Imaging Defects in a Plate with Full Non-Contact Scanning Laser Source Technique

Abstract

Previously, we used contact receiving transducers in the scanning laser source (SLS) technique for imaging defects on a plate in order to achieve a high signal-to-noise (SN) ratio. Herein, we developed a fast non-contact defect imaging technique that employs the SLS technique for in-line product inspection. Leaky Lamb waves from a plate were detected with a sufficiently large SN ratio by using low-frequency air-coupled transducers. Spurious images caused by reflected waves in the plate were removed by synthesizing images from multiple receivers. Defect images were compared for different repetition frequencies of laser emission. Images were found to be distorted at high repetition frequencies (2/3 kHz) owing to reverberations in the plate.