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The shear strength of materials under shock-loading conditions has long been recognized as being of the utmost importance, as this can be related to deformation mechanisms and ballistic performance. As such, a number of experimental techniques have been developed to study this component of shock loading, and in this paper, we concentrate on the embedded lateral stress gauge. We acknowledge that as the requirements of the target assembly necessitate that the target be sectioned, the gauge glued in place, and the target reassembled, it is possible that the gauge response is dominated by its local environment. As a consequence, we have, therefore, asked three questions—can the gauge be placed in a known stress condition and measure the correct lateral stress and shear strengths; do the results from the lateral stress gauge agree with other non-invasive techniques; and finally, if we do trust the results from lateral gauges, can they be placed in context with other known shock-induced materials responses. In all three cases, we believe that the answer is yes, and we present evidence for all three situations. [ABSTRACT FROM AUTHOR]

The response of single crystalline tantalum to one-dimensional shock loading has been investigated as a function of crystalline orientation to the loading axis. Results show that this has a significant effect, particularly on the Hugoniot elastic limit (HEL). [100] and [111] HELs are near identical with the [110] HEL having the lowest strength. This is contrary to predictions obtained by applying the Schmid factor analysis, where the ordering was expected to be (highest strength first) [111], [110], with the [100] orientation being the softest. Adopting a more appropriate model based on uniaxial strain conditions, as was previously done successfully for FCC aluminum and copper, did not rationalize our observations. We show that a non-Schmid effective stress model, incorporating twinning/anti-twinning asymmetry, has much greater success in reproducing the experimental relative HELs magnitudes. Using this model, we make a quantitative estimation of the magnitude of non-Schmid effects and compare these to equivalent low temperature, quasi-static estimates from the literature. [ABSTRACT FROM AUTHOR]

The microstructural and mechanical response of materials to shock loading is of the utmost importance in the development of constitutive models for high strain-rate applications. However, unlike a purely mechanical response, to ensure that the microstructure has been generated under conditions of pure one dimensional strain, the target assembly requires both a complex array of momentum traps to prevent lateral releases entering the specimen location from the edges and spall plates to prevent tensile interactions (spall) affecting the microstructure. In this paper, we examine these effects by performing microhardness profiles of shock loaded copper and tantalum samples. In general, variations in hardness both parallel and perpendicular to the shock direction were small indicating successful momentum trapping. Variations in hardness at different locations relative to the impact face are discussed in terms of the initial degree of cold work and the ability to generate and move dislocations in the samples.

A series of plate impact experiments have been conducted to study the spall strength of the three principal crystallographic orientations of single crystal aluminium ([100], [110] and, [111]) and ultra-pure polycrystalline aluminium. The samples have been shock loaded at two impact stresses (4 GPa and 10 GPa). Significant differences have been observed in the elastic behaviour, the pull-back velocities, and the general shape of the wave profiles, which can be accounted for by considerations of the microscale homogeneity, the dislocation density, and the absence of grain boundaries in the single crystal materials. The data have shown that there is a consistent order of spall strength measured for the four sample materials. The [111] orientation has the largest spall strength and elastic limit, followed closely by [110], [100], and then the polycrystalline material. This order is consistent with both quasi-static data and geometrical consideration of Schmid factors. [ABSTRACT FROM AUTHOR]

Quasi-Taylor wave loaded plate impact experiments have been used to study the effects of bulk grain size on the shock and spall response of pure copper samples. Using line-Velocity Interferometry System for Any Reflector velocimetry, an increased particle velocity dispersion was measured for larger bulk grain size samples resulting in lower mean peak particle velocities and a lower reload velocity following spall. Spall voids were located in a surface layer of similarly sized grains for both bulk grain size materials tested. The spall strength was observed to vary with the tensile strain-rate but was unaffected by the bulk grain size. Post-mortem studies of voids in samples showed narrow void planes for the small bulk grain size materials and a larger spread of voids in the larger bulk grain size materials consistent with the free surface particle velocity data and observed particle velocity dispersion. [ABSTRACT FROM AUTHOR]

The mechanical response of niobium and molybdenum during one dimensional shock loading in the weak shock regime is investigated in terms of the Hugoniot elastic limit (dynamic yield) and spall (tensile) strengths. Results indicate that although both metals have high elastic limits of ca. 2GPa, their responses are very different. Deformation in the weak shock regime in niobium is controlled by both the motion and generation of dislocations, resulting in high spall (dynamic tensile) strengths and ductility. In contrast, molybdenum has low spall strength and ductility, which suggests lower dislocation mobility in this metal. We have also shown that the strain-rate in the rising part of the shock front is related to the stress amplitude by the fourth power, as first shown by Swegle and Grady. Although we have not been able to elucidate further on the power relation, we believe that the scaling factor A is related to a materials ability to accommodate shock imposed plasticity via slip and dislocation generation. Overall, we have used arguments about the Peierls stress in body centred cubic metals to explain these results, with niobium (low Peierls stress) having a high dislocation mobility, resulting in behaviour showing some similarities to face centred cubic metals. Molybdenum, with its much higher Peierls stress has a much lower dislocation mobility, and hence lower spall strengths and ductility. [ABSTRACT FROM AUTHOR]

The effect of prior cold work on the shock response of tantalum has been investigated via plate impact. As-received and 50% cold-rolled material has been studied to determine the Hugoniot Elastic Limit (HEL), shear strength evolution behind the shock front, and spall strength. Results show that there is a significant drop in both HEL and shear strength due to cold-rolling, but as the thickness of the target (or time) increases, results converge between the two states. Results suggest that this is due to the cold-rolling process moving dislocations away from the surrounding interstitial solute atoms that collect there, thus reducing the initial stress to initiate yield. In other words, the main contribution of cold-rolling is to increase the population of mobile dislocations within the microstructure rather that just increase the dislocation density as a whole. In contrast, the spall strength in both states appears almost identical. It is suggested that the high Peierls stress prevents a large increase in dislocation density during rolling and hence reduces any post rolling strengthening that might be observed in the spallation response. Finally, we observe a significant change in spall response below a pulse width of 150 ns. We believe that this represents a change from a nucleation and growth of ductile voids type mechanism to one based on ductile fracture of atomic planes. The fact that at these low pulse durations, results appear to trend towards the theoretical strength of tantalum would lend support to this hypothesis. [ABSTRACT FROM AUTHOR]

Lateral stress and shear strength behind the shock front have been measured in three face centered cubic alloys. Results show different behaviors according to microstructure. A hardening response has been shown in pure nickel, while in stainless steel (SS) 304L, hardening has been shown to be minimal. This has been explained in terms of the stacking fault energy effecting the motion and generation of dislocations. In the high stacking fault energy nickel, dislocation motion is comparatively easy, and hence the microstructure consists of dislocation cells, with a pronounced hardening postshock. In SS 304L with its lower stacking fault energy, dislocation motion is more restricted, and hence deformation is dominated by deformation twins. The behavior behind the shock front correlates with the low degree of hardening observed by others during mechanical testing of preshocked samples. The aluminum alloy 6082-T6 also shows a low degree of hardening behind the shock front. This is believed to occur due to the presence of fine intermetallic particles suppressing the formation of dislocation cells but rather forming a random distribution throughout the microstructure. [ABSTRACT FROM AUTHOR]

Manganin stress gauges in lateral orientation have been used to monitor the shock response of Ti–6Al–4V when loaded either parallel to or radial to the long axis of the original bar stock studied in this investigation. Materials characterization has shown that the c-axis of the hexagonal unit cell is preferentially orientated radially to the axis of the bar. Shear strengths measured along the long axis of the bar were found to be in agreement with previous data in the literature, while strength in the radial direction was found to be significantly lower. It was also noted that the lateral stress, when measured in the radial direction, displayed a pronounced drop in the lateral stress after reaching the peak shock stress unlike the longitudinal orientation. This decrease is indicative of an increase in shear strength behind the shock front. In both instances, it is postulated that extensive deformation twinning during the early stages of deformation in the shock and thereafter c+a slip and dislocation tangling builds up over a longer time period, resulting in the higher degree of hardening noted. [ABSTRACT FROM AUTHOR]