M. U. Anderson

Shock-induced chemical reactions in titanium-silicon powder mixtures of different morphologies: Time-resolved pressure measurements and materials analysis

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.

Shock-compression response of an alumina-filled epoxy

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...

Pressure measurements in chemically reacting powder mixtures with the Bauer piezoelectric polymer gauge

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.

Particle velocity and stress measurements in low density HMX

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.

Time‐resolved shock compression of porous rutile: Wave dispersion in porous solids

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.

Time-resolved pressure measurements in chemically reacting powder mixtures

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.

Shock compression of Al+Fe2O3 powder mixtures of different volumetric distributions

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.

Shock response of porous 2Al+Fe2O3 powder mixtures

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

Determination of equivalent circuit for PVDF shock‐pressure gauges

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.

Discrete meso-element dynamic analysis of stress profiles in HMX explosive powder

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.