Non-contact microcrack detection via nonlinear Lamb wave mixing and laser line arrays

Abstract

In this study, a non-contact nonlinear Lamb wave mixing technique based on laser line-array excitation is developed for microcrack detection in plate-like structures. Specifically, a pulsed laser with a line-array pattern (LAP) source is created to generate two narrowband Lamb waves with distinctive frequencies; subsequently, a vibrometry is used to measure the corresponding ultrasonic responses. The presence of microcrack caused nonlinear wave mixing between the two generated Lamb waves; consequently, nonlinear mixed components are generated at the sum and difference of the two input frequencies. The novel contributions of this study are as follows: (i) a fully non-contact laser ultrasonic system with a Sagnac interferometer is developed for narrowband Lamb wave generation; additionally, a vibrometry is employed to identify the mixed components; (ii) the proposed system allows two Lamb waves with distinctive frequencies to be simultaneously generated by a single laser source; (iii) the proposed system allows the user to selectively implement the Lamb wave mode at the desired input frequency by simply adjusting the optical lens; and (iv) unlike the slit mask method, the Sagnac interferometer system yields laser LAP sources that can produce sufficient energy (exceeding 80%) to generate input waves in structures. Moreover, the performance of the developed laser ultrasonic system is experimentally validated by applying it to aluminum specimens containing microcracks. The results indicate that the proposed system can locate and detect microcracks in a plate by scanning the wave-mixing zone.