Analysis of Eddy Current Testing Signals' Frequency Responses and Conductivity-Invariant Crack Sizing

The eddy current testing (ECT) signals of a crack change with the crack's geometry and electromagnetic properties. The conductivity of the specimen and the conductivity inside or around the crack are generally unknown but play important roles in ECT responses. The objective of this study is to find characteristics features to determine the depth of cracks independently of the material conductivity and the partial conductivity of the cracks. Theoretical and numerical analysis showed that the amplitudes of the intrinsic impedance signal or the magnetic field density signal vary little with frequency. By identifying nearly conductivity-invariant features in the spectra of magnetic flux density signals and constructing a master curve that correlates the characteristic features with crack depth, we will be able to approximately estimate the depth of a crack by referring to the pre-constructed master curve.