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Observation of shock wave convergence or collision induced by shaping of a plane shock front in solids

A new procedure is proposed of generating converging or of colliding shock waves in solids. The method is based on the refraction phenomena of a plane shock front at a shaped material interface, due to the difference in shock velocity of the materials of each side. A high-pressure gas gun is used to produce plane shock waves in a composite target assembly. The assembly is composed of aluminum and of polyethylene. The conically converging shock wave is generated in a lower-impedance material, i.e., polyethylene. In this case, polyethylene material is machined to the shape of a cone, and is inserted and glued to the aluminum plate having just the same inner surface. The processes of shock convergence are observed by a compact high-speed streak camera together with a pulsed dye laser as a light source. We have performed a series of experiments by varying several parameters. The realized converging angle of shock waves is found to be about 55 - 60 degrees in polyethylene medium. It is shown that the converging wave front looks almost continuously curved, and it is not easy to discriminate the boundary of the Mach stem. In other words, the growth of the Mach region seems faster than expected. This result is attributed to the combined effects of wave convergence and Mach reflection.