Publications

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The high-pressure dynamic response of titanium dioxide (TiO 2) is not only of interest because of its numerous industrial applications but also because of its structural similarities to silica (SiO 2). We performed plate impact experiments in a two-stage light gas gun, at peak stresses from 64 to 221 GPa to determine the TiO 2 response along the Hugoniot. The lower stress experiment at 64 GPa shows an elastic behavior followed by an elastic-plastic transition, whereas the high stress experiments above 64 GPa show a single wave structure. Previous shock studies have shown the presence of high-pressure phases (HPP) I (26 GPa) and HPP II (100 GPa); however, our data suggest that the HPP I phase is stable up to 150 GPa. Using a combination of data from our current study and our previous Z-data, we determine that TiO 2 likely melts on the Hugoniot at 157 GPa. Furthermore, our data confirm that TiO 2 is not highly incompressible as shown by a previous study.

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Shock experiments on low density polyurethane foams reveal evidence of reaction at low impact pressures. However, these reaction thresholds are not evident over the low pressures reported for historical Hugoniot data of highly distended polyurethane at densities below 0.1 g/cc. To fill this gap, impact data for PMDI foam with a density of 0.087 g/cc were acquired for model validation. An equation of state (EOS) was developed to predict the shock response of these highly distended materials over the full range of impact conditions representing compaction of the inert material, low-pressure decomposition, and compression of the reaction products. A tabular SESAME EOS of the reaction products was generated using the JCZS database in the TIGER equilibrium code. In particular, the Arrhenius Burn EOS, a two-state model which transitions from an unreacted to a reacted state using Arrhenius kinetics, as implemented in the shock physics code CTH, was modified to include a statistical distribution of states. Hence, a single EOS is presented that predicts the onset to reaction due to shock loading in PMDI-based polyurethane foams. This methodology was also used to predict the anomalous compaction of PMDI foams over published data sets from 0.087 to 0.87 g/cc, and solid Polyurethane at a theoretical maximum density (TMD) of 1.264 g/cc. Likewise, similar modeling techniques were used to predict the performance of SX-358 foam, an RTV-based stress cushion material at a nominal density of 0.41 g/cc, and the matrix material, with properties similar to Sylgard, at 1.1 g/cc. At the start of this study, data were only available at a single impact condition below the threshold for reaction; hence, the decomposition of this material at higher pressures was revealed as a significant finding of this work. The decomposition of SX-358 at higher impact pressures to product species including solid, liquid, and gaseous molecules was estimated with thermochemical equilibrium calculations using TIGER. with thermochemical equilibrium calculations using TIGER. This modeling approach, developed for PMDI foam, was shown to predict gas gun data, acquired as part of this study, up to pressures of 14 GPa. Furthermore, additional phase transitions were predicted in the product species under shock compression. To date, this study is the first known to the authors that demonstrates and successfully predicts the decomposition of these low-density polymer-based foams using a single model applicable to a broad range of impact loading conditions.

The capability for statically pre-compressing fluid targets for Hugoniot measurements utilizing gas gun driven flyer plates has been developed. Pre-compression expands the capability for initial condition control, allowing access to thermodynamic states off the principal Hugoniot. Absolute Hugoniot measurements with an uncertainty less than 3% on density and pressure were obtained on statically pre-compressed fluid helium utilizing a two stage light gas gun. Helium is highly compressible; the locus of shock states resulting from dynamic loading of an initially compressed sample at room temperature is significantly denser than the cryogenic fluid Hugoniot even for relatively modest (0.27-0.38 GPa) initial pressures. The dynamic response of pre-compressed helium in the initial density range of 0.21-0.25 g/cm3 at ambient temperature may be described by a linear shock velocity (us) and particle velocity (up) relationship: us = C0 + sup, with C0 = 1.44 ± 0.14 km/s and s = 1.344 ± 0.025.

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