Measurement of the ultrasonic nonlinearity of kissing bonds in adhesive joints

This paper presents a study of the detectability of kissing bonds in adhesive joints using an ultrasonic inspection technique to measure nonlinearity. Kissing bonds are defined as adhesive bonding defects in which the adhesive and the substrate are not fully bonded together but are in some way connected, for example, through contact between disbonded surfaces or through coupling via a very thin layer of contaminant. The result is a region that exhibits reduced normal or shear stiffness and is difficult to detect using conventional non-destructive testing techniques. It is hypothesised that when an ultrasonic wave of large magnitude propagates through the kissing bond area, the system will behave nonlinearly. This hypothesis is the starting point for this paper, and it is tested by measuring the degree of nonlinearity exhibited by perfectly and imperfectly bonded specimens. Perfectly disbonded adhesive joints are tested over a range of compressive loads from an initial zero-load ‘open’ condition to a final highly loaded ‘closed’ condition and the degree of nonlinear behaviour measured. It is shown that adhesive joints exhibit significant nonlinearity under light compressive loads and that the level of nonlinearity decreases rapidly with compressive load. Good agreement is also observed between experimental results and a nonlinear spring model of the adhesive joint, based on its quasi-static interfacial stiffness.