Microwave NDT Sensor for Coating Thickness Inspection on CFRP Using TM110 Odd-Mode Cavity

Manual painting is a commonly used method to coat an aircraft's exterior, and it is crucial to ensure that the thickness of the coating falls within the allowable range. Accurately assessing the thickness of the coating is essential to maintaining high-quality standards. The proposed microwave-based portable nondestructive technique (NDT) sensor enables direct measurement of the coating thickness on carbon-fiber-reinforced polymer (CFRP) using resonance frequency shift. The method is based on the TM110 odd-mode electric field coupling between a dielectric-loaded cylindrical cavity and CFRP coatings through a circular ring aperture. The workings of the cavity sensor are presented using an equivalent circuit model (ECM) with even- and odd-mode electric field coupling. The performance of the sensor is validated using hardware measurements and electromagnetic (EM) full-wave simulations. In the validation process, three different commercially available coating samples—polyethylene (PE), PE terephthalate (PET), and PTFE (Teflon)—each with known coating thicknesses applied to a CFRP surface are used for simulation and measurement. Finally, a machine learning (ML) approach is used to predict the coating thickness from simulated and measured data. The performance of the sensor has an average sensitivity of approximately 325 MHz/mm and coating thickness estimation errors within 1.83% for the third-order regression model. The approach outlined in this work provides on-site assessment of coating thickness measurements for composite structures, particularly in aircraft and other applicable scenarios.