Swept-frequency eddy current testing to characterize a nonmagnetic metallic plate and a nonconductive coating over it

摘要

In the measurement of a metallic plate's dimension by eddy current testing, the plate's electrical conductivity is required but practically unavailable. This study investigated the application of swept-frequency eddy current testing (SFECT) to measure the thicknesses of the plate and a nonconductive coating over it, even without knowing the plate's conductivity. Hinted by the known analytical expressions available for the calculation of the impedance of an air-cored coil above a plate, we found that the maximum phase angle of the normalized SFECT signal is relevant to the thickness of a conductive plate and the liftoff, regardless of the conductivity, and can be applied to thickness measurement. Analytical and experimental studies also showed that the conductivity of a nonmagnetic plate can be measured nondestructively by using SFECT signals in the high frequency range.