HIGH-PRESSURE QUASI-ISENTROPIC IMPACT EXPERIMENTS**This work performed at Sandia National Laboratories supported by the U. S. Department of Energy under contract number DE-AC04-76DP00789.

A method has been developed for generating quasi-isentropic compression waves in flat plate impact experiments. Peak stresses can be in the multimegabar range, thus allowing investigations of material properties in a high-pressure, low-temperature regime which is inaccessible either quasi-statically or by conventional shock wave experiments. The key to the technique is a thin impactor with a smooth but strong gradient in shock impedance. Novel impactor fabrication and testing methods have been developed to attain the excellent uniformity and relatively close control of the shock impedance profile which are needed. In using the method, it is normally desirable to obtain time-resolved measurements of either pressure or particle velocity at two distances from the impact surface. The stress-volume loading path followed by the specimen can then be obtained from a centered-wave analysis, provided certain assumptions are valid. The data and results of several experiments are presented.