Noncontact monitoring of immersed plates by means of laser-induced ultrasounds

Abstract

This article presents the results of an experimental and numerical study where guided ultrasonic waves were used for the structural health monitoring/nondestructive evaluation of an immersed aluminum plate. Leaky Lamb waves were generated by means of a pulsed laser and detected by an array of immersion transducers. The signals were then processed using continuous wavelet transform to extract few damage-sensitive features that were fed to an unsupervised learning algorithm based on outlier analysis. The experimental setup was simulated numerically using a commercial finite element software to predict the time of arrival of the propagating modes. In order to assess the capability of the monitoring system to detect damage, four defects were devised on the plate prior to the immersion in water. We found that the noncontact probing system and the signal processing enable the detection of cracks and holes.