Accurate Thickness Measurement of Multiple Coating Layers on Carbon Fiber Composites Using Microwave Cavity Perturbation

Current techniques for evaluating the multilayered coating thickness on carbon fiber-reinforced polymer composites are suboptimal; here, we present a new nondestructive microwave method using an open cavity resonator sensor. When the open end of the cavity is positioned on a conductive polymer composite, a resonant cavity is formed. The coating perturbs the surface impedance, causing a resonance frequency shift. In the modeling, the original endplate perturbation theory for a closed resonant cavity is modified, incorporating the effect of the coating. One-, two- and multilayered coating cases are studied, and a linear relationship is revealed between the resonance frequency shift and the coating thickness change. The accuracy of the model is confirmed by electromagnetic simulation performed with CST software and actual measurements. It is shown that the proposed sensor is insensitive to the conductivity anisotropy of the composite examined, offering easy implementation. For the coating thickness estimation, errors within +/- 5% have been observed, where two reference cases are used as a simple form of calibration. The method presented here offers efficient on-site evaluation of coatings on composite structures of aircraft and other applications.