Eddy Current Probe Design and Matched Filtering for Optimum Flaw Detection

  • M. Riaziat
  • B. A. Auld
Part of the Library of Congress Cataloging in Publication Data book series (volume 2A)

Abstract

Eddy current signals obtained from variations in the probe liftoff are in general much larger in amplitude than the useful flaw signals. Small flaw signals can, however, be detected in the presence of liftoff noise if a large enough phase angle exists between them. Figure 1(a) shows how this phase discrimination can help in liftoff noise suppression. Here, the oscilloscope traces the complex impedance of the probe. The impedance plane has been rotated so that the liftoff noise lies entirely in the horizontal channel. Now if we choose to look only at the signal in the vertical channel of the scope, or the Q channel (in phase quadrature with liftoff), there will be no liftoff noise. This, however, is not a very realistic picture. Figure 1(b) is obtained when we try to detect much smaller flaws (in this case a closed crack of 20 mils in aluminum). We see that the trace of the liftoff noise has a curvature and that there are also fluctuations along the Q channel axis. Both of these effects eventually limit the detectability of small flaws. Since this contribution of liftoff to the Q channel is in practice larger than circuit noise, we define the detection figure of merit for an EC probe as
$$D = \frac{{(\Delta {Z_f})\sin \beta }}{{{{(\Delta {Z_{\ell O}})}_Q}}}.$$
(1)

Keywords

Microwave Ferrite Autocorrelation Reso Imped 

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References

  1. 1.
    B.A. Auld, “Theoretical characterization and comparison of resonant probe microwave eddy current testing with low frequency eddy current methods,” in Eddy Current Characterization of Materials and Structures, G. Birnbaum and G. Free, eds., ASTM STP 722, Philadelphia, 1981.Google Scholar
  2. 2.
    J.A. Stratton, Electromagnetic Theory, Mc-Graw-Hill, New York, 1941.MATHGoogle Scholar
  3. 3.
    A.J. Bahr and D.W. Cooley, “Analysis and design of eddy current measurement systems,” in Proceedings of the AF/DARPA Review of Progress in Quantitative NDE, 1982.Google Scholar
  4. 4.
    D.H. Michael, R.T. Wechter and R. Collins, “The measurement of surface cracks in metals by using A-C electric fields,” Proc. Royal Soc. Lond., A 381: 139–157, 1982.CrossRefGoogle Scholar
  5. 5.
    W.D. Dover, F.D.W. Charlesworth, K.A. Taylor, R. Collins and D.H. Michael, “The use of AC field measurements to determine the shape and size of a crack in a metal,” in Eddy Current Characterization of Materials and Structures, G. Birnbaum and G. Free, eds., ASTM STP, Philadelphia, 401–427, 1981.CrossRefGoogle Scholar
  6. 6.
    F.G. Muennemann and B.A. Auld, “Inversion of eddy current signals in a non-uniform probe field,” these proceedings.Google Scholar
  7. 7.
    B.A. Auld and M. Riaziat, “Quantitative modeling of flaw response in eddy current testing,” Ginzton Laboratory Report No. 3376, Stanford University, Stanford, CA, 1981.Google Scholar
  8. 8.
    J. Mavor, M.A. Jack, D. Saxton and P.M. Grant, “Design and performance of a programmable real-time recirculating delay-line correlator,” IEEE J. Elect. Cir. and Sys., 1: 137–144, 1977.CrossRefGoogle Scholar

Copyright information

© Plenum Press, New York 1983

Authors and Affiliations

  • M. Riaziat
    • 1
  • B. A. Auld
    • 1
  1. 1.Edward L. Ginzton LaboratoryStanford UniversityStanfordUSA

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