Nonlinear photothermal and photoacoustic processes for crack detection

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.