Air-coupled ultrasonic NDE of bonded aluminum lap joints

This paper describes an investigation into the feasibility of using wideband air-coupled ultrasonic transducers for evaluating the integrity of bonded aluminum lap joints. Micromachined capacitance transducers are used to generate and detect ultrasonic Lamb waves in aluminum lap-joints through an intervening air layer. The transmission of Lamb waves through the bond is affected by the bond's local state, such that scanning a pair of air-transducers allows images to be produced either with the transducers on opposite sides of the plate, or on the same side. It is shown that defects in the bond-line can be detected and imaged by such a non-contract system. Confirmation of defect positions are obtained by comparing the results of the air-coupled scans to a conventional immersion ultrasonic C-scan.