Stephen Grunschel

High Strain Rate Response of an Elastomer

Pressure‐shear plate impact experiments are used to study the nonlinear dynamic response of an elastomer at shearing rates of 105 – 106 s−1. Samples with thicknesses in the range 100 μm – 400 μm are cast between two hard steel plates. Because of the comparatively low impedance of the elastomer, longitudinal waves reverberating through the thickness of the sample — and recorded with a laser interferometer — are used to determine the isentrope of the material under uniaxial strain compression. Once the sample is fully compressed a shear wave arrives and imposes a simple shearing deformation. From the transverse velocity, measured interferometrically at the rear surface of the sandwich target, the shear stress and the transverse velocity at the rear surface of the sample are determined. These measurements provide an indication of the shearing resistance of the material under pressure. When the longitudinal unloading wave arrives from the rear surface of the target, these same measurements provide an indicati...

PRESSURE‐SENSITIVITY AND CONSTITUTIVE MODELING OF AN ELASTOMER AT HIGH STRAIN RATES

Pressure‐shear plate impact experiments have been conducted to study the pressure dependence of the shearing resistance of an elastomer (polyurea) at very high strain rates: 105–106 s−1. Two impact configurations were used. In the first, an unloading longitudinal wave reflected from the rear surface of the target assembly arrives at the sample midway through its loading by the incident shear wave. In the second, an unloading wave reflected from the free rear surface of the flyer arrives at the sample prior to the arrival of the incident shear wave. As a result, the sample is sheared at high strain rates—at both high and low pressure—during a single experiment (first case) or at high strain rates and low pressures (second case). Based on the experimental results, a constitutive model has been developed that involves a hyperelastic spring acting in parallel with an elastic spring and viscoplastic dashpot acting in series. The viscoplastic dashpot is modeled by means of a thermal activation model in which th...

PRESSURE‐SENSITIVITY AND TENSILE STRENGTH OF AN ELASTOMER AT HIGH STRAIN RATES

Pressure‐shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates: 105–106 s−1. Thin samples are cast between two hard steel plates. Longitudinal waves reverberating through the sample are used to determine the slope of the isentrope at compressive stresses greater than, say, 500 MPa—the initial pressure at impact. Shear waves measure the shearing resistance at the pressure attained after the “ring‐up” of the pressure in the sample is complete. In the current work, release wave experiments and plane wave simulations are used to extend the isentrope into the tensile regime—and ultimately to failure. The previous work is also extended to determine the pressure‐sensitivity of the materials shearing resistance at high shearing rates and low pressures. To achieve the latter, the impact configuration is designed so that an unloading longitudinal wave reflected from the rear surface of the target assembly arrives at the sample midwa...

Shearing resistance of aluminum at high strain rates and at temperatures approaching melt

High-temperature, pressure-shear plate impact experiments have been conducted to investigate rate-controlling mechanisms for plastic deformation of high-purity aluminum at high strain rates (106 s-1) and at temperatures approaching melt. The objective of these experiments was to look for a possible change in the rate-controlling mechanism of dislocation motion from thermally activated motion of dislocations past obstacles to phonon drag as the temperatures become high enough that thermal activation becomes relatively unimportant. The experimental results show an upturn in shearing resistance with increasing temperature at high temperatures, suggestive of a change in ratecontrolling mechanism. However, the upturn is too steep to be described by a usual phonon drag model with a drag coefficient that is proportional to temperature. Simulated results show that the modeling of strain rate hardening based on a phonon drag model leads to too strong an increase in flow stress with increasing strain rate in the dr...

HIGH RATE PLASTICITY UNDER PRESSURE USING A WINDOWED PRESSURE‐SHEAR IMPACT EXPERIMENT

An experimental technique has been developed to study the strength of materials under conditions of moderate pressures and high shear strain rates. The technique is similar to the traditional pressure‐shear plate‐impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample‐window interface. Experimental and simulation results on vanadium samples backed with a sapphire window show the utility of the technique to measure the flow strength under dynamic loading conditions. The results show that the strength of the vanadium is approximately 600 MPa at a pressure of 4.5 GPa and a plastic strain of 1.7%.

Plasticity Under Pressure Using a Windowed Pressure-Shear Impact Experiment

Many experimental techniques have been developed to determine the compressive strength or flow stress of a material under high strain rate or shock loading conditions [1-3]. In addition, pressure-shear techniques have been developed that allow for the measurement of the shearing response of materials under pressure [4-6]. The technique described is similar to the traditional pressure-shear plate-impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample-window interface. The velocities are measured using the normal displacement interferometer (NDI) for the normal velocity, and the transverse displacement interferometer (TDI) for the transverse velocity [7].

Plasticity at High Pressures and Strain Rates Using Oblique-Impact Isentropic-Compression Experiments

Part of LLNL’s national security mission is reliant on accurate simulations of high strain-rate plastic flow (nonreversible deformation) under conditions of high hydrostatic pressures. In an effort to help advance the predictive capability of LLNL’s multiscale modeling program a new experimental technique has been developed to provide strength properties under conditions of high strain rate (10 4 -10 6 s -1 ) and high hydrostatic pressure (1100 GPa). The oblique-impact experiments allow for the shearing response of the material to be independently measured while the material is under pressure. The strength of the material is then inferred by conducting 2-D hydrodynamic simulations to match to the experimentally measured velocity profiles. Utilizing this technique, Cu and V experiments have been conducted that establish the utility of this technique to measure strength under dynamic conditions. Introduction/Background Understanding and simulating the plastic deformation of materials at high rates under pressure is a major component of LLNL’s Stockpile Stewardship Program and is intended to simulate future NIF experiments. While progress has been made in recent years, especially for individual extremes of pressure and strain-rate [1], there is still an uncertainty in understanding the strength of materials under conditions of combined high strain-rate (10 4 – 10 6 s -1 ) and high pressure (1-100 GPa). Current strength models used in simulations include some physically based models such as the Mechanical Threshold Stress formulation [2], which has over 20 parameters. The uncertainty in the values for these parameters as well as values for the parameters in other physically based models is under question due to the inherent difficulties in conducting and extracting high-quality experimental data in the high pressure and high strain rate regimes. The experimental studies of material strength at these pressure and strain rate regimes further the understanding of the underlying physical strength mechanisms needed for accurate material strength models.

High Strain-Rate Response of High Purity Aluminum at Temperatures Approaching Melt

High‐temperature, pressure‐shear plate impact experiments were conducted to investigate the rate‐controlling mechanisms of the plastic response of high‐purity aluminum at high strain rates (106 s−1) and at temperatures approaching melt. Since the melting temperature of aluminum is pressure dependent, and a typical pressure‐shear plate impact experiment subjects the sample to large pressures (2–7 GPa), a pressure‐release type experiment was used to reduce the pressure in order to measure the shearing resistance at temperatures up to 95% of the current melting temperature. The measured shearing resistance was remarkably large (50 MPa at a shear strain of 2.5) for temperatures this near melt. Numerical simulations conducted using a version of the Nemat‐Nasser/Isaacs [1] constitutive equation, modified to model the mechanism of geometric softening, appear to capture adequately the hardening/softening behavior observed experimentally.

PRESSURE‐SHEAR PLATE IMPACT OF ALUMINUM AT ELEVATED TEMPERATURES

This study uses the pressure‐shear plate impact configuration to investigate the rate‐controlling mechanisms of the plastic response of metals at strain rates on the order of 106 s−1 and at temperatures that approach melt. In similar experiments by Frutschy and Clifton [1] on OFHC copper, the flow stress decreases with increasing temperature and increases with increasing strain rate over the full range of temperatures and strain rates examined. No conclusive evidence of a change in rate‐controlling mechanism was obtained. In the current study, temperatures that are larger fractions of the melting temperature are accessible because of the lower melting point of aluminum. So far, the shearing resistance has been measured at temperatures up to 630 °C, which is 81% of the melting temperature at the concurrent pressure. Several approaches are being explored to obtain even higher fractions of the melting temperature, possibly exceeding it.