Nonlinear photothermal and photoacoustic processes for crack detection
Laboratoire d'Acoustique de l'Université du Maine, UMR-CNRS 6613, Université du Maine, Av. Olivier Messiaen, 72085 Le Mans Cedex 9, France
2 Laboratoire de Physique de l'État Condensé, UMR-CNRS 6087, Université du Maine, Av. Olivier Messiaen, 72085 Le Mans Cedex 9, France
We show that, using the thermal and mechanical nonlinearities of cracks, it is possible to produce nonlinear effects in the process of thermoelastic laser-generation of sound at the surface of a metallic sample. Two independent laser bursts centered on two different frequencies f1 and f2 are focused on the same spot, which can be close or distant form an artificial surface crack. By recording the acoustic response of the sample, frequency spectra obtained at different spot distances from the crack are compared. We observe that the frequency components resulting from the nonlinear process of frequency mixing (f1+f2, 2f1, 2f2, ...) are detectable for a generation on the crack but are absent out of the crack. Possible future improvements and other opportunities of crack imaging or non destructive testing methods based on these nonlinear processes are discussed.
© EDP Sciences, Springer-Verlag, 2008