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Book: (Studies in Applied Electromagnetics and Mechanics 13), Non-Linear Electromagnetic Systems (Advanced Techniques and Mathematical Methods), Editors: V. Kose and J. Sievert, IOS Press, Amsterdam (1998), ISSN: 1383-7281, pp.187-192

DUAL-FREQUENCY EDDY CURRENT NDT MEASURING TECHNIQUE IMPROVED DEPTH SENSITIVITY IN METALLIC PLATES

Cs. S. Daróczi (a) and A. Gasparics(b)

(a) Hungarian Academy of Sciences, Research Institute for Materials Science (ATKI), H-1525 Budapest, P.O. Box 49, Hungary
(b) Technical University of Budapest, Department of Electromagnetic Theory (BME), H-1521 Budapest, Hungary

Eddy Current Testing (ECT) can be used to obtain information on the inhomogeneity of the electrical conductivity of different materials along the sample surface. Due to the skin-effect the penetration depth of the exciting electromagnetic field decays with the frequency, therefore the electromagnetic field induced by the surface eddy currents are usually much stronger than the field of the eddy currents flowing in the deeper regions. If the detector is placed on the same side of the sample as the exciting coil (as usually), then it will also appear the shielding-effect which rushes down the signal originated from the deep. We have applied a simultaneous dual-frequency excitation to the sample that enables to subtract the surface part of the signal and to obtain better depth sensitivity. The subtraction can be done electronically in real time to avoid the need of the repetitive measuring of (exactly) the same area of the sample at different frequencies.

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