Nonlinear optoacoustics method for crack detection and characterization

• Main Author: Mezil, Sylvain
• Language: English
• Published: Université du Maine 2012
• Subjects: [PHYS:COND:CM_GEN] Physics/Condensed Matter/Other; [PHYS:COND:CM_GEN] Physique/Matière Condensée/Autre; Nonlinear acoustic
• Online Access: http://tel.archives-ouvertes.fr/tel-01037936; http://tel.archives-ouvertes.fr/docs/01/03/79/36/PDF/2012LEMA1029.pdf

This thesis deals with crack detection by a nonlinear optoacoustic method. The samples are glass plates containing a centimeter length and micrometer thick crack. The developed method is based on the absorption of two light beams, independently modulated, and focused at the same location on the sample. This causes the generation of two waves, by thermal expansion. The first one is a thermoelastic wave at low frequency fL (~Hz), and the second is an acoustical one at high frequency fH (tens of kHz). When a crack is present in the heated zone, the thermoelastic wave can make it breathe. The crack is expected to close (open) when the intensity of the heating laser modulated at fL is high (low). This breathing influences the acoustic wave generated in the vicinity of the crack at fH. It results a nonlinear frequency-mixing process, leading to the generation of new frequencies: fH±n fL (n=1,2,...). The detection of these mixed-frequencies indicates the presence of a crack.