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The strength properties of Coors AD995 alumina is investigated in the stress range starting from the HEL (5 to 7 GPa) and up to much higher stresses. Simulating the experimental work done by Grady and Moody, we try to explain the structure of loading, unloading, reloading and cyclic loading profiles measured by velocity interferometry. Many physicists are talking about plasticity and slip systems as the mechanism behind the unique structure of the response of ceramics to shock waves, and many use pseudo elasto‐plastic strength models to describe this response. Here we try to show that the strength properties of Alumina, being a brittle material, can be derived just from the combined effects of micro‐mechanisms like pore crushing and cracks initiation and growth. Any, so called “yield”, is due to pore crushing and not to invoked slip systems.

Shock‐compression experiments have recently measured the optical reflectivity of tin to detect a phase change (melting) upon release of the strongly shocked material into a LiF anvil. In this work, we present QMD calcultions of the optical reflectivity of solid (cold) β‐phase tin, representative of the pre‐shock state, and of liquid (warm) tin, representative of a Hugoniot and a shock‐released state. Calculated differences in the optical reflectivity between the cold and warm states are compared with the measurements from shock‐compression experiments.

Comets have long been proposed as a potential means for the transport of complex organic compounds to early Earth. For this to be a viable mechanism, a significant fraction of organic compounds must survive the high temperatures due to impact. We have undertaken three‐dimensional numerical simulations to track the thermodynamic state of a comet during oblique impacts. The comet was modeled as a 1‐km water‐ice sphere impacting a basalt plane at 11.2 km/s; impact angles of 15° (from horizontal), 30°, 45°, 65°, and 90° (normal impact) were examined. The survival of organic cometary material, modeled as water ice for simplicity, was calculated using three criteria: (1) peak temperatures, (2) the thermodynamic phase of H2O, and (3) final temperature upon isentropic unloading. For impact angles greater than or equal to 30°, no organic material is expected to survive the impact. For the 15° impact, most of the material survives the initial impact and significant fractions (55%, 25%, and 44%, respectively) satisf...

Shock initiation experiments on the HMX based explosive LX‐10 (95% HMX, 5% Viton by weight) were performed to obtain in‐situ pressure gauge data, run‐distance‐to‐detonation thresholds, and Ignition and Growth modeling parameters. A 101 mm diameter propellant driven gas gun was utilized to initiate the explosive samples with manganin piezoresistive pressure gauge packages placed between sample slices. The run‐distance‐to‐detonation points on the Pop‐plot for these experiments showed agreement with previous published data and extended the range to lower pressures. The Ignition and Growth modeling parameters showed good agreement to the data over the pressure range tested. This parameter set will provide additional information to ensure accurate code predictions for safety scenarios involving the HMX based explosive LX‐10.

There is an ongoing interest in identifying inexpensive armour materials for use in the protection of personnel and vehicles. The response of AD995 under shock loading is one of the materials most extensively investigated. Over recent years, workers have reported failure occurring in various polycrystalline ceramics behind the shock front. This phenomenon has been investigated using embedded stress sensors and a recovery technique that has allowed observation of the microstructure above and below the Hugoniot Elastic Limit (HEL) and these results are brought together here to explain the observed behaviour. The failure front velocity is found to change with the applied stress, in particular it slows as the HEL is exceeded. The microstructure shows the response below the HEL is dominated by intergranular failure whilst above it, the grains exhibit plasticity (including twinning). The HEL is thus shown to be characteristic of alumina viewed as a composite with randomly oriented alumina grains.

A “damage‐initiated reaction” (DMGIR) computational model is presented for prediction of explosive response to non‐shock mechanical insults. The distinguishing feature of this model is the introduction of a damage variable, which relates the evolution of damage to the initiation of reaction in the explosive, and the growth to detonation. The DMGIR model was embedded into an existing shock‐physics computational code to utilize those existing, predictive capabilities as the basis for these non‐shock predictions. The numerical approach is presented, with comparisons of DMGIR predictions to recent experiments utilizing non‐shock initiation of the plastic bonded explosive PBXN‐5. This model has been developed, and is presently being validated for plastic bonded explosives. Extension of the DMGIR model is planned for cast‐ and moldable‐type explosives.

We present the first sensitivity study of the material isentropes extracted from ramp compression experiments. We perform hydrodynamic simulations of representative experimental geometries associated with ramp compression experiments and discuss the major factors determining the accuracy of the equation of state information extracted from such data.

Mesoscale hydrodynamic calculations have been conducted in order to gain further insight into the dynamic compaction characteristics of granular ceramics. From these calculations bulk material characteristics such as shock speed, stress and density have been obtained and compared to experimental results in order to assess the viability of the method. Insights into the grain dynamics as well as the compaction front characteristics are explored. It was found that the baseline mesoscale solution under predicted the experimental Hugoniot response. In order to better match the experimental data, the dynamic yield strength of the granular material was increased from 5 to 8 GPa. A discussion of the shortcomings in the mesoscale modeling technique, as well as future considerations, is included.

The material response of 5083‐H131 aluminum alloy subjected to dynamic loading has been investigated over the stress input range 1.5 to 8.0 GPa using plate impact shock experiments. All experiments were performed on the ARL 102 mm light gas gun. Both symmetric and asymmetric shock experiments were performed and exhibited planarity to within 1 mrad of tilt. The principal diagnostic consisted of VISAR recordings of the rear free‐surface velocity history with temporal resolution of 0.5 ns. Within this stress range considered we observe a two wave structure typical of elastic and plastic traveling waves. In prior work we reported the Hugoniot Elastic Limit (HEL), compression parameters, and shock EOS. Here we examine the data to determine the spall strength, pulse duration and tensile loading strain‐rate. We compare our results to other A15083 tempers.

The measurement of shear strength via the use of lateral stress gauges has been shown to be a viable technique in a number of materials. An experimental investigation on the intermediate‐rate behaviour and shock response of the aluminium alloy, 6082‐T6, is reported here. Results obtained using the lateral stress gauge technique show that the shear strength increases with impact stress. The lateral stress behind the shock front is seen to be relatively flat, unlike many other face‐centred cubic metals and alloys, where a decrease in lateral stress indicates an increase in shear strength. This unusal response may be a reflection of the high stacking fault energy of aluminium and its alloys resulting in a reduction of the work hardening (i.e. increases in dislocation and/or twin density). Further plate impact results show that the Hugoniot of 6082‐T6 is in effect identical to that of the more widely known 6061‐T6. Split Hopkinson pressure bar results are used to provide a fuller picture of the rate‐dependant behaviour of 6082‐T6 over a range of loading rates and conditions.

The glass transition temperature (Tg) of a polyurea (Versathane P1000) is shown to be a function of mechanical strain and strain rate. For low strain rate (10−1/s), tensile testing, Tg values increase from −58 to −52 °C with strain. These results are interpreted as an increase in phase mixing. After annealing at 100 °C, the Tg decreases to a baseline value of −63 °C as an indication that the strain induced phase mixing is reversible. For one dimensional plate impact experiments, the Tg increases from −58 to −54 °C with strain. After annealing, the Tg value at the high strain level is about 4 °C higher than the baseline value. The impact may have caused some permanent change in the morphology. For a conical‐shaped steel impact experiment, Tg values also increase with strain from −58 to −50 °C. The Tg at the impact center after an annealing cycle is about 4 °C greater than the baseline value, indicating somewhat less than full reversal of the mixing.

When highly polished metal surfaces melt upon release after shock loading, they exhibit features that suggest significant surface changes accompany the phase transition. The reflection of light from such surfaces changes from specular (pre-shock) to diffuse upon melting. Typical of this phenomenon is the loss of signal light in velocity interferometer system for any reflector (VISAR) measurements, which usually occurs at pressures high enough to melt the free surface. Unlike many other potential material phase-sensitive diagnostics (e.g., reflectometry, conductivity), that show relatively small (1%-10%) changes, the specularity of reflection provides a more sensitive and definitive (>10x) indication of the solid-liquid phase transition. Data will be presented that support the hypothesis that specularity changes indicate melt in a way that can be measured easily and unambiguously.

Laser driven shock measurements have been performed on precompressed and preheated water samples in order to reach states lying above the standard water Hugoniot in the pressure versus temperature diagram, which are representative of the thermodynamic parameters of water in the detonation products of high condensed explosives. In this experimental system, water is used as both target sample and window medium for VISAR diagnosis. We report the first experiments performed with the LCD's laser system at low shock pressure, on water samples preheated up to 300 °C and precompressed up to 300 bar. The results are used to check the predictions of the CARTE thermochemical code, and compared with the Sesame equation of state and with molecular Monte Carlo calculations.

Laser shocks are most often used to produce uniaxial stress and strain into materials by irradiating a spot diameter conventionally admitted at least three times larger than the thickness of the shocked sample. By reducing the laser spot versus the sample thickness, 2D lateral waves are created earlier and their crossing during propagation stages generates traction which can yield to voids into materials (near the front loaded face). This phenomenon has been evidenced by an experimental study, including VISAR measurements which exhibit the signature of the fracture generated by these 2D effects. Numerical simulations with the explicit finite element code RADIOSS clearly evidence the origin of the 2D effects on VISAR measurements. This different mode of damaging materials by laser lateral waves can act simultaneously with the classical laser spallation produced by the uniaxial propagation (shock wave reverberation crossing the unloading). This opens new discussed prospects for the development of 2D damage ...

This work uses density functional molecular dynamics simulations of fluid helium at high pressure to examine how shock wave experiments with precompressed samples can help characterizing the interior of giant planets. In particular, we analyze how large of a precompression is needed to probe a certain depth in a planet's gas envelope. We find that precompressions of up to 0.1, 1.0, 10, or 100 GPa are needed to characterized 2.5, 5.9, 18, to 63% of Jupiter's envelope by mass., Comment: Submitted As Proceedings Article For The American Physical Society Meeting On Shock Compression Of Condensed Matter, Hawaii, June, 2007

Quasi‐static and dynamic characterization of nickel based superalloy Waspaloy® has been performed at the University of Cassino. Quasi‐static tensile tests have been carried out on both round bar specimens, to obtain the flow stress curve at low strain rates, and hourglass specimens, to investigate damage evolution with plastic strain. The mechanical behavior at high strain rates has been obtained by means of a direct tension split Hopkinson Bar, which allows the characterization of the material up to failure. Experimental results show that when strain rates increases, the failure strain increases while the yield strength decreases, in some intervals of the range considered. This singular behavior has been modeled and implement in a Finite Element Method commercial code in order to perform numerical simulations of experimental ballistic tests carried out at the Politecnico di Torino, using an airgun facility. Good agreement has been found between FEM simulations and experimental results.

Within the next few years hard X-ray Free Electron Lasers will come on line. Such systems will have spectral brightnesses ten orders of magnitude greater than any extant synchrotron, with pulse lengths as short as a few femtoseconds. It is anticipated that large-scale optical lasers capable of shock-compressing matter to multi-megabar pressures will be sited alongside the X-ray source. We discuss how such systems can further our knowledge of shocked and isochorically heated matter, in particular investigating the potential to perform poly crystalline diffraction and the creation of warm dense matter. © 2007 American Institute of Physics.

Lattice level measurements of material response under extreme conditions are required to build a phenomenological understanding of the shock response of solids. We have successfully used laser produced plasma x-ray sources coincident with laser driven shock waves to make in-situ measurements of the lattice response during shock compression for both single crystal and polycrystalline materials. Using a detailed analysis of shocked single crystal iron which has undergone the α - ε phase transition we can constrain the transition mechanism to be consistent with a compression and shuffle of alternate lattice planes. © 2007 American Institute of Physics.

Over the past few years we have been pioneering the use of sub-nanosecond X-ray diffraction to determine the phase and compression of shocked crystals. It is well known that the deviation of atoms from their ideal lattice sites due to thermal motion reduces the integrated intensity within diffraction peaks - the so-called Debye-Waller effect, and thus it is pertinent to investigate whether line ratios might be sufficiently sensitive to be used as a viable temperature diagnostic. Clearly the matter is not completely straight-forward, as the Debye frequency of a solid also varies under compression. In our initial investigations we have calculated the ratios of intensities of high-order reflections assuming various forms of the Gruneisen parameter, and have also compared these results with those obtained from Molecular Dynamics simulations. Given the photon energies of nanosecond X-ray pulses that can currently be produced, we comment on the experimental feasibility of the technique.

Cubic, single-crystal, transparent Gd{sub 3}Ga{sub 5}O{sub 12} has a density of 7.10 g/cm{sup 3}, a Hugoniot elastic limit (HEL) of 30 GPa, and undergoes a continuous phase transition from 65 GPa to a quasi-incompressible (QI) phase at 120 GPa. Only diamond has a larger HEL. The QI phase of Gd{sub 3}Ga{sub 5}O{sub 12} is more incompressible than diamond from 170 to 260 GPa. Electrical conductivity measurements indicate the QI phase has a bandgap of 3.1 eV. Gd{sub 3}Ga{sub 5}O{sub 12} can be used to obtain substantially higher pressures and lower temperatures in metallic fluid hydrogen than was achieved previously by shock reverberation between Al{sub 2}O{sub 3} disks. Dynamic compression achieves pressures, densities, and temperatures that enable investigation of ultracondensed matter at conditions yet to be achieved by any other technique. The prototypical example is observation of minimum metallic conductivity (MMC) of dense fluid hydrogen at 140 GPa, nine-fold compression of liquid density, and {approx}3000 K [1-3]. The high pressure and density and relatively low temperature are achieved by multiple-shock compression [2]. Temperature T is relatively low in the sense that T/TP{sub F} {approx} 0.01, where T{sub F} is the Fermi temperature. The time scale of compression is sufficiently long tomore » achieve thermal equilibrium and sufficiently short so the process is adiabatic. Similar results are obtained for oxygen [4] and nitrogen [5]. Fluid Cs and Rb undergo the same transition at 2000 K near their liquid-vapor critical points [6]. All five elemental fluids have essentially the same value of MMC and the density dependences of their semiconductivities scale with the quantum-mechanical charge-density distributions of the respective atoms [5]. Liquid H{sub 2} is one of the most compressible of all materials. In this paper, we report that the dielectric crystal Gd{sub 3}Ga{sub 5}O{sub 12} (GGG) transitions to a virtually incompressible phase at 120 GPa shock pressure.« less

An accurate equation of state (EOS) for planetary constituents at extreme conditions is the key to any credible model of planets or low mass stars. However, very few materials have their high pressure (>few Mbar) EOS experimentally validated, and even then, only on the principal Hugoniot. For planetary and stellar interiors, compression occurs from gravitational force so that material states follow a line of isotropic compression (ignoring phase separation) to ultra-high densities. An example of the hydrogen phase space composing Jupiter and one particular Brown Dwarf is shown. At extreme densities, material states are predicted to have quite unearthly properties such as high temperature superconductivity and low temperature fusion. High density experiments on Earth are achieved with either static compression techniques (i.e. diamond anvil cells) or dynamic compression techniques using large laser facilities, gas guns, or explosives. The ultimate goal of this multi-directorate and multi-institutional proposal was to develop techniques that will enable us to understand material states that previously only existed at the core of giant planets, stars, or speculative theories. Our effort was a complete success, meeting all of the objectives set out in our proposals. First we focused on developing accurate Hugoniot techniques to be used formore » constraining the equation of state at high pressure/temperature. We mapped out an accurate water EOS and measured that the ionic->electronic conduction transition occurs at lower pressures than models predict. These data and their impact are fully described in the first enclosed paper ''The Equation of State and Optical Properties of Water Compressed by Strong Shock Waves.'' Currently models used to construct planetary isentropes are constrained by only the planet radius, outer atmospheric spectroscopy, and space probe gravitational moment and magnetic field data. Thus these data, which provide rigid constraints to these models, will have a significant impact on a broad community of planetary and condensed matter scientists, as well as our fundamental understanding of the giant planets. We then developed and tested precompressed and multiple shock techniques on water. Scientists around the world have teamed with us to conduct these complex and seminal high density experiments which allow access to the extreme core states of giant plants. Double shock experiments using a variety of anvils to compress water to densities higher and temperatures lower than accessible by single shock Hugoniot techniques. First a clear determination of the EOS and optical properties of the anvils needed to be measured. These properties for LiF and A1203 are written up in the second attached article, ''Shock-Induced Transformation of Sapphire and Lithium Fluoride into Semiconducting Liquids.'' An example double shock data record for water is shown. This data is being written up for publication.« less

In the three years of this project, we have provided a complete database of the electrical conductivity of planetary materials to 180 GPa. The electrical conductivities of these planetary materials now provide a basis for future modeling of planets taking into account full magnetohydrodynamics. By using a full magnetohydrodynamics simulation, the magnetic fields of the planets can then be taken into account. Moreover, the electrical conductivities of the planetary materials have given us insight into the structure and nature of these dense fluids. We showed that simple monoatomic fluids such as hydrogen, nitrogen, and oxygen at planetary interior conditions undergo a common metallization process which can be explained on a simple basis of their radial charge density distributions. This model also shows that the metallization process is actually rather common and likely to take place in a number of materials such as carbon monoxide which is also present within planetary objects. On the other hand, we have also showed that a simple two component fluid like water and methane take on much different behaviors than say nitrogen due to the chemical interactions within these systems. The dynamics of an even more complex system, ''synthetic Uranus'' are still being analyzed butmore » suggest that on some levels the behavior is very simple, i.e. the electrical conductivity is essentially the same as water, but the local dynamics are very complex. This project has shed much light on the nature of electrical transport within planetary interiors but also has shown that understanding chemical processes in the complex fluids within planetary interiors to be very important. Understanding those local interactions and processes is required to gain further insight into planetary interiors.« less

The UCu5-x Pdx system exhibits non-Fermi liquid (NFL) behavior in thermodynamic and transport properties at low temperatures for Pd concentrations 0.9 less than or approximately x less than or approximately 1.5. The local structure around the U, Cu, and Pd atoms has been measured for

A new absolute equation of state measurement technique is described and demonstrated measuring the shock state and the refractive index of MgO up to 226 GPa. This technique utilizes steady shock waves and the high-pressure transparency of MgO under dynamic shock compression and release. Hugoniot measurements performed using this technique are consistent with the previous measurements. A linear dependence of the shocked refractive index and density is observed up to 226 GPa, over a magnitude greater in pressure that previous studies. The transparency of MgO along the principal Hugoniot is higher than any other material reported to date. We observe a significant change in the refractive index of MgO as the Hugoniot elastic limit is exceeded due to the transition from uniaxial to hydrostatic strain. Measurements of the elastic-plastic two-wave structure in MgO indicate a nucleation time for plastic deformation.

The shock Hugoniot of single crystal copper is reported for stresses below 66 GPa. Symmetric impact experiments were used to measure the Hugoniots of three different crystal orientations of copper, [100], [110], and [111]. The photonic doppler velocimetry (PDV) diagnostic was adapted into a very high precision time of arrival detector for these experiments. The measured Hugoniots along all three crystal directions were nearly identical to the experimental Hugoniot for polycrystalline Cu. The predicted orientation dependence of the Hugoniot from molecular dynamics calculations was not observed. At the lowest stresses, the sound speed in Cu was extracted from the PDV data. The measured sound speeds are in agreement with values calculated from the elastic constants for Cu.

We present a method of interpolation of thermodynamic functions calculated by means of a multiphase equation of state for metals. The method takes into account melting, evaporation and sublimation phase transitions. It can be also applied in metastable regions of phase diagram including those under negative pressures. The method allows one to unambiguously determine the phase state of a given point of phase diagram. Several applications of the tabular multiphase equations of state are considered: simulation of the initial stage of electrical explosion of metal wires, formation of striations, interaction of intense laser pulses with matter etc. Information about phase state in every point of the flux allows us to study phase transition waves, apply different destruction criteria and analyze processes in metastable regions.