Eddy current testing in thickness measurement of material with multi-layer structure

The fundamental law of the variation of the probe´s impedance in thickness measurement of layer structure material using eddy current sensors including the liftoff phenomena has been found that is the variation of the conductivity of the material which is nearest to the probe of the sample with certain thickness has the critical effect in the induced variation of the probe´s impedance in eddy current testing at multilayer material. Therewith the paper prove it by the operation results of the analytical model of multilayer structure in eddy current testing. And accordingly it explains different appearances of experiments well in multilayer material eddy current testing.