Ultrasonic Lamb wave–based disbond detection and localization in adhesive lap joints using laser Doppler vibrometer

Abstract

Adhesive lap joints in thin metal sheets are widely used in automotive, marine, and aerospace industries due to their advantages in weight reduction and effective load transfer. However, defects from manufacturing, environmental exposure, and adhesive aging can cause disbonds, threatening structural integrity. This study presents a streamlined experimental methodology for disbond detection and contour assessment in adhesive lap joints using a laser Doppler vibrometer (LDV). The approach reduces A-scan requirements and computational effort compared to conventional high-resolution imaging methods, which rely on dense A-scan acquisition and heavy data processing. Ultrasonic wavefield analysis reveals disruptions and reflections near disbonded areas. Initial detection is based on contact acoustic non-linearity, where higher harmonic content in Lamb wave signals indicates disbond presence. Upon confirmation, the joint is scanned along multiple parallel paths, capturing out-of-plane A-scan responses via LDV. The Hilbert transform is applied to obtain signal envelopes, from which time-of-flight (ToF) information is extracted. Disbond lengths are first estimated by analyzing ToF delays of the $A_0$ mode as it passes through disbonded regions. Then, the ToF of reflected wave components is used to identify precise disbond edge locations, allowing accurate localization along each scanning path. Integrating these results across all paths enables reconstruction of the disbond contours. This novel and simplified technique allows accurate defect characterization with reduced scanning and processing requirements, offering a promising and efficient tool for structural health monitoring of thin-sheet adhesive lap joints.