Publications

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In order to generate new properties of metals exposed to high pressure states, it is desirable to study samples loaded in one-dimensional strain. Previous work to obtain these ideal conditions, involve a technique where the sample was recovered at late times to examine its microstructure. In those experiments, the shock-loading was produced by impacting the sample with a flyer plate. In the present work, we modified the sample recovery assembly and optimized it for ramp wave loading. We describe the 2-D calculations performed with the ALEGRA MHD code that led to improved recovery assembly efficiency. Preliminary comparisons of the simulations with measurements of the sample deformation from an experiment indicate excellent agreement. © 2009 American Institute of Physics.

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While isentropic compression experiment (ICE) techniques have proved useful in deducing the high-pressure compressibility of a wide range of materials, they have encountered difficulties where large-volume phase transitions exist. The present study sought to apply graded-density impactor methods for producing isentropic loading to planar impact experiments to selected such problems. Cerium was chosen due to its 20% compression between 0.7 and 1.0 GPa. A model was constructed based on limited earlier dynamic data, and applied to the design of a suite of experiments. A capability for handling this material was installed. Two experiments were executed using shock/reload techniques with available samples, loading initially to near the gamma-alpha transition, then reloading. As well, two graded-density impactor experiments were conducted with alumina. A method for interpreting ICE data was developed and validated; this uses a wavelet construction for the ramp wave and includes corrections for the ''diffraction'' of wavelets by releases or reloads reflected from the sample/window interface. Alternate methods for constructing graded-density impactors are discussed.

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A systematic computational and experimental study is presented on impact generated debris resulting from record-high impact speeds recently achieved on the Sandia three-stage light-gas gun. In these experiments, a target plate of aluminum is impacted by a titanium-alloy flyer plate at speeds ranging from 6.5 to 11 km/s, producing pressures from 1 Mb to over 2.3 Mb, and temperatures as high as 15000 K (>1 eV). The aluminum plate is totally melted at stresses above 1.6 Mb. Upon release, the thermodynamic release isentropes will interact with the vapor dome. The amount of vapor generated in the debris cloud will depend on many factors such as the thickness of the aluminum plate, super-cooling, vaporization kinetics, the distance, and therefore time, over which the impact-generated debris is allowed to expand. To characterize the debris cloud, the velocity history produced by stagnation of the aluminum expansion products against a witness plate is measured using velocity interferometry. X-ray measurements of the debris cloud are also recorded prior to stagnation against an aluminum witness plate. Both radiographs and witness-plate velocity measurements suggest that the vaporization process is both time-dependent and heterogeneous when the material is released from shocked states around 230 GPa. Experiments suggest that the threshold for vaporization kinetics in aluminum should become significant when expanded from shocked states over 230 GPa. Numerical simulations are conducted to compare the measured x-ray radiographs of the debris cloud and the time-resolved experimental interferometer record with calculational results using the 3-D hydrodynamic wavecode, CTH. Results of these experiments and calculations are discussed in this paper.

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Explosive charges placed on the fuze end of a drained chemical munition are expected to be used as a means to destroy the fuze and burster charges of the munition. Analyses are presented to evaluate the effect of these additional initiation charges on the fragmentation characteristics for the M121A1 155mm chemical munition, modeled with a T244 fuze attached, and to assess the consequences of these fragment impacts on the walls of a containment chamber--the Burster Detonation Vessel. A numerical shock physics code (CTH) is used to characterize the mass and velocity of munition fragments. Both two- and three-dimensional simulations of the munition have been completed in this study. Based on threshold fragment velocity/mass results drawn from both previous and current analyses, it is determined that under all fragment impact conditions from the munition configurations considered in this study, no perforation of the inner chamber wall will occur, and the integrity of the Burster Detonation Vessel is retained. However, the munition case fragments have sufficient mass and velocity to locally damage the surface of the inner wall of the containment vessel.

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The Grady-Kipp fragmentation model provides a physically based method for determining the fracture and breakup of materials under high loading rates. Recently, this model has been implemented into the CTH Shock Physics Code and has been used to simulate several published experiments. Materials studied in this paper are AerMet 100 steel and a 90% tungsten alloy. The experimental geometry consists of a right circular cylinder filled with an explosive main charge that is initiated at its center. The sudden expansion of the resulting detonation products causes fracture of the cylinder. Strain rates seen in the cylinder are on the order of 10{sup 4} s{sup {minus}1}. The average fragment sizes calculated with the Grady-Kipp fragmentation model successfully replicate the mean fragment size obtained from the experimental fragment distribution. When Poisson statistics are applied to the calculated local average fragment sizes, good correlation is also observed with the shape of the experimental cumulative fragment distribution. The experimental fragmentation results, CTH numerical simulations, and correlation of these numerical results with the experimental data are described.