There have been many applications of adhesively bonded structures in the aerospace industry and a need for nondestructive evaluation of bond strength is evident. Standard strength measurement of a lap joint requires loading until the weakest link fails. That weak link may be an adhesive interface failure, as shown by the fracture surface of an AF-191 specimen (Fig. 1), or a cohesive failure of the adhesive layer itself, as shown in the delamination of a two layer FM-300 adhesive containing carrier cloth (Fig. 2). In recent years there were several attempts to use ultrasonic measurements (e.g., velocity, attenuation, or reflection coefficients) to evaluate bond strength. The weak link that affects strength, however, may not significantly affect these linear parameters.
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