Lamb-wave sparse-frequency interdigital-transducer-based scanning laser Doppler vibrometry for quantitative depth-wise visualization of defects in plates

Abstract

The Lamb-wave-based scanning laser Doppler vibrometry (SLDV) technique, which uses continuous excitation, is considered a promising method for visualizing defects in plate-like structures. To visualize defects, the technique commonly uses the A0 mode because of its sensitivity to thickness variations, i.e., the wave speed changes according to the thickness. However, for thick plates, the A0 mode is found to be less sensitive to thickness variations relative to thin structures. Thus, to address this issue, interdigital transducer (IDT)-based SLDV has recently been proposed to efficiently generate a symmetric mode for damage detection in thick plates. In this study, with the use of an IDT-based SLDV, we demonstrate that the detectability of shallow defects in a thick plate (6 mm) in the S0 mode is superior to that with the A0 mode. In addition, we show that by using just a single guided-wave mode, it is difficult to detect defects at different depths for a 6-mm-thick carbon steel plate. To improve the damage detection capability over the entire range of thickness variation of a thick carbon steel plate, we select three excitation frequencies (200, 450, and 600 kHz) based on the analysis of wavenumber sensitivity and the degree separation between modes (DSM). Subsequent to the fusion of the images corresponding to the three frequencies, we demonstrate that the defect detectability in a thick carbon steel plate using sparse-frequency IDT-based SLDV is improved over that with the use of the single guided-wave mode. Our findings can significantly contribute to advancements in the nondestructive testing of plates using SLDV.