High-Rate Forming: First Paper: Experiments on Clamped Circular Blanks Subject to an Underwater Explosive Charge

Abstract

The results of experiments on the dynamic free stretch forming of peripherally clamped circular blanks of brass and mild steel subject to an underwater explosive charge are given for use in connection with studies on explosive metal forming. The results are presented in six parts. part 1: Some results for the effect of hydrostatic head above the charge on the polar deflection suffered by a blank are given and it is shown that for some otherwise fixed sets of parameters, at a depth of about 18 in below the free water surface the polar deflection is greatest. Illustrations of how strain distribution and the plastic work done on a blank vary with hydrostatic head are also given. In particular it is shown that the energy received from an explosive charge by a blank may greatly exceed the fraction expected by reference to the solid angle subtended by the blank at the point charge. With the aid of some blank speed measurements an attempt is made to relate the kinetic energy acquired by the blank to the plastic work done on them. part 2: The measured maximum speed of deformation over the central portion of blanks is shown to be less than 300 ft/sec (rupture excluded). For some circumstances that two phases of motion exist is incontrovertible, and the mechanism responsible for these, we believe, is as follows. In the first phase the speed is dependent upon stand-off distance and is a consequence of the primary shock wave and subsequent cavitation and diffraction effects that occur as a result of the shock wave impinging on the blank. The second phase (arising only with the larger stand-off distances) occurs some time later and may be accompanied by a greater deformation velocity; it is thought to be due to a water hammer or similar effect. part 3: An attempt is made to present generalized results for the maximum polar deflection incurred as a function of both charge weight and hydrostatic head. part 4: This shows how the density of a curtain of air-bubbles, measured by aerator pressure, inserted at the walls of the confining tank reduces the polar deflection or damage a blank may sustain. part 5: A short empty cardboard cylinder (sealed at each end with a layer of polythene) inserted between the blank and charge leads to increased polar deflection, other things remaining the same. part 6: Deals with gas-bubble pulsation and generally confirms the Willis formula, namely, that for charges not too near a free water surface or an underwater blank, the period of oscillation is proportional to (charge weight)1/3 and (the ambient pressure at the charge)−5/6.