J. E. Vorthman

Release isentropes of overdriven plastic-bonded explosive PBX-9501

Continuous release isentropes for the plastic-bonded explosive PBX-9501 are obtained from velocity interferometer system for any reflector measurements at a high-explosive/LiF interface. Forward calculations from a tabular representation of the isentropes to the measured u(t) data at the interface are iterated to yield isentropes that give agreement with the data. Curves for the isentropes and for the isentropic gamma, γS=−(∂lnP/∂lnV)S are presented. Because isentropes from different overdriven states differ, a crude estimate of the Gruneisen parameter is obtained. An overall representation of the data is achieved with this Gruneisen parameter and a single isentrope through the Chapman–Jouguet state.

Release isentropes in overdriven PBX 9502

Experiments have been done to obtain product isentropes of the plastic bonded TATB explosive PBX 9502. A thin aluminum flyer was thrown at an explosive disk so as to generate a state in excess of the Chapman-Jouguet pressure. Because the flyer is thin, the shock wave is followed by a release wave that lowers the pressure in the explosive products. The explosive disk was backed by a LiF tamper/window. Particle velocity at the explosive—window interface was measured with a velocity interferometer (VISAR) and measured particle velocity records were used to construct isentropes. Unexpected behavior is seen in the shape of the isentropes. Experimental details and the resulting isentropes will be presented.

Phase transition and spall behavior in β-TiN

We have undertaken a study of Sn in an effort to understand the spall and phase transition behavior of this material. The initial results, obtained with tin having 1–2% impurities, show the phase transformation from the β to the γ phase during the shock event with kinetic effects evident. Preliminary modeling suggests that the reverse transition occurs somewhat more rapidly than the forward transition. Records of the experimental free surface velocity show a signal usually considered indicative of spall, but analysis of a recovered sample calls that interpretation into question for the lower-pressure experiments. At higher shock pressures, flash x-ray images of the samples confirm that the samples did spall as expected.

Microstructure of depleted uranium under uniaxial strain conditions

Uranium samples of two different purities were used for spall strength measurements. Samples of depleted uranium were taken from very high purity material (38 ppm of carbon) and from material containing 280 ppm carbon. Experimental conditions were chosen to effectively arrest the microstructural damage at two places in the development to full spall separation. Samples were soft recovered and characterized with respect to the microstructure and the form of damage. This allowed determination of the dependence of spall mechanisms on stress level, stress state, and sample purity. This information is used in developing a model to predict the mode of fracture.

MULTIPLE-WAVE EFFECTS ON EXPLOSIVES DECOMPOSITION RATES

We are studying the changes in decomposition rates that occur when following-shock or rarefaction waves are imposed on shocked, reacting explosives. The requisite data are acquired with an electromagnetic-gauge technique providing both stress and particle velocity measurements at several points in the reactive flow. Global decomposition rates are calculated with a revised Lagrange analysis that allows the treatment of multiple waves by the fitting of computational path lines to wave-front features. Another analytic improvement eliminates some differentiating of data. The results of two multi-wave experiments on a plastic-bonded HMX explosive are examined to assess the relative dependence of reaction rates on the first-shock state and on current-state changes caused by subsequent waves.

High pressure response of a high-purity iron

A series of time-resolved experiments has been done using a 50-mm-diameter light gas gun to characterize the shock uniaxial strain and high-strain-rate response of a high purity iron. The high-strain-rate uniaxial stress response of the iron was characterized using a Split-Hopkinson Pressure Bar. Shock wave experiments were done both with and without windows with a VISAR diagnostic to obtain time-resolved particle velocity data. The shock experiments with windows yield information on elastic/plastic response, transformation kinetics, sound speed, and quasi-elastic release. Experiments were conducted without windows both below and above the α-to-e phase transformation to study the effect of phase-change kinetics and the release transition on the spallation response in iron. Results of the shock and Hopkinson bar experiments are compared and contrasted to previous research on iron.

Spall wave-profile and shock-recovery experiments on depleted uranium

Depleted Uranium of two different purity levels has been studied to determine spall strength under shock wave loading. A high purity material with approximately 30 ppm of carbon impurities was shock compressed to two different stress levels, 37 and 53 kbar. The second material studied was uranium with about 300 ppm of carbon impurities. This material was shock loaded to three different final stress level, 37, 53, and 81 kbar. Two experimental techniques were used in this work. First, time-resolved free surface particle velocity measurements were done using a VISAR velocity interferometer. The second experimental technique used was soft recovery of samples after shock loading. These two experimental techniques will be briefly described here and VISAR results will be shown. Results of the spall recovery experiments and subsequent metallurgical analyses are described in another paper in these proceedings.

Precursor Suppression by Shear Stress Relaxation in U‐Nb(6‐wt%)

U‐Nb(6‐wt%) exhibits plastic yield strength of a few‐tenths of a GPa that can vary depending upon the starting microstructure and heat‐treatment. However, when a several‐mm thick specimen of U‐Nb(6‐wt%) is shock loaded in the range between 1.5 and 10 GPa, no elastic precursor is observed in interferometer measurements at the rear free surface. The absence of the elastic precursor and other features of the compression and release measurements are explained by assuming shear stress‐relaxation rate is dependent upon the shear stress. The resulting stress waves are unsteady and broaden so that shear stress relaxation can occur in the front preventing the plastic yield point from being reached. U‐Nb(6‐wt%) is known to twin in quasi‐static compression and shear‐induced, rate‐dependent twinning is likely the underlying cause for shear stress relaxation in our experiments. Recent experiments in which U‐Nb(6‐wt%) was heavily cold‐rolled ( work‐hardening to ∼25% strain) display no evidence of a precursor, admitting...

Spallation studies on shock loaded U-6 WT PCT NB

Several spallation experiments have been performed on the 6 wt pct alloy of uranium using gas gun driven normal plate impacts with VISAR instrumentation and soft recovery. The nominal shock pressures achieved were 28, 34, 42, 50, 55, and 82 kbar. This paper will focus on spallation modeling, e.g. using the 1 D characteristics code CHARADE to simulate the free surface particle velocity. The spallation model involves the ductile growth and coalescence of voids. Metallographical examination of recovered samples and details of the experimental apparatus are discussed in a separate paper.

Spall response of U-Nb (6%) alloy

Experiments were done to measure the spall strength of U-NB (6%) alloy. Gas gun experiments were done to three different peak stress states, representing no spall, incipient spall, and complete spall. An experimental technique was developed to allow measurements of time-resolved particle velocity at the sample free surface, and soft recovery for post-experiment metallographical analysis. Elementary analyses were done on the time-resolved data to yield approximate spall strength. Preliminary results are described.