Bimodal Microwave Method for Thickness Estimation of Surface Coatings on Polymer Composites

Two new microwave sensors, an open cylindrical cavity resonator with a T‐shaped excitation element and a flanged coaxial line sensor, are used for the nondestructive thickness measurement of surface coatings on a unidirectional carbon fiber‐reinforced polymer (CFRP) laminate. Both sensors are found insensitive to the anisotropy of the composite substrate, offering easy implementation. Three commercially available plastic films, polyethylene (PE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC), are used to simulate the coating. In the setup, the microwave sensors are directly mounted on the 4 mm‐thick composite plate. The coating affects the surface impedance of the composite, causing changes to the signal reflection. Calibration is performed by the measurement of samples with known coating thicknesses. For the cavity resonator, a linear relationship is obtained between the resonance frequency shift and the thickness; the difference between the measured and actual thickness values is around 17 μm. For the coaxial line sensor, the phase variation increases with increasing thickness. For both sensors used, the estimation error for a coating thickness in the region of 200 μm can be well within 10%.