Microwave cavity resonator sensor with an octagonal cross section for thickness measurement of coatings on carbon fibre composites

Abstract

Manual painting is commonly conducted on an aircraft's exterior, and the coating thickness should be within the allowable range. For quality control, the thickness requires accurate assessment. However, the current non-destructive testing techniques for metallic substrates cannot be well suited to the widely used carbon fibre-reinforced polymer composites, which are less conductive. Here we present a new microwave cavity resonator sensor, in which an octagonal cross section (an intermediate shape between a cylinder and a sphere) is adopted to easily eliminate mode degeneracy. A design strategy incorporating electromagnetic simulation is presented, and an accurate model is developed for the extraction of the resonance frequency and quality factor from the asymmetrical responses. In the tests, the open cavity is placed on a composite sample, and coating thickness change causes cavity wall impedance perturbation, leading to resonance frequency shift. From the experiments, a linear relationship is seen between the coating thickness difference and resonance frequency shift, thereby facilitating the estimation process. As the resonant electromagnetic fields are similar to those of the cylindrical TE011 mode, insensitivity to the conductivity and anisotropy of the composite is also demonstrated, enabling convenient implementation. Reasonable accuracy is obtained for thicknesses up to 2 mm. In the prediction of two- and multi-layered coating cases, errors within ±5% are obtained. The sensor developed offers efficient on-site measurement of paint coatings on carbon fibre composite structures.