Michael D. Furnish

Magnetically driven isentropic compression experiments on the Z accelerator

Isentropic compression experiments (ICE) have been performed on the Z accelerator facility at Sandia National Laboratory. We describe the experimental design that used large magnetic fields to slowly compress samples to pressures in excess of 400 kbar. Velocity wave profile measurements were analyzed to yield isentropic compression equations of state (EOS). The method can also yield material strength properties. We describe magnetohydronamic simulations and results of experiments that used the “square short” configuration to compress copper and discuss ICE EOS experiments that have been performed with this method on tantalum, molybdenum, and beryllium.

Time-resolved particle velocity measurements at impact velocities of 10 km/s

Abstract Hypervelocity launch capabilities (9 – 16 km/s) with macroscopic plates have become available in recent years. It is now feasible to conduct instrumented plane-wave tests using this capability. Successfully conducting such tests requires a planar launch and impact at hypervelocities, appropriate triggering for recording systems, and time-resolved measurements of motion or stress at a particular point or set of points within the target or projectile during impact. We have conducted the first time-resolved wave-profile experiments using velocity interferometric techniques at impact velocities of 10 km/s. These measurements show that aluminum continues to exhibit normal release behavior to 161 GPa shock pressure, with complete loss of strength of the shocked state. These experiments have allowed a determination of shock-wave window transparency in conditions produced by a hypervelocity impact. In particular, lithium fluoride appears to lose transparency at a shock stress of 200 GPa; this appears to be the upper limit for conventional wave profile measurements using velocity interferometric techniques.

Isentropic compression of irradiated stainless steel on the Z accelerator

We have performed quasi-isentropic compression experiments on radiation-damaged stainless steel. The samples were dynamically loaded by Sandia National Laboratory’s Z accelerator with a ramp compression wave. Sample/window interface velocities were recorded with VISAR. The velocity histories suggest a sudden volume reduction of the material above 40 kbar caused by the collapse of nanosized voids. This is predicted by a theoretical model of void collapse based on the emission of vacancy-type dislocations loops. We compare the results of these experiments to hydrodynamic calculations performed using a constitutive model which is derived from the atomistic void collapse mechanism.

Variability in Dynamic Properties of Tantalum: Spall, Hugoniot Elastic Limit and Attenuation

A suite of impact experiments was conducted to assess spatial and shot‐to‐shot variability in dynamic properties of tantalum. Samples had a uniform refined ∼ 20 micron grain structure with a strong axisymmetric [111] crystallographic texture. An HEL amplitude of 2.8 GPa (corresponding to Y ≈ 1.5 GPa) was observed. Free‐surface spall experiments showed clear wave attenuation and spallation phenomena. Here, loading stresses were ∼ 12.5 GPa and various ratios of impactor to target thicknesses were used. Spatial and shot‐to‐shot variability of the spall strength was ±20%, and of the HEL, ±10%. Experiments conducted with smaller diameter flyer plates clearly showed edge effects in the line and point VISAR records, indicating lateral release speeds of roughly 5 km/s.

Dynamical properties measurements for asteroid, comet and meteorite material applicable to impact modeling and mitigation calculations

Abstract We describe methods for measuring dynamical properties for two material categories of interest in understanding large-scale extraterrestrial impacts: iron-nickel and underdense materials (e.g. snow). Particular material properties measured by the present methods include Hugoniot, release paths and constitutive properties (stress vs. strain). The iron-nickel materials lend themselves well to conventional shock and quasi-static experiments. As examples, a suite of experiments is described including six impact tests (wave profile compression/release) over the stress range 2 − 20 GPa, metallography, quasi-static and split Hopkinson pressure bar (SHPB) mechanical testing, and ultrasonic mapping and sound velocity measurements. Temperature sensitivity of the dynamic behavior was measured at high and low strain rates. Among the iron-nickel materials tested, an octahedrite was found to have behavior close to that of Armco iron under shock and quasi-static conditions, while an ataxite exhibited a significantly larger quasistatic yield strength than did the octahedrite or a hexahedrite. The underdense materials pose three primary experimental difficulties. First, the samples are perishable; they can melt or sublimate during storage, preparation and testing. Second, they are brittle and crushable; they cannot withstand such treatment as traditional machining or launch in a gun system. Third, with increasing porosity the calculated Hugoniot density becomes rapidly more sensitive to errors in wave time-of-arrival measurements. Carefully chosen simulants eliminate preservation (friability) difficulties, but the other difficulties remain. A family of 36 impact tests was conducted on snow and snow simulants at Sandia, yielding reliable Hugoniot and reshock states, but limited release property information. Other methods for characterizing these materials are discussed.

STATISTICS OF THE HUGONIOT ELASTIC LIMIT FROM LINE VISAR

Material heterogeneity appears to give rise to variability in the yield behavior of ceramics and metals under shock loading conditions. The line‐imaging VISAR provides a way to measure this variability, which may then be quantified by Weibull statistics or other methods. Weibull methods assign a 2‐parameter representation of failure phenomena and variability. We have conducted experiments with tantalum (25 and 40 μm grains) and silicon carbide (SiC‐N with 5 μm grains). The tantalum HEL variability did not depend systematically on peak stress, grain size or sample thickness, although the previously observed precursor attenuation was present. SiC‐N HEL variability within a single shot was approximately half that of single‐point variability in a large family of shots; these results are more consistent with sample‐to‐sample variation than with variability due to changing shot parameters.

Impact of AD995 alumina rods

Gas guns and velocity interferometric techniques have been used to determine the loading behavior of AD995 alumina rods 19 mm in diameter by 75 mm and 150 mm long, respectively. Graded-density materials were used to impact both bare and sleeved alumina rods while the velocity interferometer was used to monitor the axial-velocity of the free end of the rods. Results of these experiments demonstrate that (1) a time-dependent stress pulse generated during impact allows an efficient transition from the initial uniaxial strain loading to a uniaxial stress state as the stress pulse propagates through the rod, and (2) the intermediate loading rates obtained in this configuration lie between split Hopkinson bar and shock-loading techniques.

The effects of shock stress and field strength on shock-induced depoling of normally poled PZT 95/5

Shock-induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in a number of pulsed power devices. Several experimental and theoretical efforts are in progress in order to improve numerical simulations of these devices. In this study we have examined the shock response of normally poled PZT 95/5 under uniaxial strain conditions. On each experiment the current produced in an external circuit and the transmitted waveform at a window interface were recorded. The peak electrical field generated within the PZT sample was varied through the choice of external circuit resistance. Shock pressures were varied from 0.6 to 4.6 GPa, and peak electrical fields were varied from 0.2 to 37 kV/cm. For a 2.4 GPa shock and the lowest peak field, a nearly constant current governed simply by the remanent polarization and the shock velocity was recorded. Both decreasing the shock pressure and increasing the electrical field resulted in reduced current generation, indicating a retardation of the depoling kinetics.

Dynamical behavior of tantalum

We have performed four dynamic impact tests on tantalum to determine its high‐pressure yield and viscoelastic properties. Our experiments used compressed gas gun techniques to produce a combination of shocks, reshocks and releases over the pressure range 0–12 GPa in samples 5.0 and 7.3 mm thick. Profiles were recorded using VISAR (velocity interferometry) techniques. Elastic precursors suggest a yield strength of 0.95 GPa, which is somewhat above literature values. As with other metals, release waves do not show a perfect elastic‐plastic behavior, indicating a slight Baushinger effect. Lagrangian sound velocities for singly shocked states are consistent with earlier results (about 4.5 km/sec).

Assessing Mesoscale Material Response via High‐Resolution Line‐Imaging VISAR

Of special promise for providing dynamic mesoscale response data is the line‐imaging VISAR, an instrument for providing spatially resolved velocity histories in dynamic experiments. We have prepared a line‐imaging VISAR system capable of spatial resolution in the 10 – 20 micron range. We are applying this instrument to selected experiments on a compressed gas gun, chosen to provide initial data for several problems of interest, including: (1) pore‐collapse in single‐crystal copper (70 micron diameter hole; 2 different versions); and (2) response of a welded joint in dissimilar materials (Ta, Nb) to ramp loading relative to that of a compression joint.