Detection of surface crack in film-coated metals using an open-ended coaxial line sensor and dual microwave frequencies

Surface crack detection in metals is important to ensure the safety of the metallic structures. Because the electromagnetic wave technique can detect the surface crack in coated metals, many researches on that method have been conducted. However, the method to determine the optimal frequency for the detection of surface cracks has not been introduced. This study was conducted to find this optimal frequency and to improve detection sensitivity. As a result of scanning the film-coated metal including the surface crack using an open-ended coaxial line sensor at microwave frequencies ranging from 14 to 18 GHz, dual frequencies increasing or decreasing the reflection coefficients were found. This result could be explained based on the capacitance model of the microwave reflection. Consequently, the detection sensitivity was improved by 32.0–97.7%. The surface crack under 0.2 mm coating film was stably detected with 0.37–0.43 dB of the reflection coefficient.