Abstract
ABSTRACT The application of Eddy Current Testing (ECT) to ferromagnetic materials is challenging owing to the difficulties stemming to ferromagnetism. Due to the non-linear magnetic permeability, the use of a strong external DC magnetic field is often necessary in order to bring ferromagnetic materials close to magnetic saturation where permeability is minimised; strongly improving eddy current’s penetration in the material volume. In this work, the main application studied is ECT on a ferromagnetic tube with an Inside Diameter (ID) coil. The effect of different levels of the DC magnetic field on the coil impedance changes is investigated both experimentally and by simulation. It is shown that DC field has a systematic influence both to magnitude and phase of the Eddy Current signal. In addition, for the different defect depths, a critical level is observed, where the signals from discontinuities become visible and stand out from the noise of non-linearity. Moreover, for a range of magnetisation field level, the changes of phase angle are distinct for different defects’ depths allowing thus the application of phase analysis for defect characterisation.
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