Half-wave coaxial resonator sensor for coating thickness measurement on carbon fibre reinforced polymer composites

In this work, the development of a microwave sensor for precise dielectric coating thickness measurement on carbon fibre-reinforced polymer (CFRP) composites is presented. A non-destructive testing (NDT) based microwave half-wave coaxial resonator (HWCR) sensor is proposed to directly measure the coating thickness on the CFRP surfaces by detecting the changes of resonance frequency shift. This sensing mechanism is based on the change in capacitive response, which is jointly induced by the fringing fields and the parallel-plate capacitance between the sensor’s capacitive tip and the CFRP surface, and giving rise to measurable frequency shifts. The working principle of the sensor is described using an equivalent circuit model (ECM) and validated through both ECM and full-wave electromagnetic (EM) simulations. Additionally, a neural network (NN)-based machine learning (ML) strategy is employed to calibrate the measured data and improve the accuracy of coating thickness prediction. The PET coating exhibits the lowest sensitivity and highest estimation error among the tested materials (PTFE, PET, and PE), with values of 436.3MHz/mm and 0.96%, respectively, in the 0–1.5mm thickness range, and 1761MHz/mm and 1.27%, respectively, in the 0–0.15mm range.