Influence of the base temperature of photothermal modulation on the characteristics of optoacoustic nonlinear acoustic signal

The traditional laser ultrasonic non-destructive testing technology has been able to achieve non-contact nondestructive testing of cracks with a width of submillimeter, but it is not sensitive to smaller cracks. However, the nonlinear laser ultrasonic testing technology can realize the nondestructive testing of microcracks. Among them, the nonlinear optoacoustic frequency-mixing technology based on thermal modulation has been widely concerned by scholars. In this paper, an optoacoustic nonlinear frequency-mixing experiment platform was built. On the one hand, the relationship between the characteristics of the nonlinear optoacoustic signal and the power of the excitation source and the pump source was studied. On the other hand, the nonlinear phenomenon affected by base temperature was discussed. Besides, the influences of the base displacement generated by the base temperature on the transformation of the nonlinear mechanism in micro cracks was studied, which will be considered to provide a reference for further research on the contact mechanism of microcracks in nonlinear optoacoustic frequency-mixing technology, and also provide a theoretical basis for the semi-quantitative and quantitative characterization of cracks by using nonlinear optoacoustic methods.