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The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objective of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolved measurements of stress pulses obtained with piezoelectric polymer (poly-vinyl-di-flouride) pressure gauges provided extraordinary sensitivity for determination of rate-dependent shock processes. Both techniques showed clear evidence for shock-induced chemical reactions in medium-morphology powders, while fine and coarse powders showed no evidence for reaction. It was observed that the medium-morphology mixtures experience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastically deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fracture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the overriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock-induced solid-state chemistry. The present work in conjunction with our prior studies, demonstrates that the initiation of chemical reactions in shock compression of powders is controlled by solid-state mechanochemical processes, and cannot be qualitatively or quantitatively described by thermochemical models.

The response of highly porous powder compacts and powder mixtures to high pressure shock compression loading is of considerable interest for synthesis and processing of metals, ceramics and superhard materials. This technical note reports the first successful use of the Bauer piezoelectric polymer stress-rate gauge for measurements of shock wave velocity and stress-wave profiles in porous powder compacts. A powder mixture of 5Ti+3Si shows strong chemical reaction at a pressure of 2.5 GPa, while a powder mixture of 3Ni+Al shows no evidence for reaction at 4.7 GPa. A measurement of compaction of a powder compact of rutile at 5.5 GPa shows that it is not compacted to the solid density state. Although pressure increases due to chemical reaction products in condensed phases are modest and difficult to detect, shock wave velocities provide a sensitive measure of the existence of chemical reaction. The increase in shock speed can be described in terms of constant pressure processes which are descriptive of “ballotechnic” reactions, i.e. shock-induced reactions in heterogeneous material mixtures.

The “DaMaGe-Initiated-Reaction” (DMGIR) computational model has been developed to predict the response of ideal high explosives to impulsive loading from nonshock mechanical insults. The distinguishing feature of this model is the introduction of a damage variable, which relates the evolution of damage to the initiation of a reaction in the explosive, and its growth to detonation. This model development effort treats the nonshock initiation behavior of explosives by families; rigid plastic bonded, cast, and moldable plastic explosives. Specifically designed experiments were used to study the initiation process of each explosive family with embedded shock sensors and optical diagnostics. The experimental portion of this model development began with a study of PBXN-5 to develop DMGIR model coefficients for the rigid plastic bonded family, followed by studies of the cast, and bulk-moldable explosive families, including the thermal effects on initiation for the cast explosive family. The experimental results show an initiation mechanism that is related to impulsive energy input and material damage, with well defined initiation thresholds for each explosive family. These initiation details will be used to extend the predictive capability of the DMGIR model from the rigid family into the cast and bulk-moldable families.

The plastic bonded explosive PBXN‐5 was studied under impulsive loading experiments to relate impact‐induced mechanical damage to the onset of, and the extent of reaction produced. A small diameter projectile generated shock and release conditions at the impact interface, on the microsecond time scale during the initial portion of the impulsive loading. These shock and release wave interactions generate significant damage, resulting in a porous, powder compaction‐type initiation behavior. Experimental measurements show an energy threshold for initiation of reaction which relates to impact‐induced kinetic energy. These results are implemented in the model development and validation phases of the damage‐induced reaction (DMGIR) model, which is used to simulate impact scenarios of explosives, explosive components, and explosive systems.

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...

Alumina‐filled epoxies are composites having constituents with highly dissimilar mechanical properties, resulting in complex behavior during shock compression and release. Two distinguishing characteristics are amplitude‐dependent wave structures and high release wave velocities. Recent studies examined the effects of various compositional changes on these shock properties. As expected, the strongest effects were observed when the total alumina volume fraction was reduced in steps from a nominal 43% to 0%. In the present study, compositions prepared over the same range of alumina loadings were examined at initial temperatures that were nominally −55 °C or 70 °C. Experimental configurations were identical to previous room‐temperature experiments. Laser interferometry and wave timing were used to obtain transmitted wave profiles, Hugoniot states, and release wave velocities. Initial densities were determined from thermal expansion coefficients measured for each composition. Although initial density changes ...

Shock‐induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in pulsed power devices. High electric fields are generated within a normally poled ceramic element when the depoling current is passed through a large load resistor. Under these conditions, a portion of the depoling current is retained on the element electrodes to account for capacitance. This effect is governed by the dielectric properties of both unshocked and shocked PZT 95/5 as the field develops during shock transit. Previous studies proposed either constant or relatively simple relaxing behavior for dielectric properties on either side of the shock front. However, interpretation of the corresponding experiments was complicated by possible field effects on depoling kinetics. Recent studies have used different experimental configurations to better isolate the dielectric behavior. Multiple PZT 95/5 elements are displaced both parallel and perpendicular to the direction of shock motion to allow for sequential charging of unshocked and shocked samples. These experiments show a dielectric behavior for which a simple relaxation model is inadequate.

Alumina‐filled epoxies are used to encapsulate ferroelectric ceramics in shock‐driven, pulsed power devices. Device performance is strongly influenced by the shock compression and release properties of the encapsulant, which must be adequately understood in order to develop a capability for numerically simulating the operation of these power sources. In previous studies, Hugoniot states and release velocities were measured in reverse‐impact experiments using laser interferometry (VISAR) at stresses up to 5 GPa. In addition, wave profiles were obtained in transmitted‐wave experiments at fixed impact conditions as a function of initial temperature. These experiments showed an extended wave structure having a rise time that increased with decreasing temperature. In recent studies, Hugoniot states and release velocities at stresses up to 10 GPa have been obtained in reverse‐impact experiments. Transmitted‐wave experiments have examined the effects of wave amplitude on the wave structure and also the evolution...

The shock and release behaviors of both alumina‐filled and unfilled epoxies are examined in planar impact experiments over an initial temperature range of −50 to +70 C. The materials under investigation can be used as encapsulants in applications such as shock‐activated pulsed power supplies. Characterizing the effect of initial temperature on their dynamic behavior under shock loading and subsequent release is part of a larger effort to develop predictive numerical models for the performance of these power sources. In the current study, transmitted waveforms were recorded using laser interferometry (VISAR) to examine initial temperature effects on the inelastic behavior in these materials. Current results, combined with the results of previous studies of these epoxies at ambient temperature, show a clear trend of increasing inelastic features in compressive waveforms with decreasing temperature. Temperature effects are more pronounced in the filled samples. In both cases, release wave speeds decrease wit...

The shock and release behavior of both alumina-filled and unfilled epoxies are examined in planar impact experiments. The various materials investigated can be used as encapsulants in applications such as shock-activated pulsed power supplies. Characterizing their dynamic behavior under shock loading and subsequent release is part of a larger effort to develop predictive numerical models for the performance of these power sources. In the current study, reverse-impact techniques were used to establish Hugoniot states and subsequent release profiles for each material at several impact conditions. Laser interferometry (VISAR) was used in these experiments to determine the particle velocity history at the impact interface formed by a projectile-mounted epoxy sample and a stationary fused silica target. Conventional impact configurations and VISAR were used to observe transmitted waveforms under corresponding shock loading conditions. Transmitted waveforms at multiple Lagrangian locations were also recorded in selected experiments using embedded PVDF piezoelectric films. The results show expected differences in the dynamic properties of filled and unfilled epoxies, and similarities in the properties between different filled or unfilled epoxies. Comparisons between VISAR experiments and PVDF gauge experiments may indicate a rate-dependent response in this gauge material.

The shock compression response of Al and Fe2O3 powders has been studied with time-resolved pressure measurements using the PVDF stress-rate gauges, extending the early work of Holman et al. Experiments were performed on Al and Fe2O3 powders, mixed in different volumetric distributions, corresponding to 50:50, 40:60, and 25:75 volumetric ratios. The shock-compression response demonstrates a complex effect of volumetric distribution on the densification behavior. The propagated stress wave-forms reveal a change in slope in the rise to peak pressure, indicating the influence of the differences in reactant properties. Differences in the crush strength in powder mixtures of different volumetric distributions are also observed, with the equivolumetric powder mixture showing crush-up to full density at lower pressures.

A simultaneous PVDF/VISAR measurement technique was used for isentropic-loading experiments with a polymethyl methacrylate (PMMA) specimen. The experiments used a graded density impactor accelerated onto a tantalum driver backed with PMMA and then lithium fluoride windows for each experiment. Simultaneous measurements made at each window interface provided precise transit time and particle velocity measurements which can be used to determine the stress-vs-strain loading path using Lagrangian analysis techniques. The experimental technique provides access to 40 GPa stress levels in PMMA under isentropic-loading conditions.

Porous, sometimes called distended, materials consist of solid materials and voids such that the foam or compact is less than the theoretical maximum density (TMD). Porous high explosives (HEs) are compacts made by pressing (consolidating) crystals or multicrystal-line agglomerates together. The compact density may be as little as 50% or as much as 99% of the density of the solid material. Explosives used for munitions as well as blasting are often made in this way. Shock-loading studies of porous materials, particularly metals, have been done over the years and there are a number of papers in the literature (both English and Russian) dealing with this. In this chapter, however, we will concentrate on HE materials and discuss only studies with direct application.

Discrete Element program described in the companion paper in this proceedings by Tang, Horie, and Psakhie is used to examine recent particle velocity and PVDF stress measurements in HMX explosive powder. There were two objectives. The first is to demonstrate the applicability of the meso-dynamic program to calculate real experimental measurements. The second is to test the capability of the new program to calculate the pre-ignition profiles that reveal macroscopic manifestation of large deformation of HMX grains under plane shock loading. The new program passed both tests.

Simulations with finite element models of well controlled impact experiments with x-cut quartz gauges have been performed with the transient electromechanics code SUBWAY. Comparisons of measured gauge output current with calculated output current were made for four fully-electroded gauge configurations, involving two different can spacings and potting materials. The observed good agreement between measured and calculated currents provides a basis for confidence in the basic capabilities of the code.

Piezoelectric polymer stress gauges in copper fixtures were used with the Sandia 2.5-inch bore gas gun to obtain time-resolved pressure measurements for two polytetrafluoroethylene powders having significantly different particle morphologies. The powders had approximate average particle sizes of 534 microns and 28 microns, respectively, and scanning electron microscopy revealed differences in the appearances of representative particle surfaces. The range of input stresses was from 0.13 GPa to 2.81 GPa, and the initial densities were 57% of the solid density. The “crush strength” (pressure required to compress the porous compact to solid density) was close to 1.0 GPa for the coarse material as compared to 0.6 GPa for the finer material. At an input stress of about 0.6 GPa, the risetime of the propagated stress waves in the coarse material was approximately 240 nsec compared to 50 nsec for the finer material. These measurements show the strongly rate-dependent deformation of the powders and that particle mo...

Time-resolved pressure and particle velocity measurements provide the foundation for modern descriptions of rate-dependent materials deformation processes in condensed matter subjected to high pressure shock loading. Much of that foundation has been based on the VISAR, nanosecond time-resolved particle velocity technique. It has been known for some time that full description of the rate-dependent processes requires both particle velocity and stress data, but stress sensors with nanosecond resolution have not been previously available for general usage. More recently, the piezoelectric polymer PVDF has been used for stress and stress-rate measurements in rate dependent materials response experiments. Neither method, by itself, gives unique material response models, particularly in the case of highly porous materials. The first simultaneous, time-resolved measurements with VISAR and PVDF have now been carried out. Measurements were taken on highly porous HMX that clearly show the multi-dimensional nature of HMX viscous compression behavior, followed by the onset of chemical reaction. This technique promises to provide a qualitative improvement in our ability to develop rate-dependent material descriptions of all solids including highly porous solids. Precise use of this technique will require new window materials.

This paper describes recent work to make high quality quartz gauge (temporal and spatial) shock wave measurements in a pulsed ion beam environment. Intense ion beam radiation, nominally 1 MeV protons, was deposited into material samples instrumented with shunted quartz gauges adjacent to the ion deposition zone. Fluence levels were chosen to excite three fundamentally different material response modes (1) strong vapor, (2) combined vapor and melt phase and (3) thermoelastic material response. A unique quartz gauge design was utilized that employed printed circuit board (PCB) technology to facilitate electrical shielding, ruggedness, and fabrication while meeting the essential one dimensional requirements of the characterized Sandia shunted quartz gauge. Shock loading and unloading experiments were conducted to evaluate the piezoelectric response of the coupled quartz gauge/PCB transducer. High fidelity shock wave profiles were recorded at the three ion fluence levels providing dynamic material response da...

Magnetic particle velocity gauges and PVDF stress rate gauges have been used to measure the shock response of low density HMX explosive (1.24) g/cm3. In experiments done at LANL, magnetic particle velocity gauges were located on both sides of the explosive. In nearly identical experiments done at SNL, PVDF stress rate gauges were located at the same positions. Using these techniques both particle velocity and stess histories were obtained for a particular experimental condition. Loading and reaction paths were established in the stress‐particle velocity plane for each input condition. This information was used to determine that compacted HMX has an impedance close to that of Kel‐F and also that a global reaction rate of ≊0.13 μs−1 was observed in HMX shocked to about 0.8 GPa. At low input stresses the transmitted wave profiles had long rise times (up to 1 μs) due to the compaction processes.

Broadband impedance measurements of a PVDF shock‐pressure gauge are used to build an equivalent circuit for the gauge. The essential components are a gauge capacitance and a low‐loss transmission line. Component features are consistent with the physical characteristics. With knowledge of this circuit, troublesome oscillations can be anticipated and prevented.

Time-resolved pressure measurements have been conducted on 2Al+Fe[sub 2]O[sub 3] powder mixtures using the (PVDF) stress-rate gauge. These measurements were made on samples which were 53% of solid density. Measurements were made at pressures from 0.67 to more than 10 GPa utilizing both impact loading with a compressed gas gun and direct contact high explosive loading. The sample is pressed to the desired density in a copper capsule. PVDF gauges were positioned in front of and behind the powder sample in direct contact with the sample. These gauges measure the input and propagated stress-rate and are used for a precise measurement of velocity through the 4 mm thick sample. In the case of high explosive loading, gauges are also installed on the explosive side and on the capsule side of a metal driver plate to measure the shock velocity through the driver so that the driver pressure can be determined. The responses of 2Al+Fe[sub 2]O[sub 3] under shock compression appears to demonstrate a more complex behavior than other materials. At approximately 4.6 GPa, the material compresses to beyond solid density. There was no evidence of chemical reaction. [copyright] 1994 American Institute of Physics

Rutile (TiO2) samples at 60% of solid density have been shock‐loaded from 0.21 to 6.1 GPa with sample thickness of 4 mm and studied with the PVDF piezoelectric polymer stress‐rate gauge. The technique uses a copper capsule to contain the sample which has PVDF gauge packages in direct contact with the front and rear surfaces. A precise measure is made of the compressive stress wave velocity through the sample, as well as the input and propagated shock stress. The initial density is known from the sample preparation process, and the amount of shock‐compression is calculated from the measurement of shock velocity and input stress. Shock states and re‐shock states are measured. The observed data are consistent with previously published high pressure data. It is observed that rutile has a ‘‘crush strength’’ near 6 GPa. Propagated stress‐pulse rise times vary from 234 to 916 nsec. Propagated stress‐pulse rise times of shock‐compressed HMX, 2Al+Fe2O3, 3Ni+Al, and 5Ti+3Si are presented.

PVDF piezoelectric polymer stress‐rate gauges have been used to detect and record stress pulses input to and propagated through powder mixtures of 5Ti+3Si at densities of 53%. Data are obtained for the porous solid ‘‘crush‐up’’ and in the chemically reacting state. Wave speed is determined to an accuracy of 0.1% and serves as a sensitive and overt indication of chemical reactions. Compressed‐gas gun and high explosive loading experiments show a crush strength of about 1 GPa. Strong exothermic chemical transformation is indicated by large increases in wave speed to expanded volume states. The degree of reaction is approximately 50%. The pressure measurements are supplemented by studies of shock treated powder mixtures preserved for post‐shock analysis which determine the effect of particle size and morphology on reaction threshold and degree of reaction. The materials response is consistent with Graham’s CONMAH conceptual model of shock‐induced solid state chemistry reaction.

Alumina-filled epoxies are composites having constituents with highly dissimilar mechanical properties, resulting in complex behavior during shock compression and release. A previous study examined the shock properties of a particular composition in some detail. In the current study, the effects of compositional variations on shock properties were examined. Planar-impact experiments producing states of nearly equal strain were conducted to investigate the effects of changes in the size and shape of alumina particles, and in the total volume fraction of alumina. Laser interferometry and wave timing were used to obtain transmitted wave profiles, Hugoniot states, and release wave velocities. In addition, wave profiles and velocities were obtained in “thin-pulse” experiments that examined the combined effects of compression and release properties in different compositions. Changes in the size and shape of alumina particles were found to have little effect except in the viscous spreading of wave profiles durin...

Alumina-filled epoxies are composites having constituents with highly dissimilar mechanical properties. Complex behavior during shock compression and release can result, particularly at higher alumina loadings. In the current study, a particular material containing 43% alumina by volume was examined in planar-impact experiments. Laser interferometry was used to measure particle velocity histories in both reverse-impact and transmitted-wave configurations. Hugoniot states and release-wave velocities were obtained at shock stresses up to 10GPa, and represented smooth extensions of previous data at lower stresses. Surprisingly high release-wave velocities continued to be the most notable feature. Measured profiles of transmitted waves show a gradual transition from viscoelastic behavior at high shock stresses to a more complex behavior at lower stresses in which viscous mechanisms produce a broadened wave structure. This wave structure was examined in some detail for peak stress dependence, evolution towards...