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High-precision Hugoniot measurements of heterogeneous materials are a key tool in creating accurate equations of state, but such measurements are prone to larger uncertainties than their homogeneous counterparts. To efficiently reduce these uncertainties one must estimate the relative contributions of different error sources. Sample cracking is one likely source. Here we estimate its contribution through modeling, using computed tomography scans of powdered single crystal quartz samples to estimate typical packing variation within and between samples. Samples were prepared using the same materials and methods. Cracks were prominent in the samples. We describe a simple model to estimate shock transit time variation for a single shot due to cracking, and estimate that up to 3% variation can be attributed to these structures. This suggests that decreasing variation requires addressing such packing heterogeneities during target fabrication rather than experimental diagnostics.

We used several variations of the shock compression method to measure the longitudinal sound velocity of shocked tantalum over the pressure range 37–363 GPa with a typical uncertainty of 1.0 %. These data are consistent with Ta remaining in the bcc phase along the principal Hugoniot from ambient pressure to ≈300 GPa, at which pressure melting occurs. These data also do not support the putative melting phenomena reported below 100 GPa in some static compression experiments.

The self-emissions of Loctite 326, Stycast 1266, and LiF were studied to determine their suitability for spectroscopy- and pyrometry-based shock temperature measurements. Stycast 1266 is anecdotally known to be a bright emitter; we observed significant non-thermal emission persisting over 400 ns. Loctite 326 was found to have weaker emission at the same shock conditions, which depends on the degree of cure of the glue. Uncured Loctite emitted more brightly. LiF was found to assist cure of the Loctite and to provide an emission-free temporal window suitable for making temperature measurements. From this, we conclude that glues must be carefully considered, and ideally avoided, in shock experiments with T >2000 K. If glues cannot be avoided, care must be given to the wavelengths used to determine T, to avoid complications from non-thermal emission.

Multi-million atom non-equilibrium molecular dynamics (MD) simulations give significant insight into the transient processes that occur under shock compression. Picosecond X-ray diffraction enables the probing of materials on a timescale fast enough to test such effects. In order to simulate diffraction patterns, Fourier methods are required to gain a picture of reciprocal lattice space. We present here results of fast Fourier transforms of atomic coordinates of shocked crystals simulated by MD, and comment on the computing power required as a function of problem size. The relationship between reciprocal space and particular experimental geometries is discussed. © 2007 American Institute of Physics.

In situ picosecond X-ray diffraction has proved to be a useful tool in furthering our understanding of the response of shocked crystals at the lattice level. To date the vast majority of this work has used single crystals as the shocked samples, owing to their diffraction efficiency, although the study of the response of polycrystalline samples is clearly of interest for many applications. We present here the results of experiments to develop sub-nanosecond powder/polycrystalline diffraction using a cylindrical pinhole camera. By allowing the incident X-ray beam to impinge on the sample at non-normal angles, the response of grains making a variety of angles to the shock propagation direction can potentially be interrogated. © 2007 American Institute of Physics.

We respond to the Comment by Errandonea et al. [Phys. Rev. B 92, 026101 (2015)] on their reinterpretation of our published data [Nguyen et al., Phys. Rev. B 89, 174109 (2014)]. In the original paper, we argued that there is no solid-solid phase transition along the Hugoniot at 2.1 Mbars. There is, however, a softening of the shear modulus starting at 2.6 Mbars. Errandonea et al. [Phys. Rev. B 92, 026101 (2015)] reinterpreted our data and concluded that there is a structural change near 2.3 Mbars on the Hugoniot. We will explore the differences and agreements in the two interpretations of our data.

We measured the longitudinal sound velocity in Mo shock compressed up to 4.4 Mbars on the Hugoniot. Its sound speed increases linearly with pressure up to 2.6 Mbars; the slope then decreases up to the melting pressure of ∼3.8 Mbars. This suggests a decrease of shear modulus before the melt. A linear extrapolation of our data to 1 bar agrees with the ambient sound speed. The results suggest that Mo remains in the bcc phase on the Hugoniot up to the melting pressure. There is no statistically significant evidence for a previously reported bcc→hcp phase transition on the Hugoniot.

The electrical conductivity of water was measured at high pressures (70 to 180 GPa) and temperatures (4000 to 11 000 K) using a reverberating shock wave technique. The measured electrical conductivity of water varies from 39 to 200 Ω−1 cm−1 between 70 and 180 GPa. The relatively weak pressure dependence of the electrical conductivity is consistent with water being fully ionized chemically and the primary conduction mechanism is highly mobile protons. The results are in contrast to hydrogen, in which electrons are the dominant charge carriers.

X-ray-absorption fine-structure measurements of the local structure in UCu{sub 4}Pd are described which indicate a probable lattice-disorder origin for non-Fermi-liquid behavior in this material. Short Pd-Cu distances are observed, consistent with (24{plus_minus}3){percent} of the Pd atoms occupying nominally Cu sites. A {open_quotes}Kondo disorder{close_quotes} model, based on the effect on the local Kondo temperature T{sub K} of this interchange and some additional bond-length disorder, agrees quantitatively with previous experimental susceptibility data, and therefore also with specific heat and magnetic resonance experiments. {copyright} {ital 1998} {ital The American Physical Society }

We use double pass absorption spectroscopy to examine shock induced reactions in situ in cyclohexane and benzene at pressures up to 33.1 GPa. Reactions in cyclohexane begin by 27 GPa and complete by 33.1 GPa. Reactions in benzene are observed to begin by 12 GPa and are complete by 18 GPa. Absorption spectra indicate that the first reaction in cyclohexane occurs within or near the shock front, and that a metastable local equilibrium is reached in the post-shock state. A second process may be observed upon reshock at the lower pressures, suggesting a new equilibrium is reached post-reshock as well. Absorption bands are consistent with the formation of short radicals or fragments upon decomposition; however, spectral resolution is too low to confirm this mechanism.

Fine ferromagnetic particles (1–10 μm) of SmCo5 were compacted using shock pressures of 1.7–27 GPa (17–270 kbar). One‐piece disks were produced with pressures in the range 2–9 GPa. X‐ray‐diffraction measurements revealed no changes in the composition of the compacted SmCo5 for shock pressures below 15 GPa. Magnetic properties of the shocked samples were measured with a superconducting quantum interference magnetometer. In the pressure range 2–9 GPa, the shocked specimens exhibit high intrinsic coercivity Hci. Below 2 GPa, lower values of coercivity were observed, presumably caused by incomplete bonding of particles. Above 10 GPa the intrinsic coercivity drops suddenly and levels off at higher pressures. The remnant magnetization Br, exhibits a broad peak at 6 GPa and then falls off rapidly at higher pressures. The saturation magnetization Ms, has a peak near 10 GPa that correlates with the drop in coercivity. The hardness coefficient a, derived from a fit of the phenomenological ‘‘law of approach’’ formul...

The equation of state of glow discharge polymer (GDP) was measured to high precision using the two-stage light gas gun at Lawrence Livermore National Laboratory at pressures up to 70 GPa. Both absolute measurements and impedance matching techniques were used to determine the principal and secondary Hugoniots. GDP likely reacts at about 30 GPa, demonstrated by specific emission at 450 nm coupled with changes to the Hugoniot and reshock points. As a result of these reactions, the shock pressure in GDP evolves in time, leading to a possible decrease in pressure as compression increases, or negative compressibility, and causing complex pressure profiles within the plastic. Velocity wave profile variation was observed as a function of position on each shot, suggesting some internal variation of GDP may be present, which would be consistent with previous observations. The complex temporal and possibly structural evolution of GDP under shock compression suggests that calculations of compression and pressure based upon bulk or mean measurements may lead to artificially low pressures and high compressions. Evidence for this includes a large shift in calculating reshock pressures based on the reflected Hugoniot. These changes also suggest other degradation mechanisms for inertial confinement fusion implosions.

The unusual magnetic fields of the planets Uranus and Neptune represent important observables for constraining and developing deep interior models. Models suggests that the unusual non-dipolar and non-axial magnetic fields of these planets originate from a thin convective and conducting shell of material around a stably stratified fluid core. Here, we present an experimental and computational study of the physical properties of a fluid representative of the interior of Uranus and Neptune. Our electrical conductivity results confirm that the core cannot be well mixed if it is to generate non-axisymmetric magnetic fields. The molecular dynamics simulations highlight the importance of chemistry on the properties of this complex mixture, including the formation of large clusters of carbon and nitrogen and a possible mechanism for a compositional gradient, which may lead to a stably stratified core.

Lead is a material of fundamental scientific interest because of its relatively low melting point and ductility. Alloys of lead are used for practical reasons which include the desire to have a slightly harder form of the metal to be able to handle without damage. A practical question is how lead and alloys with small amounts of alloying element respond to dynamic loading. Here we report the results of fundamental shock wave compression experiments on an alloy of lead that contains approximately 3.5 wt.% antimony. Good results for several dynamic properties have been obtained, and are reported here.

In the present paper we address the phenomenon by which embedded manganin gauges between soda‐lime plates always fail to record the full stress history when the shock level is above 5.5 GPa. Our explanation for this early failure is related to the fracture wave generated in the backing glass plate which affects the gauge or its leads, in such a way as to disconnect them. 1D numerical simulations and several plate impact experiments were made in order to support our assumption.

To further understand the deformation mechanisms associated with the dynamic yield point and shock initiation of single crystals of explosives, shock wave loading experiments were performed on oriented crystals of RDX at impact stresses of approximately 1 GPa. Defect content in the crystals and the surface polishing technique were varied and the resulting materials were characterized by optical microscopy. In previous experiments on (100) oriented crystals, multiple loading inflections in the proximity of the dynamic yield point were thought to be due to cracks introduced during experiment assembly. Experiments were performed to try to reproduce these features on other crystal orientations by deliberately inducing cracks and/or choosing crystals with voids. Previously observed inflections in the proximity of the dynamic yield point can be interpreted as discrete yielding events potentially caused by one or more defects that eventually lead to a disperse elastic‐plastic transition as observed in randomly oriented powders or perhaps sufficiently defective single crystals. The intent of this effort is to understand the effects of orientation dependent crystal plasticity and large defects, such as surface finish, cracks, and voids, on dynamic deformation separately so that the efficiencies and importance of deformation mechanisms in the initiation process can begin to be delineated for energetic molecular single crystals.

The interaction of high‐power pulsed‐laser beam with metal targets in air from 1.06 μm, 5 ns, 3 J/pulse max, Nd:YAG pulsed laser is investigated together with hydrodynamic theories of laser‐supported detonation (LSD) wave and multi‐material reactive Euler equations. The high speed blast wave generated by the laser ablation of metal reaches maximum velocity of several thousand meters per second. The apparently similar flow conditions to those of reactive shock wave allow one to apply the equations of motion for energetic materials and to understand the explosive behavior of metal vaporization upon laser ablation. The characteristic time at which planar to spherical wave transition occurs is confirmed at low (20 mJ/pulse) to higher (200 mJ/pulse) beam intensities. The flow structure behind the leading shock wave during the early planar shock state is confirmed by the high‐resolution multi‐material hydrocode originally developed for shock compression of condensed matter.

Plate impact experiments have been performed to determine the dynamic behavior of conventional and high strength mortars. Longitudinal stresses have been directly measured by means of embedded polyvinylidene fiuoride (PVDF) gauges up to 1 GPa. A 200 mm‐cal. powder gun enable us to measure longitudinal stresses at several point from the impact surface, simultaneously. The longitudinal stress profile of conventional mortar, which static compression strength is 35 MPa, shows a two‐wave structure associated with the compaction of pores and the onset of damage around 200 MPa. Although the longitudinal stress profile of high strength mortar, which static compression strength is 120 MPa, does not show a two‐wave structure, longitudinal stress to phase velocity relationship indicates the onset of pore collapse around 500 MPa.

For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.

Composite materials have been of significant interest due to widespread application of anisotropic materials in aerospace and civil engineering problems. For example, composite materials are one of the important types of materials in the construction of modern aircraft due to their mechanical properties. The strain rate dependent mechanical behaviour of composite materials is important for applications involving impact and dynamic loading. Therefore, we are interested in understanding the composite material mechanical properties and behaviour for loading rates between quasistatic and 1×108 s−1. This paper investigates modelling of shock wave propagation in orthotropic materials in general and a specific type of CFC composite material. The determination of the equation of state and its coupling with the rest of the constitutive model for these materials is presented and discussed along with validation from three dimensional impact tests.

A series of plate impact experiments was performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to determine the dynamic tensile, or spall strength of a hypabyssal kimberlite and a siltstone. Two distinct impact configurations were employed, over a range of impact velocities up to 185 m/s. VISAR and manganin gauges were used to monitor the spalling of the material. Thicker targets were found to be less suited for spall measurements as the signal appeared to be degraded by the increased amount of material traversed before reaching the measuring plane. A comparison with a theoretical scheme relating the spall strength to the Hugoniot elastic limit is performed, and while the agreement is not perfect it is of the same order of magnitude.

We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear‐deformation and overdriven rehybridization shock wave regimes.

Recently, pressure tuning of the thermoelectric figure of merit has been reported for several materials Bi2Te3 based thermoelectric materials [2],[10],[12]. In order to investigate the bulk properties of Bi2Te3, Sb2Te3, and their solid solution in detail, we have performed structural studies up to 20 GPa. Our diffraction results show that all three compounds transform from the ambient pressure structure to a high pressure phase between 7 and 10 GPa. In addition, these diffraction results have been converted to Vinet and Holzapfel equations of state to test the claim of electronic topological transitions in these structures [3].

In conventional Split Hopkinson Pressure Bar (SHPB) experiments the striker bar is a single rod and the sample is loaded at one strain rate. In this study, we present results from a system that uses a striker bar formed from two rods of different materials. A two level loading pulse is generated, subjecting the sample to two different strain‐rates: either a low strain‐rate followed by a higher strain‐rate (“step up”) or vice versa (“step‐down”). The sample does not experience repeat loading or significant unloading between the two regimes. The “step‐down” loading sequence, of equal durations at 2300 s−1 and 400 s−1, has been applied to iron and copper samples. History independence is observed for iron, as expected from its BCC structure.

In order to study the SDT process in a granular non‐ideal explosive (NIE) an experimental technique has been developed that allows the granular explosive to be shock initiated at a well controlled “tap density”. The granular NIE was contained in a PMMA cone and a planar shock was delivered to the explosive through a buffer plate of varying material. A combination of piezoelectric probes, ionisation pins, PVDF stress gauges and a high speed framing camera were used to measure the input shock pressure and shock and detonation wave positions in the explosive. Four trials were performed to characterise the run to detonation distance versus input pressure relationship (Pop plot) of the granular NH4 NO3 based explosive. Input pressures ranged from close to the 4 GPa predicted CJ pressure of the granular explosive down to 1.4 GPa, giving run distances up to ∼12 mm for the lowest pressure. The data indicates a steady acceleration of the input shock to detonation velocity, implying significant reaction growth at t...

A gas gun has been used to investigate the impact initiation of rods of a mixture of 72 wt% PTFE (polytetrafluoroethylene) and 28 wt% aluminum powders. The rods were sabot‐launched in vacuum into steel anvils at impact velocities ranging from 468 to 969 m/s. A framing camera was used to observe the time sequence of events following impact. At the lowest impact stress of 25 kbar no light was observed. Above the initiation threshold, the initiation time dropped from 74 μs just above threshold to 14 μs at 64 kbar. These results are compared with earlier rod impact experiments for a similar material in which the only major difference is a smaller aluminum particle size.

Shock recovery experiment on sillimanite was conducted using a two‐stage light‐gas gun to clarify origin of the peculiar nano‐frgmentation observed in mullite. Recovered sillimanite was examined by X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). Shock recovered sillimanite showed amorphization by XRD measurements as seen in mullite. However, microtexture observed by TEM showed characteristic planar deformation structures and following amorphization, which are considerably different from those observed in mullite. Importance of oxygen vacancies in crystal structure of mullite for the nano‐fragmentation was inferred from comparison to the present result on sillimanite.

Improved relationships were developed in this study for the thermodynamic properties of solid and liquid silicon and silicon dioxide for use with JAGUAR thermo‐chemical equation of state routines. Analyses of experimental melting temperature curves for silicon and silicon dioxide indicated complex phase behavior and that improved coefficients were required for solid and liquid thermodynamic properties. Advanced optimization routines were utilized in conjunction with the experimental melting point data to establish volumetric coefficients for these substances. The new property libraries resulted in agreement with available experimental values, including Hugoniot data at elevated pressures.

A reactive force field (ReaxFF) is used with molecular dynamics to probe the chemistry induced by intense heating (‘accelerated cook‐off’) of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Large‐system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. Using small, 32‐molecule simulations, we calculate the reaction rate as a function of temperature and compare the Arrhenius‐predicted activation energy with experiment. Decomposition product evolution (mainly N2, H2O, CO2 and graphitic carbon clusters) is followed using a 576‐molecule larger simulation, which also illustrates the effect of system size on both carbon clustering and reaction rate.

We explore various relations for the detonation energy and velocity as they relate to the inverse radius of the cylinder. The effective detonation rate‐inverse slope relation seen in reactive flow models can be used to derive the familiar Eyring equation. Generalized inverse radii can be shown to fit large quantities of cylinder results. A rough relation between detonation energy and detonation velocity is found from collected JWL values. Cylinder test data for ammonium nitrate mixes down to 6.35 mm radii are presented, and a size energy effect is shown to exist in the Cylinder test data. The relation that detonation energy is roughly proportional to the square of the detonation velocity is shown by data and calculation.

MnAs1−xSbx material is one of the ferromagnets which can contribute to a magnetic refrigeration system that is useable at room temperature. The consolidation experiments of MnAs1−xSbx (x = 0.068, 0.073) powder are carried out by a shock compaction method using underwater shock wave with shock pressure of 16 GPa. The magnetic entropy change, ΔSmag, of annealed sample (x = 0.073) after press caused by a magnetic field of 1 T is 27.8 J/Kkg at the maximum value, and the sample annealed after press undergoes a first‐order ferromagnetic to paramagnetic transition at 304.05 K.

The evolution and decay of aluminum and aluminum monoxide emission signatures following a laser ablation event were used to compare the relative reaction rates of three aluminum based materials. Time‐resolved emission results of oxide‐free, C13F27CO2H‐passivated materials were compared with uncoated, oxide passivated aluminum nanoparticles and those coated with the same acid used in for passivation C13F27CO2H. Excited state Al and AlO emission is reduced in time for the oxide free material when compared to coated, 50 nm, oxide passivated particles mixed on an equal active Al: C13F27CO2H ratio. This is interpreted as an increase in the reaction rate afforded by the elimination of the oxide coating and proximity of oxidizing species in the SAM‐based nanocomposite.

A strong electric field can be generated when the shock‐induced depoling current from a normally poled PZT 95/5 sample is passed through a large resistive load. The portion of total depoling current that is retained on the sample electrodes to account for capacitance is governed by the dynamic dielectric properties of both unshocked and shocked material. Early studies used measured load currents from single samples to assess models for dielectric response. In more recent studies, we used shock‐driven circuits in which multiple PZT 95/5 elements were displaced both parallel and perpendicular to the shock motion. This allowed both load and charging currents to be measured for individual elements that are subjected to shock compression and release at different times. In the present study, these techniques have been utilized to examine dielectric properties in PZT 95/5 samples at initial temperatures from −56 to 74 °C. Significant changes in permittivity with temperature are observed in both unshocked and sho...

We extend the Walsh and Christian (1955) analysis in a manner which allows a reasonable estimation of the expected deviation from the mirror image approximation due to solid liquid phase transition. This estimate is limited to a phase transition that occurs at shock unloading. This analysis provides a good estimate to metals with low melting pressure such as Tin and Lead.

The precursor decay anomaly in single‐crystal lithium fluoride has been evaluated for estimated dislocation densities at the leading edges of the followers on the Sano's decay curve, which is much lower than the Asay's decay curve. The equation for the density derived reveals that the density depends only on the slopes of the decay curves for particle velocity and stress, irrespective of the form of the follower and the slope at the leading edge. The density distribution estimated has a maximum at the beginning of the decay process, and the maximum density is about 1/22 times as large as that estimated on the Asay's decay curve. Thus, the anomaly is reduced significantly. By estimating the densities at the rears of the near‐steady precursors on the Sano's decay curve, it is also shown that a large number of dislocations are multiplied in the vicinity of the leading edges of the followers. The multiplication of dislocations results from initial dislocations near the impact surface and at the subgrain boundaries as well as those in the bulk.

Direct free energy calculations for liquid metals from ab initio potential surfaces are very computationally intensive. Here we investigate Monte Carlo methods that involve sampling on the surface defined by a reference system. This allows for large gains in efficiency because the random walk is carried out on the (much faster) reference potential, and the ab initio potential is only evaluated on a small subset of uncorrelated configurations. We investigate the feasibility of these methods, by first applying them to liquid copper using an embedded atom potential, with a 1/r12 pair potential as the reference system. We find that the free energy perturbation series converges well and is numerically tractable. Second and third order perturbation theory give results that are weakly dependent on the reference system parameters. A preliminary application to an ab initio potential surface for liquid magnesium shows that we can obtain free energies accurate at the meV/atom level with only ∼100 evaluations of the ...

Results of shock‐wave experiments with free and pre‐stressed samples of K8 crown glass, K14 crown glass and fused quartz are presented. Controlled confinement pressure on the specimen in the range of 200 MPa to 300 MPa was provided by installing a shrink‐fit metal sleeve on the lateral surface of the sample disk. The shock compression pulses of approximately triangular profile were created using thin aluminum impactors and PMMA base plates. The peak shock stresses in the range of 5.5 GPa to 8.5 GPa were sufficient to initiate the failure waves, whereas following unloading stopped the cracking. Results of measurements of the free surface velocity histories show that pre‐stressing results in earlier braking of the failure wave. Thus, the compressive transversal stress increases the failure threshold that is in agreement with existing criteria of compressive fracture.

New advanced investigations of aluminum properties at extreme conditions include experiments and theoretical calculations. Experimental researches on shock and isentropic compression of aluminum at high pressures up to 500 GPa have demonstrated smooth monotonic dependencies, resulting in more “stiff” compression curve and shock adiabat for aluminum that it was figured in previously. Adiabatic‐expansion measurements of shocked aluminum have brought new accurate data for the region of hot expanded liquid. Theoretical “ab initio” QMD calculations in the region in the vicinity of the critical point established the reference domain of thermodynamic data. This complex of data serves as a fundament for a new multi‐phase EOS for aluminum.

Previous research at room temperature and quasi‐static loading rates has shown that above the glass transition conditions of their binders, PBX Brazilian disc specimens fail by the localisation of strain in binder rich areas, followed by debonding of the binder from the largest energetic crystals. Crystal fracture is rare and the process is predominantly intergranular. In contrast we show, for a UK PBX, that when the temperature is reduced to below the glass transition condition of the binder system, failure becomes predominantly transgranular. Such data are required for the development and validation of PBX micromechanical models. This paper outlines the current state of research and details the important observations to date.

The incidental effects of gaps in detonating explosives have been observed for over two centuries, yet the root cause of peripheral damage due to these features has been a partial mystery. To evaluate such damage for PBX 9501, a test series has been performed that examines single and multiply‐directed detonations crossing gaps of varying widths and angles relative to the detonation wave fronts. Damage is evaluated with steel witness plates and quantified through trench profiling and photographic analysis. In addition, streak camera traces are used to track detonations along explosive material surfaces and across gaps. Such traces allow the quantification of timing delays due to the gap reinitiation process. For some reinitiation tests, a second detonation wave is directed to interfere at varying times with the post‐gap initiation process of the first wave, thus allowing complex wave‐wave interactions to be investigated in detail. With these cumulative observations, further insight into the mechanism of ex...

A modification of REBO potential has been proposed for the molecular dynamics simulation of a TATB‐like condensed explosive whose molecule initially consists of four different atoms. TATB‐like means bulk properties of initial state and parameters at CJ point similar to those of real TATB. Parameters of the potential are subdivided into two groups that are responsible for CJ parameters and reaction zone width. The possibility of formation of intermediate detonation products allows variation of reaction zone characteristics without changing CJ parameters. Provided are a number of test MD calculations on the thermodynamic properties of both the original explosive and detonation products, parameters at CJ point, reactions rates and reaction zone width as dependent upon the potential parameters as well as the evaluation of critical diameter. Mechanism of the detonation initiation proper to heterogeneous explosives has been investigated.

Experiments performed on the Z machine at Sandia Labs used magnetically generated planar ramp waves to quasi‐isentropically compress pre‐heated solid tin across the equilibrium β‐γ phase boundary. Velocity history measurements at a tin/window interface exhibited features that could be consistently related, through simulations, to the β‐γ structural transformation occurring in the bulk tin. The simulations used a homogeneous phase‐mixture model with a γ‐phase energy offset that was adjusted to match the measured velocity feature. This determined the phase‐boundary pressure from experiment and the phase‐boundary temperature from the β‐phase equation of state. Due to wave interactions, measurements using sapphire windows were more difficult to interpret than those using LiF windows and thus led to results with larger uncertainty. Results did not display the hysteretic overshoot seen in static and shock experiments on other phase‐transforming materials.

We are studying high‐pressure laser produced shock waves in silicon (100). To examine the material dynamics, we are performing pump‐probe style experiments utilizing 600 ps and 40 fs laser pulses from a Ti:sapphire laser. Two‐dimensional interferometry reveals information about the shock breakout, while third harmonic light generated at the rear surface is used to infer the crystalline state of the material as a function of time. Sustained third harmonic generation (THG) during a ∼100 kbar shock breakout indicate that the rear surface remains crystalline for at least 3 ns. However, a decrease in THG during a ∼300 kbar shock breakout suggests a different behavior, which could include a change in crystalline structure.

Work carried out on the development of a single‐stage implosion‐driven hypervelocity launcher is presented. Explosives surrounding a thin‐walled tube filled with helium works similar to the pump tube of a conventional light gas gun. Implosion of the tube drives a strong shock that reflects back and forth between the projectile and the implosion pinch, generating very high temperatures and pressures. Experiments to evaluate the implosion dynamics and performance of the pump tube were carried out, with attention given to the helium fill pressure, diameter of the pump tube, thickness of the explosive layer, and the presence of a tamper. Simple analytic models were used to approximate the performance of the launcher; their advantages and limitations are discussed. Experiments with an implosion‐driven launcher demonstrated muzzle velocities of 4 km/s with 5‐mm‐diameter aluminum projectiles, agreement with analytical models of performance is discussed. Projectile integrity was verified by high‐speed photography and other diagnostics.

The molecular dynamics simulation is applied to study the influence of nanoprecipitates on the microscopic mechanisms of the spall fracture initiation and corresponding plastic deformation. The systems under study are single crystals doped with nanoclusters of another metal. The calculation results for the Cu‐Al system is presented. Three models are considered: the triaxial uniform expansion, the shock wave propagation and the reflection of release wave in the impactor‐target model and the model of an edge dislocation interaction with a nanocluster. Different characteristic consequences of the nanoprecipitate presence on material response are analyzed.

We present analytic solutions to two test problems that can be used to check the hydrodynamic implementation in computer codes designed to calculate the propagation of shocks in spherically convergent geometry. Our analysis is restricted to fluid materials with constant bulk modulus. In the first problem we present the exact initial acceleration and pressure gradient at the outer surface of a sphere subjected to an exponentially decaying pressure of the form P(t) = P{sub 0}e{sup -at}. We show that finely-zoned hydro-code simulations are in good agreement with our analytic solution. In the second problem we discuss the implosions of incompressible spherical fluid shells and we present the radial pressure profile across the shell thickness. We also discuss a semi-analytic solution to the time-evolution of a nearly spherical shell with arbitrary but small initial 3-dimensional (3-D) perturbations on its inner and outer surfaces.

In order to get simultaneous high speed streak and framing images of exploding metal wires in a water environment, we have employed a CW green laser as a back light source of shadow photography and a laser beam splitter as a device separating images of exploding wires. By filtering the light emitted from the exploding wires with the help of a laser line band pass filter, the images could become much finer than those pictures taken without the filter. The stability of the cylindrical plasma column could be monitored by the framing pictures, and the evolution of the shock wave and plasma expansion could be measured precisely by the streak pictures. The measured data were applied to understand dense plasma characteristics, e.g. electrical conductivity or thermodynamic properties of the dense metal plasma.

Microstructural effects including material anisotropy, impurities, grain size, and texture alter a material's response to dynamic loading through wave front dispersion and inelastic processes. The spatial variations created by these effects are a challenge for inertial confinement fusion (ICF) as they may seed instabilities, which could reduce thermonuclear yield, if not controlled through material processing. To this end, laser‐driven confined shock experiments have been conducted on Be to characterize its dynamic strength properties and usefulness as an ICF ablator. Disks of Be 3 mm in diameter and 100 to 250 microns thick in the form of single crystal, rolled foil, and equal channel angular extruded were dynamically loaded to 100's kbar while the material behavior was measured with in‐situ diagnostics. Clear two‐wave structures were observed in free surface velocity records, providing a comparison of flow stress and other dynamic properties between Be types.

Internal blast refers to explosion effects in confined spaces, which are dominated by the heat output of the explosive. Theoretical temperatures and pressures may not be reached due to heat losses and incomplete gas mixing. Gas mixing can have the largest effect, potentially reducing peak quasi‐static pressure by a factor of two due to lack of thermal equilibrium between products and atmosphere in the space, separate from the effect of incomplete combustion of excess fuel when that atmosphere is air. Chamber and test geometry affect gas mixing, which has been inferred through temperature and pressure measurements and compared to calculations. Late‐time combustion is observed for TNT compared to HMX.

We have been developing the physically based QinetiQ Porter-Gould (P-G) model for the mechanical response of PBXs over a number of years and applying it to the solution of real scenarios involving impact and blast. The main difficulty with these models is predicting the intermediate strain rate regime where the relaxation time for the polymer is of the same order as the duration of the loading (e.g. as in a Hopkinson bar test). The other main issue is the ability of the model to predict the stress/strain data as a function of temperature up to and through the glass transition temperature. The paper presents predictions from the QinetiQ P-G model compared to quasi-static compression and Hopkinson bar compression test data and discusses the results in terms of requirements for future developments of the model. © 2007 American Institute of Physics.