Photoacoustic interaction mechanism with surface defects in freeform surface materials

The interaction mechanism study between defects in variable thickness freeform surface materials and photoacoustic signals can effectively improve the characterization ability of laser ultrasound. In this paper, we mainly focus on flat aluminum plates, curved aluminum plates and gas turbine blades with variable thickness freeform surface, and emphasize the interaction process between defects and photoacoustic signals with different frequencies from the perspective of energy spectrum. Firstly, the position and shape of defects can be obtained directly from the photoacoustic field. Secondly, by the change of the energy spectrum after passing through defects, the interaction process between defects and the photoacoustic signals is analyzed, and the depth characterization of defects is realized. The study found that after the interaction between the photoacoustic field and defects, the defects break the law of energy proportional attenuation, resulting in a change in the ratio of lowfrequency and high-frequency energy, which is independent of material thickness and structure. More specifically, defects act more on the low-frequency region of photoacoustic signals. In addition, the energy attenuation of the energy spectrum increases gradually with the increase of the defect depth, while the attenuation is more sensitive in the thin area. The research in this paper mainly focuses on variable thickness freeform surface specimens and emphasizes the interaction process between defects and photoacoustic signals with different frequencies.