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Research Papers

Thermo-Elastic Model of Epicenter Displacement by Laser Pulse Irradiated on Metallic Surfaces

[+] Author and Article Information
Thanh Chung Truong

Department of Aerospace Engineering,
Korea Advanced Institute of
Science and Technology,
291 Daehak-ro, Yuseong-gu,
Daejeon 34141, South Korea
e-mail: thanhchung@kaist.ac.kr

Ayalsew Dagnew Abetew

Department of Aerospace Engineering,
Korea Advanced Institute of
Science and Technology,
291 Daehak-ro, Yuseong-gu,
Daejeon 34141, South Korea
e-mail: ayoudag@kaist.ac.kr

Jung-Ryul Lee

Department of Aerospace Engineering,
Korea Advanced Institute of
Science and Technology,
291 Daehak-ro, Yuseong-gu,
Daejeon 34141, South Korea
e-mail: leejrr@kaist.ac.kr

Jeong-Beom Ihn

Structures Technology,
Boeing Research & Technology,
Seattle, WA 98124
e-mail: jeong-beom.ihn@boeing.com

1Corresponding author.

Manuscript received June 15, 2017; final manuscript received September 11, 2017; published online October 16, 2017. Assoc. Editor: Wieslaw Ostachowicz.

ASME J Nondestructive Evaluation 1(2), 021001 (Oct 16, 2017) (6 pages) Paper No: NDE-17-1045; doi: 10.1115/1.4038030 History: Received June 15, 2017; Revised September 11, 2017

In recent years, there is a much interest in developing of nondestructive testing (NDT) systems using the pulse-echo laser ultrasonics. The key idea is to combine a low-power and short-pulsewidth laser excitation with a continuous sensing laser; and use a scanning mechanism, such as five degrees-of-freedom (5DOF)-axis robot, laser mirror scanner, or motorized linear translation or rotation scanner stage, to scan the combined beam on the structure. In order to optimize the parameters of the excitation laser, a realistic theoretical model of the epicenter displacement in thermo-elastic regime is needed. This paper revisits and revises the study of Spicer and Hurley (1996, “Epicentral and Near Epicenter Surface Displacements on Pulsed Laser Irradiated Metallic Surfaces,” Appl. Phys. Lett., 68(25), pp. 3561–3563) on thermo-elastic model of epicenter displacement with two new contributions: first, we revised Spicer’s model to take into account the optical penetration effect, which was neglected in Spicer’s model; and second, the revised model was used to investigate the effect of laser rise time and beam size to the epicenter displacement. We showed that a pulse laser with short rise time generates an equivalent surface displacement with a pulse laser with long rise time, except a “spike” at the beginning of the epicenter waveform; also when the laser beam size increases, the epicenter displacement decreases. These two conclusions were then validated by experiments.

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Figures

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Fig. 1

(a) Epicenter displacement produced by a short pulse laser and measured using an optical sensing beam. (b) Temporal profile g(t), spatial profile f(r), and absorption profile h(z) of the laser pulse.

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Fig. 2

Comparison of epicenter displacement with and without consideration of optical penetration: (a) aluminum material and (b) CFRP composite material

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Fig. 3

Epicenter displacement produced by laser pulse irradiated on aluminum surface with different rise time of pulse laser

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Fig. 4

Epicenter displacement produced by laser pulse irradiated on aluminum surface with different beam size

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Fig. 5

Experiment setup to measure the epicenter pulse-echo laser ultrasonics

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Fig. 6

(a) Beam profile result of 2-mm diameter laser beam and (b) beam profile result of 4-mm diameter laser beam

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Fig. 7

Experimental epicenter waveform with varied laser rise time

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Fig. 8

Experimental epicenter waveform with varied laser beam radius with a constant energy

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Fig. 9

Peak-to-peak amplitude of the normalized experimental waveform versus normalized amplitude of theoretical epicenter displacement with varied laser rise time

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Fig. 10

Comparison ratios of amplitude changes when changing laser beam radius between theory and experiment

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