Nonlinear contactless optoacoustic technique for crack detection and evaluation
Résumé
Nonlinear techniques for non destructive testing provide cracks detection with high sensitivity. The frequency-mixing technique [J. Appl. Phys. 106, 036101 (2009)] is based on the absorption of two laser beams independently modulated at frequencies $f_L$ and $f_H$ (with $f_L{\ll}f_H$) and focused at the same spot, generating thermoelastic and acoustic waves, respectively. When the focusing is on a crack, the thermoelastic wave causes crack breathing, resulting in the generation of the frequency sidelobes $f_H{\pm}n f_L$ ($n=1,2,...$), absent otherwise. The technique can be implemented all-optically through detection by deflectometry or interferometry [Opt. Lett. 36, 3449 (2011)]. Here we report two-dimensional crack imaging with a 50 ${\mu}$ m resolution and a 40 dB range contrast. The theoretical model [J. Appl. Phys. 107, 124905 (2010)] explains frequency-mixing phenomena. Here we report an extension of the theory, including possible hysteresis in the crack breathing motion. The fit of the extended theory to the measured amplitudes and phases of the sidelobes provides opportunity to determine crack parameters. This research is supported by ANR project ANL-MEMS ANR-10-BLAN-092302.
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