Microwave Coaxial Line Cavity Resonator Sensor for Thickness Measurement of Coatings on Fiber-Reinforced Polymer Composites

A novel microwave coaxial line cavity resonator sensor has been developed to measure the thickness of coatings on carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) composites, which are widely used in aerospace and wind energy industries. Due to the symmetric electromagnetic field distribution, the sensor is insensitive to the anisotropy of the composite. Unlike the existing microwave sensors that only work for CFRP, this sensor demonstrates promising results for GFRP composites, expanding its applicability. Experimental results show a consistent increase in the resonance frequency with increasing coating thickness, with prediction errors of 1.87% for CFRP and −0.40% for GFRP, respectively. Furthermore, a wireless measurement system, including a low-cost, portable microwave analyzer and a single-board computer, is proposed to enhance the use for on-site coating assessment of fiber-reinforced polymer composites.