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Abstract
An ultrasonic nondestructive technique for the quantitative determination of the cohesive properties of adhesive joints based on the measurement of the reflection coefficient from the top adhesive/adherend interface and the bondline transit time has been developed. The method requires access to only one side of the joint and, for joints with typical aerospace geometries, it can be implemented using a single transducer with a centre frequency below 50 MHz. The technique has been used to determine the longitudinal bulk wave velocity in aluminium-epoxy-aluminium joints to within ±6% of the nominal values determined from bulk samples. The bondline thickness of the samples tested was evaluated to within micrometer accuracy, and thickness variations within the scan area were detected to much better than micrometer accuracy. The method has been tested successfully on joints made with two-part epoxies and with film adhesives containing a “scrim” carrier, and it has also been shown that the different standard adherend preparation procedures have a negligible effect on the results. The method, therefore, promises to provide a reliable, nondestructive means of measuring the cohesive properties of a bonded joint and represents a significant advance on the currently available technology.