Amongst others, the cause of this can be surface contaminations with a thickness of an atomic layer, leading to a reduced interlaminar shear-strength. As a consequence the part of the disturbed microstructure in the total volume of interaction of the NDT technique applied is extremely small and hence a low signal-to-noise ratio is obtained.

For some years researchers try to overcome the situation discussing the applicability of nonlinear ultrasonic phenomena. The bond is modelled by a distribution of springs whose spring constants are distant-dependent and whose magnitudes represent the local bond strength. As a consequence, the insonification by ultrasound at frequencies in the MHz range provides the generation of second and higher harmonics in a defective bond area in reflection and transmission.

We have carried out transmission experiments, and indeed found such a behaviour in various bonds. Analysing the measurements, we found that the sum of the amplitudes of the higher harmonics is a measure of bond strength. Furthermore, mixing of two ultrasonic frequencies can be performed, as the individual points of the contact area of the bond can be viewed as an ensemble of mechanical diodes, the transfer function of which is nonlinear. Of course, momentum and energy must be conserved in the mixing process with the result that the sum and difference frequencies are emitted at the bond length under well-defined angles in agreement with experimental results. This technique has been demonstrated on several joined materials, especially on nickel coated aluminum. Besides reporting our results, an overview on NDT of adhesive bonds is given.