a welded joint in dissimilar materials (Ta, Nb) to ramp loading relative to that of a compression joint. The instrument is capable of resolving details such as the volume and collapse history of a collapsing isolated pore.

The shock response of heterogeneous materials involves highly fluctuating states and localization effects that are produced by mesostructure. Prior studies have examined this shock behavior in randomized inert and reactive media. In this work, we investigate the shock behavior in a porous lattice consisting of hexagonally packed layers of 500 {micro}m tin spheres impacted at 0.5 km/s. This ordered geometry provides a well-defined configuration to validate mesoscale material modeling based on three-dimensional CTH calculations. Detailed wave fields are experimentally probed using a line-imaging interferometer and transmitted particle velocities are compared to numerical mesoscale calculations. Multiple shock fronts traverse the porous layers whereby particle-to-particle interactions cause stress bridging effects and the evolution of organized wave structures.

This report summarizes a multiyear effort to establish a new capability for determining dynamic material properties. By utilizing a significant reduction in experimental length and time scales, this new capability addresses both the high per-experiment costs of current methods and the inability of these methods to characterize materials having very small dimensions. Possible applications include bulk-processed materials with minimal dimensions, very scarce or hazardous materials, and materials that can only be made with microscale dimensions. Based on earlier work to develop laser-based techniques for detonating explosives, the current study examined the laser acceleration, or photonic driving, of small metal discs (''flyers'') that can generate controlled, planar shockwaves in test materials upon impact. Sub-nanosecond interferometric diagnostics were developed previously to examine the motion and impact of laser-driven flyers. To address a broad range of materials and stress states, photonic driving levels must be scaled up considerably from the levels used in earlier studies. Higher driving levels, however, increase concerns over laser-induced damage in optics and excessive heating of laser-accelerated materials. Sufficiently high levels require custom beam-shaping optics to ensure planar acceleration of flyers. The present study involved the development and evaluation of photonic driving systems at two driving levels, numerical simulations of flyer acceleration and impact using the CTH hydrodynamics code, design and fabrication of launch assemblies, improvements in diagnostic instrumentation, and validation experiments on both bulk and thin-film materials having well-established shock properties. The primary conclusion is that photonic driving techniques are viable additions to the methods currently used to obtain dynamic material properties. Improvements in launch conditions and diagnostics can certainly be made, but the main challenge to future applications will be the successful design and fabrication of test assemblies for materials of interest.

In order to provide real-time data for validation of three dimensional numerical simulations of heterogeneous materials subjected to impact loading, an optically recording velocity interferometer system (ORVIS) has been adapted to a line-imaging instrument capable of generating precise mesoscopic scale measurements of spatially resolved velocity variations during dynamic deformation. Combining independently variable target magnification and interferometer fringe spacing, this instrument can probe a velocity field along line segments up to 15 mm in length. In high magnification operation, spatial resolution better than 10 {micro}m can be achieved. For events appropriate to short recording times, streak camera recording can provide temporal resolution better than 0.2 ns. A robust method for extracting spatially resolved velocity-time profiles from streak camera image data has been developed and incorporated into a computer program that utilizes a standard VISAR analysis platform. The use of line-imaging ORVIS to obtain measurements of the mesoscopic scale dynamic response of shocked samples has been demonstrated on several different classes of heterogeneous materials. Studies have focused on pressed, granular sugar as a simulant material for the widely used explosive HMX. For low-density (65% theoretical maximum density) pressings of sugar, material response has been investigated as a function of both impact velocity and changes in particle size distribution. The experimental results provide a consistent picture of the dispersive nature of the wave transmitted through these samples and reveal both transverse and longitudinal wave structures on mesoscopic scales. This observed behavior is consistent with the highly structured mesoscopic response predicted by 3-D simulations. Preliminary line-imaging ORVIS measurements on HMX as well as other heterogeneous materials such as foam and glass-reinforced polyester are also discussed.