Papers by Stephen Grunschel
Aps Shock Compression of Condensed Matter Meeting Abstracts, Jun 1, 2007
Pressure-shear plate impact experiments have been conducted to study the mechanical response of a... more Pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates: 10^5 - 10^6 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 pressure at impact. Release wave experiments, combined with plane wave simulations, are used to extend the isentrope into the tensile regime. Because the shearing resistance of polyurea depends strongly on pressure, two approaches are used to investigate the regime of high shearing rate and low pressure. First, an unloading longitudinal wave reflected from the rear surface of the target assembly is made to arrive at the sample midway through its loading by the incident shear wave. As a result, the sample is sheared at high strain rates and both high and low pressure during a single experiment. Second, the thickness of the flyer and front plates are selected such that the compressive pulse passes through the cast-in-place sample before the shear wave arrives, allowing the shearing resistance to be measured at zero pressure. Results of these experiments and their simulation will be presented.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
Many experimental techniques have been developed to determine the compressive strength or flow st... more 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
Aps Shock Compression of Condensed Matter Meeting Abstracts, Jun 1, 2009
An experimental technique has been developed to study the strength of materials under conditions ... more 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 copper and vanadium samples backed with a sapphire window will be presented to show the utility of the technique to measure flow strength under dynamic loading conditions. The samples were impacted with a Ta10W flyer at approximately 200 m/s.
Many experimental techniques have been developed to determine the compressive strength or flow st... more 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 . 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 .
Pressure-shear plate impact experiments have been conducted to study the mechanical response of a... more Pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates:10^5,10^6,-1. To measure the pressure-sensitivity of polyurea's shearing resistance, an impact configuration was designed to reduce the pressure during the shear wave loading of the sample by having an unloading longitudinal wave reflected from the rear surface of the target
High-temperature, pressure-shear plate impact experiments were conducted to investigate the rate-... more 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
ABSTRACT Pressure-shear plate impact experiments have been conducted to study the mechanical resp... more ABSTRACT 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 material's 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 midway through its loading by 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.
ABSTRACT High-temperature, pressure-shear plate impact experiments have been conducted to investi... more ABSTRACT 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 (10^6s-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 becomes 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 rate-controlling 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 drag-dominated regime.
ABSTRACT This study uses the pressure-shear plate impact configuration to investigate the rate-co... more ABSTRACT 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.
Metallurgical and Materials Transactions A, 2007
This study uses the pressure-shear plate impact configuration to investigate the rate-controlling... more This study uses the pressure-shear plate impact configuration to investigate the rate-controlling mechanisms of the dynamic plastic response of aluminum at strain rates of approximately 10 6 s -1 and at temperatures that approach melt. To achieve this combination of high strain rates and high temperatures, pressure-shear plate impact experiments are being conducted with modifications introduced by Frutschy [1] to enable the experiments to be conducted at high temperatures. So far, the shearing resistance has been measured at temperatures up to 906 K (632°C), which is 81 pct 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.
High Pressure Research, 1992
Pressure-shear plate impact experiments have been conducted to study the mechanical response of a... more Pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates: 10^5 - 10^6 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 pressure
Experimental Mechanics, 2012
For Kolsky bar testing beyond strain-rates of 10,000/s, it is useful to employ bars with diameter... more For Kolsky bar testing beyond strain-rates of 10,000/s, it is useful to employ bars with diameters of only a few millimeters or less. Furthermore, very small (sub-millimeter) systems are compatible with micron-sized specimens, to be used, for example, for the determination of mesoscale properties. However, at these sizes, traditional strain-gage measurements of the longitudinal waves within the bars become impractical. In this paper we describe the application of optical measurement techniques to two Kolsky bars, with 3.2 and 1.6 mm diameters. A transverse displacement interferometer is used to measure the displacement of the mid-point of the incident bar and provide measurements of the incident and reflected pulses. Similarly, a normal displacement interferometer is used to measure the displacement of the free-end of the transmitter bar and provide a measurement of the transmitted pulse. The new methods are used to characterize the behavior of 6061-T6 aluminum at rates greater than 100,000/s. The feasibility of application to smaller bars is also discussed.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
The use of optical measuring techniques for small diameter Kolsky bar experiments is discussed. T... more The use of optical measuring techniques for small diameter Kolsky bar experiments is discussed. The goal is to develop methods that can eliminate the need for more commonly used strain gages which become impractical as bar sizes decrease. The basic approach taken here is to adapt interferometer-based methods, used commonly in pressure-shear plate impact experiments, to high-rate Kolsky bar experiments. A Normal Displacement Interferometer (NDI) is used to measure the motion of the free end of the transmitter bar and provide a measurement of the transmitted pulse. Similarly, the incident and reflected pulses are measured with a Transverse Displacement Interferometer (TDI) utilizing a diffraction grating at the midpoint of the incident bar. Both techniques are applied to 1.59 mm diameter steel pressure bars. In the case of the transmitter bar, measurements are also made with the traditional strain gage instrumentation and comparisons between the two are made. The incident bar measurements made via TDI are validated with a simple bar impact against a single incident bar, i.e., without a specimen or transmitter bar. The possible application of these methods to smaller systems is also discussed.
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Papers by Stephen Grunschel