Nondestructive evaluation with fully non-contact air-coupled transducer-scanning laser Doppler vibrometer Lamb wave system

Ultrasonic Lamb waves have been proved useful for nondestructive evaluation (NDE) due to their abilities to propagate a long distance with less energy loss as well as their high sensitivity to small defects on the surface or inside the structure. However, there are still many challenging tasks for Lamb wave based NDE due to the complexity involved with Lamb waves propagation and the complexity caused by coupling layer used in traditional contact-type transducers. This paper established a fully non-contact Lamb wave NDE system by using a non-contact air-coupled transducer (ACT) which can eliminate the need for couplant/adhesive and actuate pure fundamental A0 mode Lamb wave; and a non-contact scanning laser Doppler vibrometer (SLDV) for sensing which can provide high spatial resolution wavefield data. Through ACT, pure A0 mode was actuated at selected ACT incident angle based on Snell’s law. By SLDV sensing, multi-dimensional wavefields for one-dimensional or two-dimensional wave propagation were obtained and further used for Lamb waves’ characterization analysis. A specimen with a through-thickness crack was manufactured and adopted to evaluate the inspection capability of the ACT-SLDV system. Waves with normal incident and aligned incident w.r.t the longitudinal dimension of the crack were investigated, and our results showed that cracks were detected successfully in both cases. Moreover, crack lengths can be quantitatively evaluated for both situations with 10% error.