Investigation of aluminium plate damage visualization by through-the-thickness ultrasound generated and detected by lasers

Abstract

In this study, the full-field pulse-echo ultrasonic wave propagation imaging (FFPE-UWPI) technique is proposed for three-dimensional damage evaluation. To realize this objective, a full-field pulse-echo ultrasonic propagation imaging (FFPE-UPI) system has been developed using Q-switched Nd: YAG laser to generate diverse mode ultrasounds by thermoelastic mechanism at the impinging point of generation laser beam, laser Doppler vibrometry (LDV) sensor for sensing ultrasound and two axis transition stage for raster sca n of two laser beams, so that the FF-PE-UPI system allows scanning of both ult rasonic excitation and sensing laser beams simultaneously, this generation and sensing principle is repeated over the scan area. Contrary to the conventional ultrasonic propagation imaging which has imaged in-plane guided wave propagated from a single ultrasonic source or multiple source, the proposed system makes full-field ultrasound as large as the scanned area and visualizes longitudinal wave propagation through the thickness. Thus, the system allows us to evaluate the damage three dimensionally with more accurate sizing and shaping. To verify the effectiveness of the system, an aluminium plate of 4 mm with a 2 mm thickness at the milled area was tested. From the imaging results obtained from the FF-PE-UWPI technique, the damage on the tested specimen was successfully detected, localized and clearly visualized.