Quantitative analysis of voids in multi-layer bonded structures based on transmitted laser ultrasonic waves

A laser ultrasonic method based on transmission has been investigated to characterize voids in the bonded layer and its corresponding quantitative strategy has been proposed to feed back accurate manufacturing information on multi-layer metal bonded structures. Characteristics of laser ultrasonic waves obtained at epicentre in a bonded joint were analyzed and interaction of laser ultrasound with voids in the bonded layer was explained with aids of simulation results and experimental data. The longitudinal wave amplitude gradually increases and then decreases with the increase of distances off epicentre, while the shear wave amplitude shows a monotonic decline with distances off epicentre rising. Moreover, the relative sensitivity has been proposed to quantitatively measure the sizes of voids and its variation is from −2.48 dB to −2.44 dB with defects of 3 mm to 15 mm in diameter. The laser ultrasonic C-scan result based on shear waves with transmission can find the small void with 3 mm size and other natural defects. The proposed quantitative method is effective for measurement of void sizes. As a result, laser ultrasonic C-scans on basis of transmitted shear waves jointed with the proposed quantitative method have great potential for quantitative characterization of voids in bonded structures.