Features of shock Hugoniot measurements of underdense materials

An important challenge pertaining to equation of state model improvements for underdense materials is the quality of experimental measurements. A recent analysis of errors associated with historical explosively-driven experiments yielded large values (>10%) – due, in part, to the near-equivalencies of the shock and mass velocities, which give rise to large errors in density, and spatiotemporal limitations on diagnostics. Other challenges include material heterogeneity and high shock temperatures, both of which contribute to difficulties in experimental measurements. Shock heating, for exmaple, can dominate the compressive response resulting in “anomalous compression.” In the anomalous regime, the Hugoniot curves often bend back, with increases in shock pressure resulting in no volume change, even at modest porosities. We present a summary of features of shock Hugoniots of porous polymers over a range of initial densities, and show the interplays between initial density, shock stress, and normal vs. anomalous regimes. We also describe how penetrating x-ray-based imaging can be used to drive down errors. Recommendations will be made for future improvements and their impacts on theoretical model development.