Research Papers

Pressure-Dependent, Infrared-Emitting Phenomenon in Hypervelocity Impact

[+] Author and Article Information
Jonathan M. Mihaly

Graduate Aerospace Laboratory,
California Institute of Technology,
Pasadena, CA 91125
e-mail: jmmihaly@caltech.edu

Jonathan D. Tandy

University of Leceister,
Leicester, UK
e-mail: jt245@leicester.ac.uk

A. J. Rosakis, M. A. Adams, D. Pullin

Graduate Aerospace Laboratory,
California Institute of Technology,
Pasadena, CA 91125

1Corresponding author.

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. Editor: Yonggang Huang.

J. Appl. Mech 82(1), 011004 (Jan 01, 2015) (9 pages) Paper No: JAM-14-1242; doi: 10.1115/1.4028856 History: Received June 02, 2014; Revised October 17, 2014; Accepted October 21, 2014; Online November 19, 2014

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.

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Fig. 2

CDF of an IR image pixel grayscale distribution and the p = 95% grayscale used to define the image threshold value

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Fig. 3

(a) Cropped IR image before grayscale level thresholding, (b) IR image after grayscale thresholding based on the p = 95% grayscale level, and (c) R-theta plot of the boundary pixels in the IR image and definition of the experimentally observed radius, Rexp

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Fig. 4

Radii of IR cloud expansion measured for each IR image considered

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Fig. 1

IR images for four experiments with a range of ambient chamber pressures. Times in microseconds indicate time after impact. Images shown with false color to add contrast.

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Fig. 5

Optimum value of K (lowest RMS error) versus α determined empirically by considering all five experiments for h = 1.5 mm target plates. The results for K versus α determined using only the two higher chamber pressure experiments are also presented.

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Fig. 6

Minimum RMS error as a function of α

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Fig. 7

Measured expansion radii versus the predicted radii using the Taylor dimensional analysis and empirically determined values for K and α

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Fig. 8

Predicted radii of the IR cloud expansion for each IR image using the Taylor blast wave dimensional analysis and empirically determined values for K and α

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Fig. 10

LSL system results for a double-plate target configuration. Two h = 0.5 mm target plates, with 50 mm separation, are impacted by a 5.59 mg nylon cylinder at 6.53 km/s. Timestamps shown indicate image time after impact.

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Fig. 11

Concurrent IR and LSL image results for a 6.53 km/s impact on a double-plate target configuration consisting of two 0.5 mm aluminum plates separated with a 50 mm stand-off distance

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Fig. 9

Measured expansion radii versus the corresponding predicted radius as a function of pressure for each impact experiment




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