Damage imaging using non-contact air-coupled transducer/laser Doppler vibrometer system

In this work, a rapid imaging technique is proposed for imaging damage in metallic plates using a zero-lag cross-correlation imaging condition in the frequency domain. A fully non-contact, single-side access, hybrid inspection system employing air-coupled transducer (ACT) for the generation of anti-symmetric Lamb wave mode and laser Doppler vibrometer (LDV) for dynamic visualization of the Lamb wavefield is used to experimentally verify the proposed technique on three identical aluminum plates with same notch geometry but with various orientations at different locations. The notches (10mmx5mm) are grooved in the aluminum plates using electric discharge machining with a maximum depth of 40% of plate's thickness and with the following orientations 90 degrees, 60 degrees, and 45 degrees with respect to the horizontal axis. A damage image is constructed by cross-correlating the forward and backward propagating wavefields in the aluminum plates which are separated by analyzing the actual laser Doppler vibrometer measured wavefield using a frequency-wavenumber filtering post-processing technique. The experimental results demonstrate a strong capability of guided wave zero-lag cross-correlation imaging condition technique in localizing, sizing, and detecting the orientation of relatively small defects in the isotopic material.