C. Scott Alexander

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.

Simulation and analysis of Magnetically-Applied Pressure-Shear (MAPS) experiments

Magnetically-Applied Pressure-Shear (MAPS) is a new experimental technique to measure material shear strength at high pressures and has been developed for use on MHD driven pulsed power platforms [1]. By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse velocity interferometry system (VISAR) from which the sample strength is determined [2]. This paper presents and analyzes the 2D MHD simulations used to design the MAPS experiments.

Pressure-shear experiments on granular materials.

Pressure-shear experiments were performed on granular tungsten carbide and sand using a newly-refurbished slotted barrel gun. The sample is a thin layer of the granular material sandwiched between driver and anvil plates that remain elastic. Because of the obliquity, impact generates both a longitudinal wave, which compresses the sample, and a shear wave that probes the strength of the sample. Laser velocity interferometry is employed to measure the velocity history of the free surface of the anvil. Since the driver and anvil remain elastic, analysis of the results is, in principal, straightforward. Experiments were performed at pressures up to nearly 2 GPa using titanium plates and at higher pressure using zirconium plates. Those done with the titanium plates produced values of shear stress of 0.1-0.2 GPa, with the value increasing with pressure. On the other hand, those experiments conducted with zirconia anvils display results that may be related to slipping at an interface and shear stresses mostly at 0.1 GPa or less. Recovered samples display much greater particle fracture than is observed in planar loading, suggesting that shearing is a very effective mechanism for comminution of the grains.

Design and optimization of magnetically-applied pressure-shear (MAPS) experiments on Sandia's Z machine

Magnetically-applied pressure-shear (MAPS) is a recently developed technique to directly measure material shear strength using magnetohydrodynamic (MHD) pulsed power platforms [1,2]. The method has been prototyped on Sandias Veloce pulser, but was limited to strength measurements at modest pressures of ~10 GPa. Our present aim is to develop MAPS into a robust technique to measure strength at higher pressures achievable on Sandias Z pulsed power machine.

Application of a Computational Glass Model to the Shock Response of Soda-Lime Glass

This article details the implementation and application of the glass-specific computational constitutive model by Holmquist and Johnson (J Appl Mech 78:051003, 2011) to simulate the dynamic response of soda-lime glass under high rate and high pressure shock conditions. The predictive capabilities of this model are assessed through comparison of experimental data with numerical results from computations using the CTH shock physics code. The formulation of this glass model is reviewed in the context of its implementation within CTH. Using a variety of experimental data compiled from the open literature, a complete parameterization of the model describing the observed behavior of soda-lime glass is developed. Simulation results using the calibrated soda-lime glass model are compared to flyer plate and Taylor rod impact experimental data covering a range of impact and failure conditions spanning an order of magnitude in velocity and pressure. The complex behavior observed in the experimental testing is captured well in the computations, demonstrating the capability of the glass model within CTH.

Magnetically applied pressure-shear : a new technique for direct strength measurement at high pressure (final report for LDRD project 117856).

A new experimental technique to measure material shear strength at high pressures has been developed for use on magneto-hydrodynamic (MHD) drive pulsed power platforms. By applying an external static magnetic field to the sample region, the MHD drive directly induces a shear stress wave in addition to the usual longitudinal stress wave. Strength is probed by passing this shear wave through a sample material where the transmissible shear stress is limited to the sample strength. The magnitude of the transmitted shear wave is measured via a transverse VISAR system from which the sample strength is determined.

Expansion into vacuum of a shocked tungsten carbide-epoxy mixture.

The behavior of a shocked tungsten carbide / epoxy mixture as it expands into a vacuum has been studied through a combination of experiments and simulations. X-ray radiography of the expanding material as well as the velocity measured for a stood-off witness late are used to understand the physics of the problem. The initial shock causes vaporization of the epoxy matrix, leading to a multi-phase flow situation as the epoxy expands rapidly at around 8 km/s followed by the WC particles moving around 3 km/s. There are also small amounts of WC moving at higher velocities, apparently due to jetting in the sample. These experiments provide important data about the multi-phase flow characteristics of this material.