Thickness Measurement of Metal Plates Using Swept-Frequency Eddy Current Testing and Impedance Normalization

Electromagnetic nondestructive testing signals generally change with the electromagnetic properties of the test objects. In the determination of metal thickness by electromagnetic nondestructive testing methods, the material's electrical conductivity and magnetic permeability are required but practically unavailable or inexact. This paper investigated the use of swept-frequency eddy current testing (SFECT) and impedance normalization to measure a metal plate's thickness, without the prior knowledge of its exact conductivity and permeability. Analytical and experimental studies were conducted separately on nonmagnetic and ferromagnetic metal plates. For nonmagnetic metal plates, it was found that the plate thickness can be determined by using the extreme values (maximum or minimum) of the normalized SFECT impedances' phases. For ferromagnetic metal plates, phase and resistance of very low frequency normalized impedance were found to be 'permeability-independent,' and a conductivity-insensitive function was constructed. The thickness of a ferromagnetic plate was evaluated by using this function. This paper concludes that it is possible to measure a metal plate's thickness by SFECT and impedance normalization even without the prior knowledge of the conductivity and permeability of the test object.