Compressive strength measurements in aluminum for shock compression over the stress range of 4–22GPa

Abstract

Measurements of the high-pressure compressive strength are presented for several aluminum alloys shocked to 22GPa. Five well-characterized aluminum materials were studied, including 6061 alloy with three average grain sizes (50, 30, and <5μm), pure aluminum 1060 (99.5% Al) with a 180-μm grain size, and ultrapure aluminum (99.9998% Al) with a 300-μm grain size. The purpose of these experiments was to investigate deformation mechanisms responsible for the apparently anomalous quasielastic recompression previously observed and to determine how the shock-induced yield strength varies with initial properties. The yield strength was estimated using combined reshock and release techniques previously developed. These results show that quasielastic recompression occurs for all materials investigated and is independent of grain size and impurity level. The shear stress and the shear strength at the shocked state were estimated from the reshock and release wave profiles. These results are consistent with previous investigations and suggest that the shear stress at the Hugoniot state is less than the yield strength. This is thought responsible for the observed quasielastic recompression. The present data, together with other reported measurements, illustrate that the yield strength of aluminum increases with applied shock stress to 90GPa. The Steinberg-Guinan strength model [Steinberg, Cochran, and Guinan, J. Appl. Phys. 51, 1498 (1980)] was used to describe these data and was found to represent the overall data trend with increasing stress, but is not an accurate representation. The collective data suggest that the increase in strength at shock states, ΔY(ΔY=Yyield−YHEL), increases with applied stress and plastic strain. A strength model was developed to describe this increase, which fits the data accurately to 55GPa and reveals that ΔY increases with shock stress in three distinct regions. It also strongly indicates that metallurgical properties, such as impurities and grain size, influence the ambient yield strength, but not the change in strength, which appears to be controlled by the shock-deformed aluminum matrix and possibly grain boundaries.