Dennis Brewster Hayes

Near-absolute Hugoniot measurements in aluminum to 500 GPa using a magnetically accelerated flyer plate technique

Hugoniot measurements were performed on aluminum (6061-T6) in the stress range of 100–500 GPa (1–5 Mbar) using a magnetically accelerated flyer plate technique. This method of flyer plate launch utilizes the high currents, and resulting magnetic fields produced at the Sandia Z Accelerator to accelerate macroscopic aluminum flyer plates (approximately 12×25 mm in lateral dimension and ∼300 μm in thickness) to velocities in excess of 20 km/s. This technique was used to perform plate-impact shock-wave experiments on aluminum to determine the high-stress equation of state (EOS). Using a near-symmetric impact method, Hugoniot measurements were obtained in the stress range of 100–500 GPa. The results of these experiments are in excellent agreement with previously reported Hugoniot measurements of aluminum in this stress range. The agreement at lower stress, where highly accurate gas gun data exist, establishes the magnetically accelerated flyer plate technique as a suitable method for generating EOS data. Furth...

Polymorphic phase transformation rates in shock‐loaded potassium chloride

Single crystals of potassium chloride (KCl) were impacted on quartz stress gauges at velocities ranging from 0.0363 to 0.0755 cm/μsec, producing plane shocks with stress amplitudes between 17 and 31 kbar. Stresses greater than 21 kbar are sufficient to induce a polymorphic phase transformation in KCl. For impact stresses above the phase transition point, the theory of continuum mechanics is used to relate measured stress history at the impact interface to transformation rate between the phases. Experimental results show that, when shocked above the measured dynamic transition pressure of 20.8±0.5 kbar, KCl rapidly transforms to a metastable nonequilibrium mixture of phases. Phase transformation to these initial metastable states proceeds at a rate greater than 500 μsec−1 for shock propagation along the 〈100〉 and 〈111〉 directions, based on the magnitude of initial transformation and the minimum response time of the measuring system. The degree of partial transformation is observed to depend only on impact ...

Shock‐compression and release behavior near melt states in aluminum

The response of porous aluminum (ρ0=1.6 g/cm3) when shock‐compressed to states of partial melting and then released has been investigated. There are no detectable changes in any of the Hugoniot properties in the region where melting should occur. However, release wave speeds in the completely compacted state of the porous aluminum specimens were found to depend critically upon the magnitude of the initial shock loading. For impact stresses below about 7 GPa, the measured release‐wave velocity is well described by the assumption that the initial release is elastic. For initial impact stresses above 7 GPa, the speed of the release wave is observed to be about 20% lower than the elastic wave velocity and approaches the bulk sound speed predicted by equilibrium thermodynamics. All calculations of wave speeds were made with a complete equation of state which was obtained by constructing semiempirical free‐energy functions for both solid and liquid pure phases. These were developed by first assuming self‐consis...

Measurement of the compression isentrope for 6061-T6 aluminum to 185GPa and 46% volumetric strain using pulsed magnetic loading

The Z accelerator at Sandia National Laboratories was used to measure the compression isentrope of 6061-T6 aluminum to 185GPa. The isentropic compression experimental technique uses a rapidly increasing, planar magnetic field to simultaneously subject multiple planar aluminum samples of different thicknesses to a ramped magnetic stress load. This magnetic stress load causes a ramped compression wave to propagate in the aluminum. Motion histories at the rear surface of each aluminum sample are measured through a LiF window using laser velocity interferometry. Backward and forward integration of the one-dimensional equations of motion are used to analyze the data. Imposing the requirement that each motion history comes from the same magnetic stress load is sufficient to determine both the stress load and the stress-strain behavior of the aluminum. Because of shocks that grow in the LiF, the usual VISAR interferometer analysis was modified. The measured compression curve obtained on different aluminum sample...

Continuous index of refraction measurements to 20 GPa in Z-cut sapphire

Isentropic compression experiments (ICE) on Sandia National Laboratories’ Z Accelerator are used to make continuous measurements of the index of refraction in single crystal Z-cut sapphire as the longitudinal stress is gradually increased to 20 GPa (uniaxial strain of about ∼3.9%). A planar, ramp compression wave, generated by a large magnetic field, propagates through a planar copper base plate. Part of the rear surface of the base plate is a free surface and part is covered with a sapphire window. Comparisons are made of velocity histories measured simultaneously at the free surface and at the copper/sapphire interface using a velocity interferometer system for any reflector (VISAR). This is sufficient to determine the index of refraction of the sapphire continuously as a function of density. Perturbations to the compression waves caused by interactions with the free surface and with the sapphire are accounted for by backward or forward solutions to the equations of motion. The effect of the unsteady na...

Unsteady compression waves in interferometer windows

Optical windows for the velocity interferometer system for any reflector perturb the measurement of specimen/window interface velocity during compression wave experiments. The perturbation is caused by the variation of window index of refraction with density. The correction for shocks and steady waves is the ratio of apparent to actual interface velocity and is called 1+Δν/ν0 in the literature. We find 1+Δν/ν0 for unsteady waves also depends only upon interface velocity and not on details of the unsteady compression wave as previously reported. The correction 1+Δν/ν0 for unsteady waves is the same as for steady waves and shocks only if the index of refraction is linear in density in which case 1+Δν/ν0 is constant equaling the zero-density intercept of the index of refraction function. Differences in LiF window performance for shocks and for unsteady waves are shown to be small compared with other experimental uncertainties.

Wave propagation in a condensed medium with N transforming phases: Application to solid‐I–solid‐II–liquid bismuth

Constitutive assumptions of local thermal and pressure equilibrium in a mixture of N transforming phases allow construction of a constitutive equation of a generalized Maxwellian form suitable for studying the kinetics of phase transformations induced by shock‐wave loading. This equation relates the pressure rate to the strain rate and the phase change rate, the latter being expressed as the time derivative of a vector which has components equal to the mass fractions of the constituent phases. A separate kinetic equation is required for evolution of this composition vector. Coefficients appearing in the constitutive equation depend only on properties that the mixture displays with a frozen composition which in turn depend directly on properties of the pure‐phase components of the mixture. The constitutive equation is applied to solid‐I–solid‐II–liquid bismuth (N=3). When wave propagation calculations on bismuth are compared to previous theory and experiments, a lower bound on the melting rate of 4 μsec−1 ...

Correcting Free Surface Effects by Integrating the Equations of Motion Backward in Space

Free surface and window interfaces perturb the flow in compression wave experiments. The velocity of these interfaces is routinely measured in shock‐compression experiments using interferometry (i.e., VISAR) and the perturbations must be accounted for before meaningful material property results can be obtained. Using the VISAR results as “initial conditions” we integrate the flow fields backward in space to the interior of the specimen where the VISAR interface has not perturbed the flow at earlier times and results can be interpreted as if the interface had not been present. This provides a rather exact correction for free surface perturbations. The method can also be applied to window interfaces by selecting the appropriate initial conditions. Applications include interpreting Z‐accelerator ramp wave experiments. The method can be applied to elastic‐plastic and quasi‐elastic materials for experiments with multiple layers and multiple reverberations.

Isentropic compression experiments on dynamic solidification in tin.

Isentropic compression experiments were performed on molten tin (initial temperature 500–600 K), using the Sandia Z Accelerator to generate magnetically driven, planar ramp waves compressing the tin across the equilibrium liquid‐solid phase boundary. Velocity interferometry measured time‐resolved wave profiles at the tin/window interface. The experiments exhibit a departure from expected liquid response, time‐dependent behavior above 8 GPa, and, at higher pressure, reduced wave speed relative to calculations using a nonequilibrium phase‐mixture model. These phenomena may be due to a nonequilibrium solidification process, but verification of this conjecture will require further work.

Recent Advances in Quasi‐isentropic Compression Experiments (ICE) on the Sandia Z Accelerator

The Z Accelerator is a pulsed power machine capable of delivering currents to loads of ∼20 MA over times of 100–300 ns. This current produces smoothly increasing, time dependant magnetic pressures that can be applied to specimens allowing quasi‐isentropes for these materials to be inferred. A new load design has been developed that allows this pressure to be uniformly applied to as many as 8 samples simultaneously. Diagnostics have recently been fielded that have resulted in an increased understanding of the magneto‐hydrodynamic effects and our confidence in the utility of this experimental configuration for EOS measurements. Efforts are also underway on Z to provide a capability for shaping the pressure profile applied to the samples which should increase useful sample thicknesses to > 1 mm by eliminating the formation of low‐level shocks. In addition to direct measurements of quai‐isentropic material response, the impulse from this loading technique has been demonstrated to launch macroscopic flyer plat...