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This paper focuses on examining the feasibility of using an explosive end-projector device to conduct shock loading experiments. The concept employs an explosive end-projector to accelerate a bonded bimetallic impactor toward a stationary target material in order to conduct complex shock loading experiments. A high-impedance material, tantalum, was specifically selected to generate high stresses in targets. The driver plate materials were varied to create an impedance spectrum from 2 to 4 which would encompass materials from aluminum to zirconium. Techniques for launching single density metal plates exist. [1] The objective of this paper is to further refine an existing explosive technique to launch bimetallic plates which would generate complex shock/reshock or shock/release waves when used as a impactor. Although a bimetallic impactor will generate complex wave loading within a target material upon impact, it must first survive intact the explosive shock acceleration stress history imposed during launch in order to obtain the terminal velocity for use as an impactor. Initial computational studies using the Lagrangian finite element code EPIC were promising. [2] Based on the concern of the plate spallation during launch [2], an air gap between explosive and the backside of the bimetallic plate was modeled to reduce the magnitude of the initial shock resulting from direct explosive-metal interaction and to further gain insight into its effects on pressure, terminal velocity, and planarity of the plate. Initially, a single, homogenous plate of aluminum was modeled to examine the effects of the air gap without the complication of bimetallic wave reverberation. The air gap did reduce the intensity of pressure by 30% with only a 4% loss in terminal velocity. Planarity was exacerbated by the air gap leading to increased warping. With the introduction of bimetallic plates and the corresponding wave reverberation pressure reductions ranged between 20% to 30% with corresponding losses in terminal velocity of between 5% to 7%. Planarity was improved for the configurations in which the high density material, tantalum, was on the free surface or impacting side of the impactor but warping increased when the order was reversed. This results from the low-impedance material serving as a buffer to allow quasi-isentropic and monotonic increase in loading of the tantalum plate. However, when the high density material was placed on the back side of the impactor, the loading history was reversed and the low density material was subjected to high pressures. Overall, the technique appears to be a feasible alternative for conducting shock loading experiments. The air gap reduces pressure with only a minimal loss in terminal velocity, and the air gap in conjunction with plate geometry changes appears to mitigate spall. The air gap does not improve planarity with the explosive end-projector design used in this investigation but optimized designs would improve the flexibility of the technique.Copyright © 2009 by ASME |
