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The eddy current tests of ferromagnetic materials are theoretically analyzed employing a Globus model for the magnetization process, and the impedance change of a coil due to the presence of flaws contained in a magnetic conductor is derived employing Green’s function technique. The results show that for an excitation field which is lower than the critical field, the impedance change is maximum when the residual magnetization (or the dc magnetic field applied for the magnetic saturation of the conductor) is absent but for a large field the impedance change becomes maximum when the appropriate dc magnetic field in which the differential permeability reaches its maximum value is applied. For small surface flaws, the volume inhomogeneity mainly contributes to the reactance change of the coil whereas the discontinuity in magnetic permeability contributes to both real and imaginary parts of the impedance change. The dependence of the impedance change on flaw shape is shown explicitly in the derived formula.

The impedance of a test coil was measured as a function of the dc current of an electromagnet, and the determination method of magnetization level with the normalized impedance diagram is discussed. An eddy current test for the outer side of a steel tube wall was also performed. Test results show that the impedance change of the test coil is brought about by the eddy current perturbation and the change of magnetization level. These effects on the flaw inspection are also analyzed.

Magnetic effects in eddy current nondestructive method are studied employing an extension of the general ideas of Globus for the magnetization processes of polycrystalline materials. The reversible permeability depends on the residual magnetization of the material. The relation between these two quantities is derived from the Globus model. An expression is derived for the impedance of a single‐turn coil above a ferromagnetic conducting half‐space. From this expression, flaw effects and liftoff effects are analyzed, and theoretical treatment for the evaluation of residual magnetization is described.