The aim of this study was to investigate the use of swept frequency eddy current testing to measure each layer's thickness in a layered structure. Theoretical inference showed the impedance signal is an integrand of shape function and generalized reflection function. Analytical study indicated that the wavelength to maximize the shape function is an indicator of a probe's thickness measurement ability. Comprehensive investigation revealed the reflection coefficient of a layered structure could be considered a modification of that of a half space. The amount of modification is a logarithmic linear function of plate thickness and thus a characteristic feature for thickness estimation. The frequency response of a double-layered structure depends, in addition to layer-wise thickness and properties, significantly on the relation of the conductivities of the two layers. In order to evaluate two closely attached layers, we introduced a novel variable, the derivative of impedance with respect to log scaled angular frequency. Spectral analysis on impedance or the frequency derivative related quantities, such as extrema of the real or imaginary parts of the variable, suggested it is possible to determine the top layer's thickness using characteristic features taken from high frequency signals, whereas the lower layer's thickness using characteristic features taken from lower frequency signals afterwards. The characteristic quantities derived from spectral analysis are conductivity independent, implying of conductivity independent measurement. The analytical findings were experimentally verified, suggested that it is possible to determine layers' thicknesses by spectral analysis of swept frequency eddy current testing signals.
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