AN IMPROVED METHOD OF MAGNETIC FLUX LEAKAGE INSPECTION FOR FAR- SIDE CORROSIVE TYPE DEFECTS OF FERROMAGNETIC SPECIMENS
Kazuyoshi Sekine and Akira Iizuka Department of Safety Engineering, Faculty of Engineering Yokohama National University, 156 Tokiwadai, Hodogayaku, Yokohama, Japan 240
Keywords: Magnetic flux leakage testing; far-side defect, corrosion pit; Hall probe
ABSTRACT
The NDT technique by measurements of magnetic flux leakage (MFL) is a standard one which has been widely used on the ferromagnetic materials, especially for the inspection of surface breaking or subsurface flaws existing on the near-side surface of the test specimen. Recently, much attention become to be paid on the application of this MFL technique to detecting or characterizing the so-called "far-side defects", for instance, the external corrosion pits or locally corrosive damages of pipes in soil and of oil storage tank bottom plates. A problem associated with this application of MFL technique for far-side defects is considered that the strength of far-side magnetic leakage field due to corrosive type defects is not high enough for reliably detecting small size of defects. Concerning this problem, the present paper describes an original method, which makes it possible to enhance the leakage field strength in the neighbourhood of far-side defect or to improve the signal amplitudes in terms of placing the magnetically supporting plate behind the Hall probe element. This enhancement effect of far- side magnetic leakage field strength by the magnetically supporting plate can be done by creating a lower magnetic reluctance path for field line within the magnetic leakage region. The advantages of this method have been verified by the experimental measurements for far-side MFL field by means of the Hall element sensor having the magnetic supporting plate, in which the signal strength for the normal components of the far- side MFL field can be higher. Finally, the possibilities of application of the proposed method to other targets in the MFL NDT techniques are discussed.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 3, pages 1627 - 1630 Publisher:Ashgate Publishing Company
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