Nonlinear non-collinear ultrasonic detection and characterisation of kissing bonds

The development of cost effective and reliable bonded structures ideally requires an NDT method to detect the presence of poor quality, weak bonds or kissing bonds. If these bonds are more compliant in tension than in compression stress-strain nonlinearities provide a possible route to detection with the use of nonlinear ultrasonic techniques. This paper focuses on the kissing bond case and the resulting contact acoustic nonlinearity of the interface. A kissing bond is created by compression loading of two aluminium blocks. Non-collinear mixing of two shear waves producing a sum frequency longitudinal wave is the method of stimulation of contact acoustic nonlinearity in this research. The parametric space of the nonlinear mixing is measured in terms of interaction angle of the input beams and the ratio of their frequencies creating a 'fingerprint' of the sample's bulk and interface properties in the region where the beams overlap. The scattering fingerprint of a classically nonlinear solid is modelled analytically and a kissing interface is modelled numerically; these results are compared with experimentally measured values. The experimental interface is tested with varied interfacial loading, resulting in an increase in scattering amplitude as load is increased. Secondary peaks in the parameter space also appeared as loading increased, as well as other changes in the fingerprint pattern.