The prediction and measurement of the ultrasonic reflection coefficient from interlayers in adhesive joints

The nature of the thin interlayer between the adhesive and the adherend in adhesive joints with aluminium adherends is one of the major determinants of the strength and durability of the joint. It is therefore of great interest to monitor the properties of this layer and predictions presented at the 1992 Ultrasonics Symposium showed that measurement of the ultrasonic reflection coefficients from the interlayer, particularly at oblique incidence, is likely to be the most attractive practical technique. This paper presents experimental reflection coefficient measurements on joints with different adherend surface preparation procedures, and hence different interlayer properties. The results agree well with the predictions made using a transversely isotropic model of the interlayer. The anisotropy of the interlayer is produced by the pore structure of the oxide which is grown on the adherend surface during the preparation procedure and it is shown that the porosity of the oxide has a major influence on the detectability of the interlayer by reflection coefficient measurements