Detection of kissing bonds with non-collinear nonlinear ultrasound

Abstract

The development of cost effective and reliable bonded structures ideally requires an NDT method to detect the presence of poor quality, weak, 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 work focuses on the kissing bond case and the resulting contact acoustic nonlinearity of the interface. The technique developed in this research is a non-collinear ultrasonic method; involving the interaction of two beams, it removes all signal related to system nonlinearity except for that produced in the region of beam overlap. Mixing of two shear waves producing a sum frequency longitudinal wave is the selected mode of interaction. The frequencies and angle between the two beams are varied during the experiment. By measuring the nonlinear mixing response as these two parameters are swept through a ‘fingerprint’ of the nonlinear properties in the interaction region can be obtained. This fingerprint is shown to contain information about the bulk material and the interface status.

To interpret the fingerprints an understanding of the bulk mixing response is first required. This was gained through modelling and experimental testing of solid metal samples. The characteristic pattern of bulk mixing could then be distinguished from that of kissing bonds which were also investigated with models and experimentally. Kissing bonds were created by compressive loading of two aluminium blocks. It was found that the bulk mixing and interface mixing occurred in overlapping regions of the parameter space and thus could not be measured independently in the compressively loaded samples.

The experimental contacting interfaces are tested with varied interfacial loading, and surface roughness. Many changes in the fingerprint patterns are observed, some suggest that non-collinear mixing does not evenly sample the area of interaction. In the process of the above investigation a new mixing phenomenon is discovered which occurs at the interface between water and the solid samples. A finite element model is developed that can replicate this mixing behaviour.

Date of Award	23 Jan 2019
Original language	English
Awarding Institution	University of Bristol
Supervisor	Anthony J Croxford (Supervisor) & Dmitry Ivanov (Supervisor)