Local and noncontact measurements of corrosion and adhesive disbond using zero-group velocity lamb modes

A non-contact laser based ultrasonic technique is proposed for detecting plate thickness variations due to corrosion and adhesive disbonds between two plates. The method exploited the resonance at the minimum frequency of the S1 Lamb mode dispersion curve. At this minimum frequency the group velocity vanishes, whereas the phase velocity remains finite. The energy deposited by the laser pulse, generates a local vibration of the plate. This vibration is detected at the same point by an optical interferometer. Due the finite wavelength of the S₁-ZGV mode, the spatial resolution is approximately limited to twice the plate thickness. First experiments show the ability to image a 1.5-mum deep corroded area on the back side of a 0.5-mm thick Duralumin plate. With the same technique we investigate the state of adhesive bonds between Duralumin and glass plates. The S₁-mode resonance is strongly attenuated when plates are rigidly bond. In the case of small adhesive layers, we observed others resonances, associated with ZGV modes of the multi-layer structure, whose frequencies and amplitudes vary with adhesive thickness.