EXPERIMENTAL STUDY OF THE VELOCITY EFFECT ON THE MAGNETIC FLUX LEAKAGE TESTING OF THE THIN STEEL STRIP
Zhongsoo Lim*, Goohwa Kim*, and Derac Son** * Research Institute of Industrial Science and Technology P. O. Box 135, Pohang, Kyungbuk, 790-600, Korea ** Physics Department, Hannam University, 133, Ojung Dong, Daejon, 306-010, Korea
Keywords: Magnetic flux leakage testing, Steel, Surface defect, NMI
ABSTRACT
The surface flaws of the thin steel strip can be inspected in various ways. But under on-line stages with high speed moving strip, it is very difficult to do the nondestructive testing. Especially for the NMI's (nonmetallic inclusions) which are embedded inside of the strip, the conventional optical testing cannot be applied. The only method which has been successfully applied for the NMI's and the certain surface defects of the steel strip at high speed condition is the magnetic flux leakage testing. But for the moving strip under magnetic saturation with the externally applied do magnetic field, the magnetic field distribution pattern could be deformed due to the movement of the strip which is an electrical conductor. Therefore for the quantitative evaluation of the leakage magnetic field for defect identification, it is necessary to know the leakage magnetic field depending on the moving speed under given environment. We have measured the leakage magnetic field from the minute defects in the steel strip under different moving speed (> 10 m/sec) using a fluxgate magnetometer (resolution is better than 1 m G) with wire core. The peak positron, peak-to-peak value and the field profile at a certain liftoff position is deformed according to the moving speed, but not linearly. The main reason for the deformation can be qualitatively explained with the motional induction current.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.full paper not received Publisher:Ashgate Publishing Company
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