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Lift-off Point of Intersection Feature in Transient Eddy Current Oscillations Method to Detect Thickness Variation in Stainless Steel

Angani, C. S ; Ramos, H. ; Ribeiro, A. L. ; Rocha, T.

IEEE Transactions on Magnetics Vol. 1, Nº 99 , pp. 1 - 8, February, 2016.

ISSN (print): 0018-9464
ISSN (online): 1941-0069

Journal Impact Factor: 1,129 (in 2008)

Digital Object Identifier: 10.1109/TMAG.2016.2531024

Abstract
Detection of corrosion and measurement of the thickness of stainless steel in industrial structures such as pipelines has been an important issue to avoid unwanted leakages. A recently proposed transient eddy current oscillations (TECO) method has been used in this study to measure thickness in non-magnetic stainless steel. TECO method works based on electromagnetic induction principle, as the conventional eddy current testing (ECT) method. An inductor (excitation coil) connected in parallel with a capacitor to generate damped oscillations, is used to induce eddy currents into the test material beneath the coil. A Hall-sensor which is fixed at the bottom side and at the axial center of the excitation coil, detects the resultant magnetic field due to the induced eddy currents. Experiments were performed on stainless steel specimens having different thicknesses to test the potential of TECO method. The results were analyzed using Fast Fourier Transform (FFT). This manuscript proposes a lift-off intersection (LOI) point in the frequency domain that can be used to eliminate the false indications due to lift-off variations during the thickness measurement of the test material. The results reveal that the proposed method can be applied to detect thickness variations in metal structures covered with non-conductive paints or insulation coatings.