This dissertation considers the usage of non-linear non-collinear mixing as a non destructive evaluation tool of aircraft materials, i.e. aluminium alloys and carbon fibre reinforced plastic composite materials. Non-collinear mixing is a non conventional ultrasonic evaluation method which is under the non-linear methods. Non-linear ultrasonic techniques exhibit higher sensitivity to defects smaller than a wavelength, that they are currently undetectable with the conventional ultrasonic methods. It occurs when two intersecting elastic waves interact in an homogenous material. When the resonance conditions are met, a third wave is generated with frequency equal to the sum of the two input frequencies. For this reason, the non-collinear mixing was selected to investigate sensitivity over kissing bonds, a type of defect of great interest in the aerospace industry mainly due to the luck of conventional procedures that can detect them. Initially, a set-up of two partially closed interfaces was designed to simulate a kissing bond; an adhesive-metallic and a metallic-metallic interface. It was shown that the kissing bond in adhesive joints was highly non-linear with inconclusive results whereas the metallic-metallic interface showed higher sensitivity. In order to investigate this further, the use of phased arrays was selected. Subsequently, basic modelling of the linear field of the phased arrays was computed as well as theoretical calculations of the optimum interaction angles, depths and sub-aperture groups. These results were later used, where experimental application of non-collinear mixing was performed with phased arrays. A successful interaction was performed along with a research of the effect of dependant variables, i.e signal gain, elapsed time, coupling and alignment. Finally, the sensitivity of non-collinear mixing was tested in carbon fibre reinforced plastic contaminated bonds with different fracture toughness values, with the signal amplitudes showing expected trends but overall limited sensitivity.
|Date of Award||19 Mar 2019|
- The University of Bristol
|Supervisor||Anthony J Croxford (Supervisor) & Simon A Neild (Supervisor)|