Characterization of multilayered structures by swept-frequency eddy current testing

The signals of the electromagnetic non-destructive testing of a multilayered structure are functions of the dimension and the electromagnetic properties of each layer in this structure. The signals of the to-be-characterized layer should be ‘extracted’ from the composite signals. This paper proposes of separating signals of different layers by frequency band, and characterizing the layer of interest by using the correspondent signals. An analytical solution derived in terms of transmission and reflection of electromagnetic waves was used to conduct simulation of swept-frequency eddy current testing of a multilayered structure. A couple of characteristic parameters were extracted from the analysis of the signals and the differences of the signal series in the frequency series. It was found that the thicknesses of the top and lower layers can be determined respectively by using high frequency and low frequency signals. The differences of the signal series in the frequency series are insensitive to the variation of air gap between two conductive layers. The maximum difference of resistance in the frequency series can be used to characterize the lower layer regardless the change of conductivity and air gap.