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In the analysis of impedance-match experiments, the release adiabat of thestandard material is often approximated by reflecting the Hugoniot in thepressure-particle velocity (P-UP) plane. In cases where it has been testedexperimentally, this reflected shock approximation (RSA) has been found to befairly accurate at pressures as high as 100 GPa. The success of the RSA isusually attributed to the lack of thermal effects, i.e., that the Hugoniot andrelease adiabat are nearly identical in the pressure-density (P-rho) plane.This report demonstrates that this explanation is not correct. The success ofthe RSA does not arise from the absence of thermal effects--it arises becauseof thermal effects. When the Hugoniot and adiabat are identical in the P-rhoplane, the adiabat lies below the reflected Hugoniot when mapped into the P-UPplane. Material strength effects also cause the exact adiabat to lie below theRSA. A thermal offset between the Hugoniot and the adiabat compensates forthese two effects, so that the RSA turns out to give good results for manymaterials, even at high pressures. However, this fortuitous cancellation oferrors does not occur in all cases. This report shows that the RSA is notaccurate for two "soft" materials, Teflon and PMMA, and for a high-strengthmaterial, tungsten. The issues discussed here apply to use of the RSA at lowpressures, not to very high pressures, where it is already well-known to beinaccurate.

Calculation of Release Adiabats and Shock Impedance Matching