Detection and sizing of disbond in multilayer bonded structure using modally selective guided wave

Bonded structures are frequently adopted in structural connections and are highly prone to degradation or decrease of interfacial strength due to adhesive aging, poor quality of surface preparation, as well as the exposure to harsh environment and external loading. This study addresses the establishment of a framework in which a modally selective ultrasonic guided wave is used for disbond identification and sizing. In this framework, the propagating and evanescent modes of ultrasonic guided waves are first obtained, followed by the excitability analysis for each ultrasonic guided wave propagating mode, providing a theoretical basis for effective wave excitation in the experiment. Then the interaction of ultrasonic guided wave with disbond is interrogated analytically using a method combining semi-analytical finite element and normal mode expansion, whereby wave transmission, wave reflection, and mode conversion can be calculated quantitatively. Taking all these aspects into account, mode 11 at around 3.85 MHz features a high propagation velocity, large mode excitability, and increasing amplitude drop with the enlargement of disbond size, and is thus selected for disbond detection. Both numerical and experimental validations are performed, in which disbonds of different lengths from 10 to 40 mm are examined, and the results well corroborate the effectiveness of the proposed framework for ultrasonic guided wave–based disbond detection.