High Pressure Hugoniot Measurements in Solids using Mach Reflections
March 2, 2012
Professor Guruswami Ravichandran
Graduate Aerospace Laboratories
California Institute of Technology
It has long been recognized that high pressure equation of state (Hugoniot) measurements in solids can be determined by subjecting the material to a one dimensional plane shock wave. Shock waves of this type are usually generated by the planar impact of two flat plates and, as a result, the shock amplitude is inherently limited by the velocity of the impacting plate. In an effort to dramatically increase the range of pressures which can be studied with available impact velocities, a new experimental technique has been developed. A target consisting of two concentric cylinders aligned with the axial direction parallel to the loading is subjected to planar impact. The target is designed such that upon initial impact, the outer cylinder will have a higher shock velocity than the inner material of interest. Conically converging shocks are generated at the interface between the two materials due to the impedance mismatch. Upon convergence, an irregular reflection occurs and the conical analog of a Mach reflection develops and grows until it reaches a steady state. The resulting high pressure Hugoniot state behind the Mach stem is measured using velocity interferometry (VISAR). The new technique is demonstrated using a planar mechanical impact generated by a powder gun to study the shock response of copper. Two systems are examined which utilize either a low impedance (6061-T6 aluminum) or a high impedance (molybdenum) outer cylinder. The results from the experiments are presented and compared to both numerical simulations and a hydrodynamic model based on shock polars used in solving gas dynamics problems. The feasibility of measuring an entire Hugoniot curve in a single experiment using full field velocity interferometry (ORVIS) is discussed.