Session N1: Poster Session II (7:00-9:00pm)

Comparison of Theory and Measurements of a Two-Stage Light-Gas Gun

We present a comparison of techniques for obtaining projectile velocity history on a two-stage launcher and discuss gun code accuracy vis-\`{a}-vis pressure gauges and the new photonic Doppler velocimetry (PDV) technique. The PDV technique itself is described in a companion paper. The PDV records were differentiated to compute acceleration and, hence, base pressure. Two acceleration episodes are revealed in the data. Base pressure values were compared with measurements from stationary pressure gauges and with predictions of a standard two-stage gun code. The agreement with the pressure gages was satisfactory. Code predictions did not account for the two acceleration stages. However, for the main acceleration episode, the predicted base pressure is in good agreement with the smoothed pressure computed from the PDV record. Both the gauge records and PDV contain short-time pressure spikes which are apparently real. Therefore, use of computed base pressure for projectile design may lead to failures if the projectile is vulnerable to pressure spikes.   

Measuring Mechanoluminescence Generated During Ballistic Impacts

In 1888, Wiedemann published the first paper where luminescence is defined as the emission of electromagnetic radiation in excess of thermal radiation. This radiation is usually in the visible portion of the electromagnetic spectrum. Since the same basic processes may yield infrared or ultraviolet radiation, such emission in excess of thermal radiation is also described as luminescence. Depending on how this radiation is produced, determines the type of luminescence. For example, all luminescence caused by a mechanical action or process is known as mechanoluminescence (ML). Such action can include the breaking of crystals, triboluminescence (TL), or simply stressing and deforming crystals, deformation luminescence (DL). High pressure studies have been performed on ZnS and CaS phosphors. Using a hydrostatic pressure chamber, relative emission intensity and emission peaks have been determined. In addition, the relative emission intensity, emission peaks, and lifetimes of ZnS and CaS phosphors have been determined using common firearms and bullets. Using various calibers of ammunition, we were able to produce TL impacts between 200 and 600~m/s. Results indicate that ZnS and CaS phosphors show promise for future use as the active element for impact sensors.   

Development and Evaluation of the Next Generation of Meteoroid and Orbital Debris Shields

Recent events such as the Chinese anti-satellite missile test in January 2007 and the collision between a Russian Cosmos satellite and US Iridium satellite in February 2009 are responsible for a rapid increase in the population of orbital debris in Low Earth Orbit (LEO). Without active debris removal strategies the debris population in key orbits will continue to increase, requiring enhanced shielding capabilities to maintain allowable penetration risks. One of the more promising developments in recent years for meteoroid and orbital debris shielding (MMOD) is the application of open cell foams. Although shielding onboard the International Space Station is the most capable ever flown, the most proficient configuration (stuffed Whipple shield) requires an additional $\sim $30{\%} of the shielding mass for non-ballistic requirements (e.g. stiffeners, fasteners, etc.). Open cell foam structures provide similar mechanical performance to more traditional structural components such as honeycomb sandwich panels, as well as improved projectile fragmentation and melting as a result of repeated shocking by foam ligaments. In this paper, the preliminary results of an extensive hypervelocity impact test program on next generation MMOD shielding configurations incorporating open-cell metallic foams are reported.   

On the Existence of Shock Instabilities at Hugoniot Pressures Beyond the Minimum Volume

Flow instabilities are among the main issues of ICF studies. Heterogeneities of material or geometry are sources of instabilities which are strongly amplified in spherical geometries. According to the theory of Dyakov, some ranges of the Equation of State (EOS) also generate or amplify instabilities in shock waves, which can be considered among the origins of Richtmyer- Meshkov instabilities [Bates, 2004]. Stability corresponds to Dyakov parameter $-11$, associated to a positive P-V slope on the Hugoniot curve beyond the minimum volume, occurring with ionization, remains unstudied as well theoretically as experimentally. We know that, on the Hugoniot curve, the volume decreases versus pressure down to a minimum and then increases with the successive levels of ionization towards asymptotic values for the volume. Recent EOS results obtained by Pain (2007) in this range of pressure about all the elements allow us to investigate now the stability conditions for $h>1$. The first question to raise is the possibility of existence of such instabilities. In the present study, we focus on the properties of several elements (aluminium, iron, copper...) in this range of pressure to try to give a first answer to this question.   

Void configuration effect on coalescence in single crystal copper under shock loading

Void coalescence is one of most critical stages during ductile fracture and is related with many factors. The configuration effect on coalescence has been investigated by means of molecular dynamics (MD) simulations in this paper. The void configuration is represented by $\theta $, which is the angle between the connected line of the voids and the shock direction. Using four different void configurations, microscopic mechanism of coalescence was observed and analyzed under 15.7GPa shock strength. The results show that the coalescence process is consistent with phenomena observed in experiment of ductile fracture. Moreover, the coalescence is most easy to occur when the $\theta $ equals 60 degree. The theory model was proposed and explained simulations results very well.   

The response of a carbon-fibre phenolic resin composite to one-dimensional shock loading

The Hugoniot of a carbon-fibre-phenolic composite has ben measured via plate impact, using manganin stress gauges as the main diagnostic. The shock velocity has a linear response with particle velocity, but the velocity itself is higher than in similar aerospace composites. Consequently, the Hugoniot in terms of shock stress is also higher than in conventional carbon composites. Differences between the measured stress and calculated hydrodynamic pressure suggest that the shear strength of this material also increases with pressure.   

Effect of Particle Morphology on the Reactivity of Explosively Dispersed Titanium Particles

The effect of particle morphology on the reaction of titanium (Ti) particles explosively dispersed during the detonation of either cylindrical or spherical charges has been investigated experimentally. The explosive charges consisted of packed beds of Ti particles saturated with nitromethane. The reaction behavior of irregularly-shaped Ti particles in three size ranges is compared with tests with spherical Ti particles. The particle reaction is strongly dependent on particle morphology, e.g., 95 $\mu $m spherical Ti particles failed to ignite (in cylinders up to 49 mm in dia), whereas similarly sized irregular Ti particles readily ignited. For irregular particles, the uniformity of ignition on the particle cloud surface was almost independent of particle size, but depended on charge diameter. As the charge diameter was reduced, ignition in the conically expanding particle cloud occurred only at isolated spots or bands. For spherical charges, although large irregular Ti particles ignited promptly and uniformly throughout the particle cloud, the smallest particles dispersed nonuniformly and ignition occurred at isolated locations. In general, particle ignition is a competition between particle heating (which is influenced by particle morphology, size, number density and the local thermodynamic history) and expansion cooling of the products.   

Initiation Studies of Annular-Shaped PBX 9502

A series of tests has been conducted to evaluate the initiation limits of annular-shaped PBX 9502. The center of the PBX 9502 annulus is packed with a donor charge (such as XTX 8004) that is sufficiently extended above the annulus opening to achieve a steady detonation prior to entering the main charge. The initiation effects of donor charge type, size, and shot temperature are examined via witness plate and streak camera visualization at the output of the explosive train. The streak camera allows for distinguishing between direct radial initiation from the donor charge and initiation from the high-pressure juncture at the witness plate surface. These tests are designed to examine the effects of non-planar initiation of insensitive explosive in a divergent geometry.   

Shock-metamorphic Transformations of Rock-forming Minerals in Condition of Step-like Shock Compression of Crystal Schist

Shock-metamorphic transformations of garnet, biotite, quartz and feldspar were studied with the use of recovery assemblies of planar geometry. Experimental samples were prepared from Southern Urals' crystal schist. The maximal shock pressures in the samples were attained upon a few reverberations of the waves between the walls of the recovery ampoule (step-like shock compression) and were equal 26, 36 and 52 GPa.and feldspar reveal the strongest transformations. Planar elements having different crystallographic orientation appear in quartz at 26 GPa. Feldspar amorphization degree grows up with shock pressure amplitude increasing.reveals strong mechanical deformations (fissuring, crumbling stripes forming). It is partly being melted, the strongest biotite melting degree is observed at 52 GPa.transformed weaker than other minerals. It becomes heavily cracked under shock wave compression. Its fissuring degree becomes greater with shock pressure growth.   

Electrical conductivity of aluminum hydride AlH3 at high pressure and temperature

A study of electrophysical and thermodynamic properties of alane AlH$_{3}$ under multi shock compression has been carried out. The increase in specific electroconductivity of alane at shock compression up to pressure 100 GPa have been measured. High pressures and temperatures were obtained with explosive device, which accelerates the stainless impactor up to 3 km/sec. The impact shock is split into a shock wave reverberating in alane between two stiff metal anvils. The conductivity of shocked alane increases in the range up to 60-75 GPa and is about 30 1/Ohm*cm. In this region the semiconductor regime is true for shocked alane. The conductivity of alane achieves approximately 500 1/Ohm*cm at 80-90 GPa. In this region conductivity is interpreted in frames of the conception of the ``dielectric catastrophe'', taking into consideration significant difference between electronic states of isolated AlH$_{3}$ molecule and condensed alane.   

Plasmonic Enhancement of Direct Optical Initiation

Current Direct Optical Initiation (DOI) detonators use a laser focused onto a thin metal layer to drive a hot plasma and/or fragments into PETN powder. Previous studies showed a dramatic decrease in laser energies required to initiate the detonation using this approach over direct laser illumination of the PETN powder. Plasmonic metal nanostructures have been shown capable of strongly coupling laser energy into adjacent materials. We have incorporated gold nanospheres into PETN powder and are investigating their plasmonic enhancement of direct optical initiation via measurements of threshold laser energies and streak camera measurements for calculation of run to detonation distances compared to other DOI schemes.   

A simple URANS approach for secondary combustion of HE detonation products

The detonation of a high explosive (HE) charge in air generates a high-speed compressible turbulent flow and detonation products (DP) - air combustion. The classical Unsteady Reynolds Averaged Navier Stokes (URANS) approach involves too many parameters impossible to identify separately for such an application. We propose a simplest formulation to describe the turbulent combustion due to DP-air interpenetration, considering that the turbulent flow contains symmetric vortices. The Reynolds averaged reactive Euler equations analysis and thermodynamic requirements lead to an equation for the turbulent entropy production due to the DP-air combustion and an equation of state similar to ideal gas for turbulent variables (turbulent pressure, energy, temperature and entropy). Considering an infinite chemistry rate, a quasi-steady solution is derived in spherical geometry and embedded in a 1-D spherical ALE code based on a Godunov scheme and HLLC solver to calculate the fireball radius and air-blast parameters. The numerical solution behaves correctly comparing to experimental data for CHNO and CHNOAl HE.   

Isentropic Compression driven by high-explosive: application to Ti-6Al-4V

We report on a n isentropic compression experiment of Ti-6Al-4V alloy based on the use of the release of detonation products from a high-explosive to generate a ramp wave compression in a multisteps target. VISAR and DLI measurements of the rear free surface velocities of the different steps allow computing the sound velocity of the material during its compression, which is characteristic of the EOS of the material. The experiment device is described and the sound velocities measurements are analyzed. The results are compared with 2-dimensional elasto-plastic simulations.   

Development of an optical system for high-speed, small-scale velocity measurements

An optical system was developed to allow the accurate focussing and alignment of small scale samples for velocity analysis with streak photography. Even at low magnifications, the small streak slits required meant that any vibration or instability in the system would greatly reduce the accuracy of the velocity measurements achieved. Therefore, the optical system was designed to reduce the effects of any vibration. A spatial mount and rotation stage were modified to allow three spatial axis and rotational freedom of a custom-made sample mount. Markers within the sample mount were used to achieve precise alignment of the sample with the optical axis of the streak camera.   

Method for pressing powdered explosives in target assemblies for gas gun-driven initiation experiments

In order to obtain an equation of state (EOS) and initiation information for a powdered explosive at the lower density ranges, it is necessary to press the material into the gas gun target assembly. Pressed pellets can be built into targets at the higher densities where they have integrity or can be machined but are difficult or impossible to use at low densities. We have a need to employ multiple magnetic gauges to measure the EOS and initiation data on low density, powdered explosives such as ammonium nitrate (AN). We have designed a ``half cell'' target assembly that has a magnetic gauge membrane glued to a triangular shaped cavity which can be loaded from the cell side. The technique uses a miniature load cell to monitor the force on a pressing stemple that fits into the triangular cavity. A force vs. density curve can be obtained for the particular powder and stemple/cell combination. Using this, the powdered explosive can be loaded to the desired density by pressing in increments to a given force. After the cell is loaded, a cover is glued on to confine the sample. Examples of pressing AN into the target assemblies at different densities and the data obtained from the gas gun driven experiments will be shown.   

Shock Compression Experiments with in situ Ellipsometry Measurements

Knowledge about the optical properties of materials at high pressure and high temperature is needed for EOS research. Ellipsometry measures the change in the polarization of a probe beam reflected from a surface. From the change in polarization, the real and imaginary parts of the time dependent complex index of refraction can be extracted. From the measured optical properties, fundamental physical properties of the material, such as emissivity, phase transitions, and electrical conductivity can be extracted. A dynamic ellipsometry measurement system with nanosecond resolution was built in order to measure all four stocks parameters. Gas gun was used to accelerate the impact flyer. Our experiments concentrated on the optical properties of 1020 steel targets with impact pressure range of 40-250 kbar. Free surface measurements as well as window-target interface measurements were preformed. Although there are intrinsic difficulties with dynamic ellipsometric measurements, distinct changes were observed for shock wave pressures larger than 130kbar, the $\alpha \to \varepsilon $ phase transition.   

Progress Towards Microwave Ignition of Explosives

Microwaves could provide a method of propellant ignition that does away with a traditional primer, making ammunition safer and suitable for Insensitive Munitions (IM) applications. By embedding a suitable material inside a propellant, it is postulated that microwaves could be used to stimulate hotspots, through direct heating or electrostatic discharge (arcing) across the energetic material. This paper reports on progress in finding these suitable materials. Graphite rod, magnetite cubes and powders of graphite, aluminium, copper oxide, and iron were irradiated in a conventional microwave oven. Temperature measurements were made using a shielded thermocouple and thermal paints. Only graphite rod and magnetite showed significant heating upon microwave exposure. The light output from arcing of iron, steel, iron pyrite, magnetite and graphite was measured in the same microwave oven as above. Sample mass and shape were correlated with arcing intensity. A strategy is proposed to create a homogeneous igniter material by embedding arcing materials within an insulator, Polymethylpentene (TPX). External discharges were transmitted through TPX, however no embedded samples were successful in generating an electrical breakdown suitable for propellant ignition.   

Impact of Aluminum Sphere On Aluminum Plate At 4~KM/S : Comparison Between Experimental And Simulations With Two Non-Linear Hydrocodes

High velocity impact of 3 mm diameter aluminum sphere against 2.1 mm aluminum target plate have been performed at impact velocity of 4000 m/s with the two stage light gas gun HERMES at THIOT-INGENIERIE laboratory. Impacts at normal and with a 32\r{ } angle tilt generated debris clouds that were collected by 1.1 mm aluminum witness plates. The visualization of the debris clouds generated after the impact has been realized by using an ultra high speed camera SIM8 developed by SPECIALISED IMAGING LIMITED. Impact simulations using Smooth Particle Hydrodynamic (SPH) solvers were performed on two commercial codes ANSYS-AUTODYN and LS-DYNA to reproduce debris clouds generation and expansion in the two angle configurations. Comparison between simulations and experimental frames taken with the ultra high speed camera are proposed. The simulated and experimental witness plate debris cloud damages are also analyzed.   

Imaging High Speed Particles in Explosive Driven Blast Waves

Researchers Mr. Charles Jenkins and Dr. Yasuyuki Horie at the High Explosive Research {\&} Development (HERD) facility at Eglin AFB with sponsorship from DTRA has successfully imaged high speed explosively driven metallic particles. The process uses an adapted, commercially available Particle Image Velocimetry (PIV) instrument. Regional and particle flow vectors are determined from particle displacement between two images taken in rapid succession. The instrument consists of a 120 mJ, pulsed Nd:YAG laser, camera system, synchronizer, and proprietary imaging software. The new PIV capability provides the ability for scientists and engineers to map explosively driven metallic particles in a blast wave. Characteristics of particle motion, interaction and dispersion can be determined by this method, providing measurements of key parameters such as particle size, shape, velocity, and concentration. This new capability to image and track small (from a few microns to as large as several hundred microns) high-speed particles without direct intervention by physical means, ensures that the particles are unchanged in their environment and provides greater measurement accuracy of particle dynamics in very short time scales. The capability can also be used to map large areas (square feet) or to zoom down at higher magnifications to study particle features such as particle agglomeration.   

Shock initiation behavior of PBXN-9 determined by gas gun experiments

The shock to detonation transition was evaluated in the HMX based explosive PBXN-9 by a series of light-gas gun experiments. PBXN-9 consists of 92 wt{\%} HMX, 2wt{\%} Hycar 4054 {\&} 6 wt{\%} dioctyl adipate with a density of 1.75 g/cm$^{3}$ and 0.8{\%} voids. The experiments were designed to understand the specifics of wave evolution and the run distance to detonation as a function of input shock pressure. These experiments were conducted on gas guns in order to vary the input shock pressure accurately. The primary diagnostics are embedded magnetic gauges which are based on Faraday's law of induction along with photon Doppler velocimetry (PDV). The run distance to detonation vs. shock pressure, or ``Pop plot,'' was redefined as \textit{log} \textit{(X*) = 2.14 -- 1.82 log (P)}, which is substantially different than previous data. The Hugoniot was refined as $U_{s}$\textit{ = 2.32 + 2.21 U}$_{p}$. This data will be useful for the development of predictive models for the safety and performance of PBXN-9 in addition to providing an increased understanding of HMX based explosives in varying formulations.   

A Verification and Validation Effort for High Explosives at Los Alamos National Lab

We have started a project to verify and validate ASC codes used to simulate detonation waves in high explosives. Since there are no non-trivial analytic solutions, we are going to compare simulated results with experimental data that covers a wide range of explosive phenomena. The intent is to compare both different codes and different HE models. The first step is to test the products equation of state used for the HE models. For this purpose, the cylinder test and 1D plate-push experiments are being used. These experiments sample different regimes in thermodynamic phase space: the cylinder test mainly gives information about the CJ isentrope while the reflected shock in the plate-push experiment results in pressure above the CJ isentrope and is sensitive to the Gruneisen coefficient. We will be presenting the results of our findings for PBX 9501.   

Modeling and Simulation of the Impact Response of Maraging Steel Linear Cellular Alloys for Structural Energetic Material Applications

A refined Johnson-Cook material strength model is developed for predicting the dynamic strain and fracture response of Maraging 250 steel at high-strain rates. Finite element simulations of rod-on-anvil impacts are carried out at velocities exceeding 100m/s and compared with experimental impact tests performed on a 7.62mm gas gun. The transient and final dimensions of the simulated and experimentally impacted rods are compared and Johnson-Cook strength parameters are modified accordingly. The newly developed Maraging 250 steel Johnson-cook strength model is then applied to simulate the impact response of multiple, 25{\%} dense linear cellular alloys (LCA) of various geometries at velocities exceeding 100m/s. Analyses of the deformation, fragmentation, and stress transfer behavior of the simulated LCAs are performed and validated through comparison of corresponding impact experiments performed on the LCAs produced via an extrusion and reduction process. Stress transfer to the interior walls varies as a function of LCA geometry, which also influences the outward fragmentation and energy retention at the cross-section of impact.   

Effects of temperature, humidity, sample geometry, and other variables on Bruceton Type 12 impact initiation of HMX-based high explosives

The drop weight impact test, developed at Bruceton Naval Research Laboratory 60 years ago, is still the most commonly used configuration for evaluating sensitivity of explosives to non-shock ignition. The standard drop weight impact test is performed under ambient conditions for temperature and humidity - variations in which are known to significantly affect the probability of reaction. We have performed a series of impact tests in an attempt to characterize the effect of temperature, humidity, sample geometry (height, mass, L/d, and pressed density), sample confinement, and impact surface properties (strength and coefficient of friction) on the probability of reaction in a drop weight impact test. Differences in the probability of reaction have been determined across a range of drop heights for each configuration. The results clearly show significant shifts in the probability of reaction and in the slope of the reaction probability curve for each of the variables.   

Reaction of Projectiles with Targets during Hypervelocity Impact

Hollow tungsten projectiles were filled with bismuth oxide or copper and shot into aluminum blocks at 2200 m/s. The blocks were cut open, and the contents and morphology of the penetration channels were examined. In the case of copper fill, the channel was found to be filled with a black foam containing closed-cell bubbles. X-ray diffraction revealed the presence of CuAl2, indicating reaction with the aluminum target. In the case of bismuth oxide, there was little foam, but the penetration channel walls had many craters, which contained nodules of bismuth metal, again indicating reaction with the target. There were variations in crater diameter apparently corresponding to the onset and termination of the reactions. The exothermic nature of the reactions produced cracks in the target blocks.   

Environmentally Responsible Energetic Materials: Another Look at the Styphnates

Lead Styphnate (lead 2,4,6-trinitroresorcinate) has many applications as a primary explosive, most notably in priming compositions. Its largest drawback, however, is the toxicity of lead. Heavy metals often feature in primary explosives, providing favourable density, bonding, and reaction products; but, the toxic nature of heavy metals makes these explosives of limited use. Current research efforts are being made to design new energetic materials (such as those based around the 5-nitrotetrazole molecule), but familiar energetics can still be of use. The styphnate anion provides many favourable energetic qualities (such as a ring structure and nitro groups), and while the lead salt has proven its usefulness, other metallic styphnates also provide a range of energetic qualities. This paper reports on ignition thresholds, energetic output, and thermal properties of the following salts of trinitroresorcinol: Barium, Bismuth, Calcium, Copper, Lithium, and Lead. Such information provides a list of characterized energetic materials, but also insight into how metal cations can control measurable energetic effects at the molecular and crystal level.   

Thermodynamic work of adhesion between HMX and a UK PBX binder system

The polar and dispersive components of the surface energy of a UK PBX's binder system have been measured using the Wilhelmy plate technique. These data can be combined with the known values for HMX to give the so-called Thermodynamic Work of Adhesion (TWA) between the two. This quantity represents the intrinsic amount of energy required to create new surface. This can be compared to the so-called Measured Work of Adhesion (MWA), which represents the total amount of energy required for debonding, i.e. TWA plus energy dissipated during deformation, which has previously been reported for this system.   

Initiation of Reactive Waves in Metallic Powder Mixtures Using a High Explosive Booster

Shock-induced reactions were initiated in a number of metallic powder mixtures including titanium-silicon, titanium-boron, and aluminum with copper oxide or molybdenum trioxide. The reactions were initiated in long cylinders ten charge diameters in length filled with compacted powders of micron-size particles. The cylinders were fitted with a high explosive booster in a donor-acceptor configuration to initiate the reaction with a strong shock. Using embedded shock pins to detect the arrival of the shock and embedded fiber-optic cables to detect luminous reactions, the propagation of the transmitted shock and ensuing reaction was observed. The strong shock from the booster was found to transmit into the powder, then steadily decay until the end of the cylinder. A luminous front was found to follow the leading shock closely for two to four charge diameters, then gradually decouple and lag behind. The shock and reactive waves were found to share some similarities with an overdriven detonation, however self-sustained, reaction-driven waves were clearly not observed. Certain mixtures showed erratic velocity fluctuations of the luminous front rather than a gradual deceleration and transformation into a diffusion flame. The behavior of these waves was interpreted in terms of shock propagation in porous media and mechanical-chemical reaction mechanisms.   

The equation of state of predominant detonation products

The equation of state of detonation products, when incorporated into an experimentally grounded thermochemical reaction algorithm can be used to predict the performance of explosives. Here we report laser based Impulsive Stimulated Light Scattering measurements of the speed of sound from a variety of polar and nonpolar detonation product supercritical fluids and mixtures. The speed of sound data are used to improve the exponential-six potentials employed within the Cheetah thermochemical code. We will discuss the improvements made to Cheetah in terms of predictions vs. measured performance data for common polymer blended explosives. Accurately computing the chemistry that occurs from reacted binder materials is one important step forward in our efforts.   

Explosive Compaction of Intermetallic-forming Powder Mixtures for Fabricating Structural Energetic Materials

A double-tube implosion geometry is used to explosively shock consolidate Ni-Al, Ta-Al, Nb-Al, Mo-Al and W-Al powder mixtures for fabricating bulk structure energetic materials, with both mechanical strength and the ability to undergo impact-initiated exothermic reactions. The shock consolidated compacts are characterized based on the uniformity of the microstructure including degree of densification and variation in constituent volume fraction as a function of the axial and longitudinal dimensions of the compacts. Near full density compacts are achieved with minor variations in mixing of constituents, and no evidence of intermetallic reaction taking place during compaction. Differential thermal analysis was performed to determine the thermal reactivity of the compacts and compare with that of unshocked statically-pressed powder compacts. The dynamic mechanical properties of the compacts are characterized using the split-Hopkinson bar, and the reactivity under impact loading was determine using rod-on-anvil experiments.   

Hot-Spot Physics and Chemistry in Energetic Materials Initiation

Research on a project concerning fundamental investigations on hot spot physics and chemistry in energetic materials initiation is presented. Areas covered include fabrication and characterization of microstructure controlled heterogeneous material, gas gun driven initiation experiments, chemiluminescence hot-spot imaging, laser shock experiments and molecular and mesoscale level computation and theory.   

Shock compressibility of C$_{70}$ fullerite at the pressure range 6 - 9 GPa

Shock compressibility of C$_{70}$ fullerite was measured with the use of pulsed-periodical source of synchrotron radiation of the Institute of Nuclear Physics SB RAS. The starting C$_{70}$ specimens were prepared by high (1 GPa) hydrostatic pressure treatment and had a density of 1.65 g/cc, a diameter of 15 mm and a thickness of 2.5-3.5 mm. Specimens were loaded by impacts of metal plates (with a diameter of 16 mm) accelerated by high explosives. Synchrotron radiation technique was used to measure the parameters of the shock-compressed fullerite. This method of measurements is based on immediate visualization of X-T diagram of shock-wave processes by measuring a degree of attenuation of synchrotron radiation by an explored material during passage of a shock wave through this material. It was obtained that the experimental Hugoniot of C$_{70}$ fullerite in the explored pressure range (6.3-9.3 GPa) is allocated below the experimental Hugoniot of C$_{60}$ fullerite [V.V. Milyavskiy et al. Diamond and Rel. Mat. 14 (2005) 1920] on pressure - specific volume plane.   

The ionization and equation of state of fluid helium at high temperatures and densities

The ionization degree is obtained from nonideal ionization equilibrium, taking into account the correlation contributions to chemical potential and the lowering of ionization energy of fluid helium due to the interactions among all particles of He, He$^{+}$, He$^{2+}$, and e, which is determined self-consistently by the free energy function. The composition and equations of state of dense helium plasma have been calculated in the range of density of 0-8.0 g/cm$^{3}$ and temperature of 4-50 kK. This provides a basis for calculating its thermodynamic, transport, optical properties, and electrical conductivities.   

On the melting temperature measurements of metals under shock compression by pyrometry

The high-pressure melting temperatures are of interest in validating equation of state and modeling constitutive equation. The determination of melting temperatures for metals at megabars by pyrometry experiments is principally associated with the one-dimensional models for heat flow through dissimilar media: Grover-Urtiew model (J. App. Phys. 1974, 45: 146-152) and Tan-Ahrens model (High Press. Res. 1990, 2: 159-182). In the present work, we analyzed the insufficiency of Grover-Urtiew model in determining melting temperatures from observed interface temperatures. Based on the Tan-Ahrens model, we extracted the upper and lower bound on melting temperature at interface pressure, and proposed that the median of the both bounds was a good approximation to the melting temperatures at interface pressure. Pyrometry experiments were performed on tantalum, and the high-pressure melting temperatures were evaluated by application of the proposed approximation. The obtained results were compared with available theoretical calculations.   

Study of naturally occurring Precambrian native iron through high-pressure M\"{o}ssbauer Spectroscopy up to 10 GPa

We report here the M\"{o}ssbauer spectroscopic studies on native iron sample obtained from Proterozoic Mica Schist of Chaibasa SinghBhum craton of Eastern India at ambient condition as well as under high pressure up to 10 GPa using Diamond Anvil Cell and 4:1 methanol -- ethanol mixture as hydrostatic pressure medium. The results were compared with the studies on metallic iron under high pressure [1,2] . A slight variation in isomer shift up to 5.6 GPa and onset of a new peak corresponding to BCC$\to $HCP transformation at 9.1 GPa might indicate the magnitude of impact experienced by the sample before attaining the thermodynamical equilibrium. [1] Pipkorn D.N., Edge C.K., Debrunner P., De Pasquali G., Drickamer H.G. and Fraunfelder H. (1964) 135, 1604. [2] Chandra usha, Mudgal Prerana, Kumar Manoj, Rawat Rajeev, Parthasarathy, Dilawar Nita and Bandyopadhyay A.K. (2005), Hyper. Inter.163, 129.   

Reconstruction of hypervelocity impact crater progenitors utilising experimental data and hydrocode modelling at micron-scales

We have used the Ansys Inc. AUTODYN software to hydrodynamically model small particle impacts into aluminium foil under the conditions of the Stardust encounter with comet 81P/Wild 2 (i.e., normal incidence, 6.1 km s$^{-1})$. We compare results of impact models, based on carefully defined particle structures inferred from experimental data, with three-dimensional crater shapes reported from Stardust. This allows us to assess the extent to which the particle's structure (and composition) is reflected in the resulting impact feature. Our aim is to improve interpretation of comet Wild 2's dust characteristics, especially sub-grain dimensions, internal porosity and overall grain density. Here we present a simulation of the formation of a complex crater seen on Stardust foil C029W,1 and demonstrate that a reasonably simple model for the impactor results in a simulated crater morphology very consistent with the measured morphology.   

Numerical Simulation on the Damage Characteristics of Ice Targets by Projectile Hypervelocity Impact

Interpretation of cratering records on planetary surfaces including the Earth has primarily been concerned with rocky surfaces, most notably the lunar surface and more recently Mars and Venus. Recently, the survey of craters on icy surfaces in the Solar System has been augmented by data from spacecraft close encounters, such as the Galileo mission to the jovian system. To fully understand these cratering records, the physics of hypervelocity impacts needs to be understood. The numerical simulation on the damage characteristics of ice targets by projectile normally hypervelocity impact has been performed using the hydro-code AUTODYN. The 1mm spherical projectile is aluminum 2017 alloy. The targets are water ice. The simulation velocities were in the range of 1km/s-10km/s. The material models are consisted of Tillotson and Polynomial equation of state, Mohr-Coulomb and Johnson-Holmqiust strength model and Johnson-Holmqiust and principle stress failure model. The damage characteristics include peak ejection angle, peak temperature and pressure, maximum crater depth and diameter etc. The simulation results are given and compared with the experimental results of Burchell 2002. The simulation results are consistent very well with the experimental results.   

Capture of cometary dust grains in impacts at 6.1 km s$^{-1}$.

The NASA Stardust mission collected freshly ejected dust from comet 81P/Wild 2 during a fly by at 6.1 km s$^{-1}$ in 2004 (in what was in effect a real life shock/recovery experiment), and returned its samples to Earth in 2006 (Brownlee et al., \textit{Science }\textbf{314}, 1711 -- 1716, 2006). The collecting media were aerogel and aluminum foil. We will present laboratory data from an extensive series of light gas gun experiments at 6.1 km s$^{-1}$ on these media. For aluminum targets, the shock pressures ($\sim $60 -- 100 GPa) extensively disrupt the impactors, leaving residue in craters. Using SEM-EDX and Raman spectroscopy we show that it is nevertheless possible in some cases to reconstruct \textit{both} projectile elemental composition \textit{and} mineralogy. Capture in aerogel is less destructive, but still modifies the impactors (shock pressures $\sim $1 GPa). The processing during aerogel capture and the resulting biases introduced in determining particle size, shape and mineralogy will be discussed. Examples of real Stardust cometary dust samples will be shown to illustrate the results.   

Multiple shock compression of diamond single crystal over 1 TPa with shaped laser pulse

Experiments on off principal Hugoniot conditions of shock compression were performed with shaped laser pulse. Experiments were done on GEKKO-XII HIPER glass laser facility at Institute of Laser Engineering, Osaka University. Single crystal diamond foils (surface orientation:[100]) were irradiated by third harmonics of Nd: Glass laser ($\lambda $: 0.35 $\mu $m) at an intensity of above 10$^{14}$ W/cm$^{2}$. The baseline of the pulse duration was 2.5 ns. We added a weak ``foot pulse'' prior to the main drive laser pulse for low entropy compression. Diamond foils were coated with Ti (thickness: 0.5 $\mu $m) on the half side of the rear surface for shock velocity measurements. Thin Au (5 $\mu $m) coatings were also made on the laser irradiation surface in order to eliminate preheating due to ablation plasmas. Two VISARs (velocity interferometer system for any reflector) were employed for measurements of shock velocity and reflectivity. We observed strong reflectivity for multiple shock compression conditions whereas no clear reflectivity was observed by single-shock compression up to 2 TPa.   

Study of iron under high pressure conditions using isentropic compression

Equation of state (EOS) data of iron and iron alloys is important for a deeper understanding of the dynamic of the earth inner core, which requires off-Hugoniot data ($\sim $3 Mbar, 5000 K), especially in the range of the high pressure-melting curve. Isentropic compression with high-energy lasers is a promising approach to reach high-pressure off-Hugoniot states. Presently two techniques are used: an indirect approach using the ramp load of a foil exploded by a laser (reservoir technique) [1] and direct ramping of the ablation pressure by shaping the temporal profile of the laser [2]. We are presenting results on isentropic compression experiments of iron using the reservoir technique and direct laser shaping. [1] Smith, Phys. Plasmas 14, 057105 (2007) [2] Swift, PRE 71, 066401 (2005)   

On Beryllium Deformation at High-Velocity Oblique Impact

One of the methods for providing intensive dynamic stresses in metals is loading by shock waves (normal or oblique). At oblique impact of metals, intensive plastic shear strains and zones of strong heating are growing in neighborhood of contact point. Shear flows with velocity gradient depending on angle and velocity of impact of plates occur for short time. Due to intensive deformation, heating in local zones causes significant softening of substance. In these areas, shear modulus and yield strength are significantly less comparing to those at normal conditions. The mentioned effects result in distortion of profile of interface between metals after impact. Regular waves, non-symmetric distorted waves, melt layers of mixed components are formed. The process of high-velocity oblique impact of beryllium samples (beryllium and stainless steel) was experimentally studied. Beryllium has high ability for wave formation without significant plastic flow of material along sliding line. During high-velocity oblique impact of beryllium and stainless steel, their welded connection was achieved.   

Turbulent Pulsations as a Way for Dynamic Straining of Solids

Although dislocations and twins are known to be the basic deformation mechanisms in solids, they cannot provide a direct coupling between micro- and macroscales of dynamic straining. Necessary intermediate mechanisms are the mesoscale turbulent pulsations and vortical structures which result from shock-induced structure instability of solids and are the main mechanisms of momentum and energy transportation. These mechanisms may be reversible or irreversible depending on the ratio of local (mesoscale) and macroscopic strain rates. The quantitative measures of transported energy are the particle velocity dispersion and velocity defect. In the paper presented, the different situations of meso-macro energy exchange are analyzed on the basis of series of shock experiments where the particle velocity dispersion and particle velocity defect are measured in real time.   

Impact Strength of Glass and Glass Ceramic

Bar impact tests, using the techniques described elsewhere in this symposium, were used to measure compressive and tensile strengths of borosilicate glass, soda lime glass, and glass ceramic. The glass ceramic was 25{\%} crystalline spinel, furnished by Corning, Inc. There are two measures of compressive strength: the peak stress that can be transmitted in unconfined compression and the steady-state strength. For both glasses, these values were similar, being about 1.8 and 1.5 GPa, respectively. The glass ceramic was almost 50{\%} stronger. Tensile failure in the glass and glass ceramic takes places via surface flaws, and thus tensile strength is an extrinsic---as opposed to intrinsic---property.   

Features of Mechanical Behaviour of Nanostructured Ceramics under Impulse Loadings

Features of mechanical behavior of nanostructured Al$_{2}$O$_{3}$ and ZrO$_{2}$-3mol. {\%} Y$_{2}$O$_{3}$ ceramics under dynamic loadings were investigated by multilevel computer simulation approach. It was studied model ceramics representative volumes with grain sizes from 50 up to 1000 nm and porosity from 0 to 10{\%} under shock waves with amplitudes up to 10 GPa. Results of simulation have shown that Hugoniot elastic limit of nanostructured oxide ceramics depends not only on the porosity, but also a ratio of size of voids to size of grains and voids distribution on the mesolevel. At identical porosity, concentration of nanovoids near grain boundaries causes the decreasing of the shear strength of nanostructured and ultrafine-grained ceramics. It is revealed, the occurrence of bimodal distributions of the local particle velocity on mesolevel precedes the nucleation of microcracks.   

The Dynamic Strength of Alumina Ceramics under Impact Loading

There are many pores, microcracks and other defaults in polycrystalline ceramics with high singularity in stress distribution. So the hydrostatic pressure and strain rate greatly affect the inelastic deformation and dynamic strength of brittle ceramics under impact loading. In this paper, normal plate impact experiments and impact recovery experiments were performed on 92.93 wt{\%} alumina using 100-mm-diameter compressed-gas gun. Free surface velocity histories show the HEL differs with the shock intensity. SEM analysis of recovered samples shows the transit of intergranular microcracks to transgranular microcracks with increasing impact loading. Based on the experiments, the evolution of the HEL of ceramics was discussed. It was proposed that the HEL was brought into correspondence with transition from intergranular to transgranular microcracking. Then a modified form for the HEL was proposed from the Drucker-Prager yielding criterion.   

Impact experiments with aluminum-helium bubbles targets

The dynamic behavior of aluminum targets with helium bubbles was investigated in plane impact experiments. The targets were obtained by melting pure aluminum with 0.15{\%} wt. $^{10}$B powder. The solid targets were irradiated at the Soreq nuclear reactor to get homogeneous helium atoms inside the aluminium boron 10 matrix according to the reaction $^{10}$B+n?$^{7}$Li+$^{4}$He. In order to get bubbles, each target was heated to an appropriate temperature during a time that was estimated from our analytic approximation of the solution to a diffusion equation with a sink. The impact experiments were performed by accelerating aluminium impactor into different targets: (1) pure aluminum, (2) Al-$^{10}$B and (3) Al-$^{10}$B with different radiuses and concentrations of helium bubbles. From the free surface velocity measurements the spall strength was calculated and analyzed. Theoretical comparison between spall creation due to voids growth and bubbles growth was made. The impacted targets were collected after the impact experiments and examined by TEM. These targets were compared to TEM pictures before the impact. The number of helium atoms in the bubbles was calculated from the electron energy lose spectrum (EELS). Comparison of bubble radiuses and concentration before and after the impact leads to the conclusion that helium atoms that were distributed in the aluminum before the impact were added to the bubbles after the impact.   

Modeling Solid Rayleigh-Taylor Growth

Intense impulses applied to solid materials result in high strain rates, strong plastic strains and significant temperature increments. Data in such regimes would allow confidence in material strength models to be extended to strain rates of 10$^{6 }$-- 10$^{7}$ s$^{-1}$. High explosives can be used to accelerate a plate with a perturbation on the side facing the HE. This results in a Rayleigh-Taylor-like perturbation growth that depends on amplitude and wavelength of initial surface perturbation, strength of the material, time dependence of the driving pressure force, and temperature of material. Such experiments have been conducted on perturbed copper plates at LANL, using the LANSCE proton radiography beam to obtain multiple frames of data for each experiment. The results of numerical simulations of these experiments using a 2-D ALE code will be presented. Comparisons of results obtained from various strength models used to describe the material behavior will be presented.   

Dynamic Behaviors of Lead Plate Driven by Collision of Head-on Sliding Detonations

The dynamic behaviors of lead plate driven by head-on sliding detonation waves were characterized. Experimental records have shown a jet like bulging in the collision region, size of which extended rapidly after the collision of the head-on detonation waves because of the obvious speed gradients of particles inside the bulging from the tip to the bottom of the bulging. Multi-layer like structures by height of the collision bulging were recorded, which should be related to the detailed structure of loading front formed in the result of the impact of two symmetric detonation fronts. The mass densities inside the bulging structure fixed by the pulsed X-ray radiography were evaluated at the level of 1{\%}$\sim $10{\%} from the initial density of lead. The dynamic strength and shock melting should have played dominate role in the formation of the initial stage and the evolution of cavitation and fragmentation process should have finished merely in several microseconds inside the continuum of melted lead under the intensive tension of release wave, in the result of which a porous or dispersed state bulging was formed.   

Ejection of debris cloud from shock-loaded tin melted on release or on compression

A triangular shock-wave of sufficient intensity propagating in a metal sample may induce melting. When it reaches the free surface, tensile stresses are generated in the liquid state and lead to the creation of an expanding cloud of liquid debris. This phenomenon called micro-spalling consists of a dynamic fragmentation process in the melted material. Relevant data are still few but important for developing robust and physics-based models. Plate impact experiments have been performed on tin to explore this phenomenon. The breakout shock pressures range from 18 to 60 GPa, corresponding to increasing levels of melting. The so- called Asay window technique is employed to infer some kinematical properties of the debris cloud. The velocity of a LiF window impacted by tin ejecta is measured using Velocity Interferometer System for Any Reflector. Influence of shock pressure and gap distance between the target and the window has been investigated. Measurement has been compared to one- dimensional hydrocode simulation using a multiphase equation of state.   

Experimental Determination of the Dynamic Behavior of a Low Alloy Structural Steel S355K2G3

An experimental characterization of the dynamic response of a low strength structural steel (S355K2G3) has been investigated using various experimental techniques. Taylor impacts, hat shaped shear tests, dynamic tensile test and plate impact experiments have been used to quantify strain hardening, shearing behavior and fracture threshold under well controlled conditions. All these tests were performed at THIOT INGENIERIE impact shock physics test facility using the single stage gas gun TITAN. Dynamic Tensile tests have been performed in a new experimental configuration which is like a small Hopkinson bar. The strain rate achieved in the specimen (6 mm diameter, 40 mm long) varies between 2.10$^{2}$s$^{-1}$ and 10$^{3}$s$^{-1}$. Velocity profiles obtained with an interferometer system lead to the determination of the stress-strain conditions in the metallic sample submitted to tensile pulse. Plate impact experiments allow the determination of fracture threshold and magnitude under uniaxial strain condition. Samples with 55 mm diameter, 10 mm thickness have been impacted by 55 mm diameter, 5 mm thickness at velocities between 178 and 463 m/s. All these results obtained under different experimental conditions and different strain rates are compared in order to determine useful parameters for plasticity and fracture models.   

Deviatoric response of the aluminium alloy, 5083

Aluminium alloys such as 5083 are established light weight armour materials. As such, the shock response of these materials is of great importance. The shear strength of a material under shock loading provides an insight into its ballistic performance. In this investigation embedded manganin stress gauges have been employed to measure both the longitudinal and lateral components of stress during plate impact experiments over a range of impact stresses. In turn, these results were used to determine the shear strength and to investigate the time dependence of lateral stress behind the shock front to give an indication of material response.   

Application of van der Waals density functional theory to study physical properties of energetic materials

An empirical correction to account for van der Waals interactions based on the work of Neumann and Perrin [J. Phys. Chem. B \textbf{109}, 15531 (2005)] was applied to first-principles calculations of energetic molecular crystals. The calculated equilibrium unit-cell volumes of FOX-7, $\beta $-HMX, solid nitromethane, PETN-I, $\alpha $-RDX, and TATB show a significant improvement in the agreement with experimental results. Hydrostatic-compression simulations of $\beta $-HMX, PETN-I, and $\alpha $-RDX were also performed. The isothermal equations of state calculated from the results show increased agreement with experiment in the pressure intervals studied. The bulk modulus and its pressure derivative were calculated by fitting to three commonly used equations of state, and the results are within the range of reported experimental values.   

Reactive Molecular Dynamics Study of TATB Detonation Products

Under shock conditions 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) reacts to form primarily gaseous N$_{2}$, H$_{2}$O, CO$_{2}$ and CO as well as solid carbon. In a previous study of TATB thermal decomposition based on molecular dynamics (MD) simulations using the ReaxFF reactive force field, we observed a large amount of amorphous (graphite-like) carbon but no diamond structures, even at high pressures. In the current study we focus in greater detail on the reaction products mixture to assess ReaxFF predictions of both the relative stabilities of diamond-rich and graphite-rich product fluids and the equilibrium stoichiometry of CO$_{2}$, CO and solid carbon at 3250 K and as a function of pressure. In these simulations, we vary systematically the initial phase of solid carbon (pure graphite vs. pure diamond), the initial oxidation state of the remaining gaseous carbon (balanced to either pure CO$_{2}$ or pure CO), and the material density. In this poster we will summarize the results of these simulations, compare the results with both experimental observations and previous theoretical models, and discuss more generally the extent to which results obtained using short-time MD simulations can influence our understanding of the long-time behavior of real high explosive product mixtures.   

Tight binding multi-scale simulations of detonating energetic materials

We present density-functional tight-binding (DFTB) molecular dynamics simulations of shock and detonation waves propagating through a series of explosives ranging from insensitive TATB to sensitive hydrogen azide and identify key differences in behavior. The simulations are performed using the Multi-Scale Shock Method (MSST) which we have extended to maintain thermodynamic equilibrium between electrons and ions to correctly treat electronic heat capacity. This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Laser-Driven Miniflyer System for Shock Compression Studies

A laser-driven miniflyer system is being set-up for small-scale shock compression experiments on inert and reactive materials. The system consists of an Nd:YAG 3 J driving laser, beam shaping optics, an impact assembly and velocity measurement diagnostics. The beam from the driving laser travels through a BK7 substrate to irradiate a thin film composite of carbon, aluminum oxide, and aluminum. Copper foils (flyers) of 25 $\mu $m, 50 $\mu $m or 100 $\mu $m thickness and 3.2 mm or 2.4 mm diameter are mounted to the aluminum layer. The rapidly expanding carbon plasma, thermally buffered by the aluminum oxide layer and physically constrained by the aluminum coating, launches the copper flyer up to 1 km/s to impact the target. VISAR and Photonic Doppler Velocimetry (PDV) are used to investigate the flyer velocity and acceleration with respect to the driving laser energy for different thicknesses and diameters of flyer. Data from this study are used as a foundation for small-scale shock impact studies performed with the laser-driven miniflyer system on inert and reactive materials.   

Dislocation density in copper and tantalum after shock loading up to 20-100 GPa

Dislocation density was investigated by X-ray radiography method in recovered copper samples having grain sizes of 0.5 and 30$\mu $m after loading by shock waves with amplitudes of 30-60GPa, and in tantalum after loading up to 20-100~GPa. The highest value was recorded in 30~$\mu $m copper loaded by pressure of 50\textit{GPa}, namely, it was 5$\cdot$10$^{11}$\textit{cm}$^{-2}.$ In ultrafine-grain copper (UFG) with grain size of 0.5$\mu $m, which had the highest initial dislocation density (1.8$\cdot $10$^{11}$~\textit{cm}$^{-2})$, it was possible to observe relatively small increment of dislocation density up to the values of $\sim $3$\cdot $10$^{11}$~\textit{cm}$^{-2}$ at \textit{P$\sim $}40-50~\textit{GPa}. In all samples, the maximum was revealed in the range of 40-50\textit{GPa} followed by drop. The authors explain this drop by annealing of defects at adiabatic heating caused by compression. The sharpest drop of dislocation density was observed in UFG copper, since the most deformed and nonequilibrium initial structure took place in it. In tantalum, monotonous growth of dislocation density was recorded up to the value of $\sim $~3$\cdot $10$^{10}$~\textit{cm}$^{-2}$ as pressure grew in shock wave. Comparison of the obtained data with results of the earlier measurements of density of twins in copper shows that strength depends on presence of the both types of defects in a structure.   

3-Dimensional X-Ray Tomography of Plastic Bonded Energetic Simulant Materials

Given that modern energetic compositions are usually composite systems, consisting of a hard energetic crystal in a softer polymer binder, knowledge of the microstructure is vital in understanding the performance and safety issues concerned. In addition, modern hydrocodes have reached the point where microstructural information can be used as direct input data for modelling purposes. The traditional approach has involved interpolation of two-dimensional sections, but x-ray tomography (XRT), allows a fully three-dimensional representation of the microstructure to be built up. In this paper, we present data on a number of inert plastic bonded stimulant materials (PBSs) to demonstrate the viability of this technique. British Crown Copyright MOD/2009.   

Hugoniot measurement of silicon up to 1.1 TPa

Crystalline structures of silicon under pressure has been studied intensively over the last five decades. Semiconducting diamond structure of silicon transforms to metallic phases above 11.7 GPa at room temperature. Melting curve of silicon as a function of pressure is not determined above 16 GPa. Hugoniot of silicon up to 200 GPa has already been known. However, no obvious kink due to shock melting on the known Hugoniot is observed. Here we report that Hugoniot measurement of silicon from 0.4 to 1.1 TPa using laser-driven shock waves. Onset pressure of shock melting will be addressed in the presentation.   

Shock Consolidation and High Strain Rate Deformation/Die Upsetting of Magnetic Powders

Materials comprised of a high magnetization soft phase exchange coupled with a high coercivity hard phase have the potential to substantially increase the highest available energy product for permanent magnet applications. Producing bulk magnets requires overcoming challenges in both consolidation with retention of nano-scale structure, and development of texture with the low rare-earth content. High strain rate severe plastic deformation provides the potential advantages in overcoming these difficulties. In the present work, melt-spun magnetic powders of NdFeB and PrFeB/alpha-Fe are used for shock compaction and novel high rate deformation experiments. Recovered samples are characterized for density, microstructure, magnetic properties, and texture. The powder packing (green) density and shock pressure are identified as important factors in achieving high density compacts, and retention of nano-scale grain size with only limited coarsening of the grains is observed.   

High-Energy-Rate Processing of Materials Using Explosives

The overall field of application of explosives substances for material processing and synthesis include: Cladding/welding of dissimilar materials; the compaction/consolidation of nanocrystalline, super-hard, high-Tc superconducting composites, metastable highly-alloyed or amorphous powdered materials; the forming of small-series of very special shape and/or very special materials plates; the cutting of metal and/or concrete structures and the synthesis of nanocrystalline, ultra-dispersed, spherical shaped, single component or multicomponent (binary and/or ternary) metal oxide particles. The very special characteristic features of this technique makes it, sometimes, the only route available to achieve singular results and a promising widespread use can be envisaged for it in a near future. Pretending to contribute for that widespread use, this paper depicted the particular cases of the explosive welding and consolidation, presenting examples of the research activity developed recently at the Department of Mechanical Engineering of the University of Coimbra.   

Growth and characterization of one-dimensional tilted carbon nanorod arrays synthesized by the catalyst-assisted oblique angle deposition technique

One-dimensional tilted carbon nanorod arrays were synthesized on molybdenum substrates by using the catalyst-assisted oblique angle deposition technique. The structures of the one-dimensional tilted carbon nanorods evolve with substrate temperatures, but otherwise identical growth conditions. The crystallographic structures, chemical compositions, and bond structures of the tilted carbon nanorods were investigated by using X-ray diffraction, energy dispersive x-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. The cross-sectional SEM image showed the multilayered tilted carbon nanorods were also obtained. The electron field emission behaviors of the obtained one-dimensional carbon tilted nanorod arrays were greatly improved with the increase of substrate temperature. Meanwhile, the sample with multiple layers of carbon tilted nanorods exhibits better field emission behaviors than those with single layer.   

A Note on Unified Statistics Including Fermi-Dirac, Bose-Einstein, and Tsallis Statistics, and Plausible Extension to Anisotropic Effect

In the light of some recent hypotheses suggesting plausible unification of thermostatistics where Fermi-Dirac, Bose-Einstein and Tsallis statistics become its special subsets, we consider further plausible extension to include non-integer Hausdorff dimension, which becomes realization of fractal entropy concept. In the subsequent section, we also discuss plausible extension of this unified statistics to include anisotropic effect by using quaternion oscillator, which may be observed in the context of Cosmic Microwave Background Radiation. Further observation is of course recommended in order to refute or verify this proposition.   

Optical and Raman microspectroscopy of nitrogen and hydrogen mixtures at high pressures

Extended phases of molecular solids formed from simple molecules have led to polymeric materials under extreme conditions with advanced optical, mechanical and energetic properties. Although the existence of extended phases has been demonstrated in N2, CO and CO2, recovery of the materials to ambient conditions has posed considerable difficulty. Recent molecular dynamics simulations have predicted that the addition of hydrogen to nitrogen may increase the stability of the cubic-gauche nitrogen polymer and thereby offer the possibility of synthesis at lower pressures and temperatures. Here we present optical and Raman microspectroscopy measurements performed on nitrogen and hydrogen mixtures to 85 GPa. To pressures of 30 GPa, large deviations in the internal molecular stretching modes of the mixtures relative to those of the pure material reveal unusual phase behavior. After an unusual phase separation near 35 GPa, a phase assemblage of consisting of a phase rich in both nitrogen and hydrogen, a phase of relatively amorphous nitrogen and a mixture of the two is observed. Near this pressure, Raman bands attributed to the N-N single bonded stretch were observed.   

Penetration Resistance in Granular Materials With and Without Fluid Injection

Investigating the different factors affecting the resistance of earth materials to penetration is important to both commercial and military applications. Both friction and resistance to deformation are involved, but the work presented in the paper focuses on friction and the effectiveness of reducing it by injecting a fluid at the interface with the penetrator. Measurement of the coefficient of friction between a granular material under pressure and a hardened steel surface sliding at velocities between 0.1 and 100 m/s are presented. Pressures above and below the crushing strength of the grains were considered. Two types of granular materials were tested - sand and glass beads, the latter a model material that allows a highly uniform bed of particles. The measurements with dry and pre-wetted beds are compared to those of initially dry beds as a fluid is being injected through the sliding steel surface.   

The effects of defects on melting under hydrostatic and shock loading

We perform molecular dynamics simulations on Cu to systematically investigate the effects of defects on melting under hydrostatic and shock loading. The defects investigated include vacancies and voids, stacking faults, dislocations, low and high energy grain boundaries, and free surfaces. Nucleation and growth of liquid during melting are characterized in terms of order parameters and diffusion coefficients to reveal the nature of defect-induced melting under contant pressure conditions. For shock-induced melting, we investigate the effect of preexisting defects including nanocrystalline Cu and single crystal Cu with voids.   

The optimisation and use of a Digital Speckle Radiography program to investigate long rod penetration of granular media

Digital Speckle Radiography (DSR) is a technique allowing full field displacement maps in a plane within an opaque material to be determined. The displacements are determined by tracking the motions of small sub-sections of a deforming speckle pattern, produced by seeding an internal layer of lead and taking flash x-ray images. An improved DSR program is discussed which can improve the often poor contrast in DSR images such that the mean and variance of the speckle pattern is uniform. This considerably improves the correlation success relative to other similar programs. A series of experiments involving the penetration of granular media by long-rod projectiles, and the improved correlation achieved using this new program, are discussed.