Flaw localization using the reassigned spectrogram on laser-generated and detected Lamb modes

Rapid, accurate inspection of metallic plates using broadband guided waves has traditionally been limited by the multi-mode, highly dispersive nature of these waves; current practice typically restricts either the type of mode generated or detected, and/or its frequency range. The current study presents an effective alternative procedure by combining a novel digital signal processing technique, the reassigned spectrogram, with laser generated and detected Lamb waves. The reassigned spectrogram is used to characterize the modal and frequency content of a single ultrasonic signal as a function of time, enabling a procedure to locate flaws in an aluminum plate specimen.