Effect of Phase Delay on Low Frequency Operation of Flux Focusing Eddy Current Probe
The operation of the Flux Focusing Eddy Current Probe has been found to yield critical information on the thickness of the material being inspected [1–2]. The design of the probe forces the low frequency magnetic fields to diffuse through the sample in order to link with the pickup coil. An attenuation of the magnetic field results such that the pickup coil output is inversely related to the material thickness . In extending the technique to thicker and/or layered materials, however, an apparently anomalous behavior is sometimes seen in which a small increase in the probe output occurs with increasing material thickness. This paper will clarify the underlying principles involved with the probe during low frequency operation and explain the apparent anomaly in terms of the phase shifting of the magnetic field with diffusion depth. A phasor addition model will be presented which accounts for the observed experimental results, and implications of the phenomena on material testing will be discussed.
KeywordsPlate Thickness Skin Depth Material Thickness Probe Output Pickup Coil
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- 1.B. Wincheski, J. R Fulton, S. Nath, N. Namkung, and J. W. Simpson, “Self-Nulling Eddy Current Probe for Surface and Subsurface Flaw Detection,” in Materials Evaluation, Vol. 52/Number 1 (January 1994).Google Scholar
- 2.B. Wincheski, M. Namkung, J. R Fulton, and S. Nath, Review of Progress in QNDE, Vol. 13B, 1611, Plenum Press, New York, 1994.Google Scholar
- 3.J. P. Fulton, B. Wincheski, S. Nath and Min Namkung, Review of Progress in QNDE, Vol. 14B, 2317, Plenum Press, New York, 1995.Google Scholar
- 4.J. R Fulton, B. Wincheski, S. Nath, and M. Namkung, “Corrosion Detection in Airframes Using a New Flux-Focusing Eddy Current Probe” in Proceeding of ASNT 1994 Spring Conference, 34, 1994.Google Scholar
- 5.J. P. Fulton and B. Wincheski, “A New Electromagnetic Technique for Quantifying Material Loss in Aircraft Lapjoints,” presented at 1995 Review of Progress in QNDE, Seattle, Washington.Google Scholar
- 6.J. D. Jackson, Classical Electrodynamics, Second Edition, John Wiley & Sons, Inc., New York, 1975.Google Scholar
- 7.H. L. Libby, Introduction to Electromagnetic Nondestructive Test Methods, Robert E. Krieger Publishing Co., Inc., Malabar, Florida, 1971.Google Scholar