Pressure-Dependent, Infrared-Emitting Phenomenon in Hypervelocity Impact
A series of hypervelocity impact experiments were conducted with variable target chamber atmospheric pressure ranging from 0.9 to 21.5 Torr. Using a two-stage light-gas gun, 5.7 mg nylon 6/6 right-cylinders were accelerated to speeds ranging between 6.0 and 6.3 km/s to impact 1.5 mm thick 6061-T6 aluminum plates. Full-field images of near-IR emission (0.9 to 1.7 μm) were measured using a high-speed spectrograph system with image exposure times of 1 μs. The radial expansion of an IR-emitting impact-generated phenomenon was observed to be dependent upon the ambient target chamber atmospheric pressures. Higher chamber pressures demonstrated lower radial expansions of the subsequently measured IR-emitting region uprange of the target. Dimensional analysis, originally presented by Taylor to describe the expansion of a hemispherical blast wave, is applied to describe the observed pressure-dependence of the IR-emitting cloud expansion. Experimental results are used to empirically determine two dimensionless constants for the analysis. The maximum radial expansion of the observed IR-emitting cloud is described by the Taylor blast-wave theory, with experimental results demonstrating the characteristic nonlinear dependence on atmospheric pressure. Furthermore, the edges of the measured IR-emitting clouds are observed to expand at extreme speeds ranging from approximately 13 to 39 km/s. In each experiment, impact ejecta and debris are simultaneously observed in the visible range using an ultrahigh-speed laser shadowgraph system. For the considered experiments, ejecta and debris speeds are measured between 0.6 and 5.1 km/s. Such a disparity in observed phenomena velocities suggests the IR-emitting cloud is a distinctly different phenomenon to both the uprange ejecta and downrange debris generated during a hypervelocity impact.
© 2015 by ASME. Contributed by the Applied Mechanics Division of ASME for publication in the Journal of Applied Mechanics. Manuscript received June 2, 2014; final manuscript received October 17, 2014; accepted manuscript posted October 21, 2014; published online November 19, 2014. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award No. DE-FC52-08NA28613. The authors would also like to thank Michael Mello for his assistance with the optomechanical design of the LSL system and Petros Arakelian for his assistance in installing the optical benches and safety feature.
Published - jam_082_01_011004.pdf