Using the Hugoniot to Approximate the Release Adiabat

Abstract

When analyzing data from uniaxial strain plate impact experiments, to obtain equation-of-state (EOS) information, material strength usually is assumed to be negligible, and consequently is ignored. However, anomalies in recent experimental data indicate that material strength may not be negligible. Quasi-isentropic (QI) compression (low strain-rate, long rise time uniaxial compression) data for aluminum¹ (0–9 GPa) exhibit a stress-volume path lying a few percent above the principal Hugoniot. Theories not accounting for strength predict the opposite behavior. The release adiabat of copper² shocked to ≈100 GPa shows equally peculiar behavior. The adiabat is below the Hugoniot for the initial ≈2/3 of the release, then crosses over for the remainder. Theories not accounting for strength predict that the release adiabat should lie above the Hugoniot. In a previous paper,³ we showed how material strength can explain the QI loading data, and that material strength along QI compression paths can be large. Here we examine the effects of material strength on the release adiabat. In particular, we discuss the magnitude of the strength correction to data from laboratory impedance matching (IM) experiments. Our analysis differs from an earlier study by van Thiel and Cochran⁴ in that we use experimental dynamic strength data to construct the release adiabat.