In situ disbond detection in adhesive bonded multi-layer metallic joint using time-of-flight variation of guided wave

Adhesive bonded joints are frequently adopted in structural applications. The adhesive aging, low quality of surface preparation, as well as the exposure to external harsh environment and loading, may degrade the quality of adhesive, leading to disbond and decrease of the interfacial strength of the bonded joints. This study addresses both numerical and experimental investigations of ultrasonic guided wave (UGW) propagating in adhesive bonded metallic waveguide, whereby disbond detection is realized based on variation of the wave arrival time of UGW. First the dispersion curves of UGWs in both intact (bonded) and disbonded joints are obtained via the Semi-Analytical Finite Element (SAFE) method, and are grouped into mode pairs of phase velocity match and mis-match, respectively. Then a model combining SAFE and Frequency Domain Finite Element (SAFE-FDFE) is developed to enable excitation of any UGW of desired single mode-frequency combination and analysis of the wave interaction with disbond. The obtained results indicate that the UGW Mode 2 generated at the low frequency range of the mis-matched group shows a good sensitivity to disbond, featuring variation of the wave arrival time induced by mode conversion. Finally, Time Domain Finite Element and a proof-of-concept experiment, with comb transducers to act as both in situ actuators and sensors made of PVDF sheets embedded into the adhesive layer, well validate the results obtained via SAFE-FDFE. The selected mode-frequency combination Mode 2 at 0.52 MHz for wave time-of-arrival-based disbond detection, compared with conventional signal-amplitude-based disbond indicator using high frequency UGWs (~several MHz), merits the advantages of better controllability of wave excitation, less wave attenuation, and higher robustness.