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Interlayer Crack Detection Results Using Sliding Probe Eddy Current Procedures

David G. Moore and Floyd W. Spencer
Sandia National Laboratories[Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-ACO4-94-AL85000.], P.O. Box 5800 MS-0615
Albuquerque, New Mexico 87185 U.S.A.
Telephone (505) 844-7095 E-mail dgmoore@sandia.gov.



Test Specimen Characteristics and Equipment Used

Outline of the Steps in the Boeing 53-30-11 Procedure:

Analysis and Inspector Results Summary

Conclusions and Observations

  1. The data gathered from this experiment clearly show that a traditional 2-parameter binary regression model provides an inadequate fit to many of the individual inspection results. In many cases, those models would indicate that a 90 percent detection crack length would be estimated as being larger than a set of the largest cracks in the test specimen, even though these large cracks were all detected. We have introduced the fitting of a 4-parameter model to capture inspection behaviors that result in hits at smaller cracks that are independent of crack length and misses at large cracks that are independent of crack length. Thirty-two percent (18 of the 56) of the individual inspections data were better fit at a statistical significance level of 0.05 by the addition of a parameter allowing for hits independent of crack size. An additional 16 percent (9 of 56) were better fit by the expanded model, but the better fit was not statistically significant.
  2. As is often the case, there was substantial inspector-to-inspector variation in this experiment. This variation indicates that fundamentally different inspections are being carried out from one inspector to another.
  3. For the interlayer inspection, many of the inspectors failed to follow the Boeing procedures. Only a few of the inspectors implemented the checks that were included in the procedures to give a better inspection in the presence of tear straps. Also, many of the inspectors only used an absolute threshold for the rise of the signal, and did not include calls based on signal loop width, as called for in the procedures.
  4. Inspectors, for the most part, did not use a non-conductive straightedge to aid in keeping the probe centered over the rivets. The nature of the experiment did not allow a direct measurement of how much, but being off-center at individual rivets was an oft-observed phenomenon.
  5. The data show that the factor of tear strap presence or absence and the factor of total skin thickness do have an effect on the outcome.



  1. Berens A P, et. al., (1988) A User's Manual for Probability of Detection Software System (POD/SS), UDR-TR-88-12, UDRI, Dayton, Ohio. USA
  2. Moore D G and Spencer F W, (1998) Detection Reliability Study for Interlayer Cracks, Airframe/Engine Maintenance & Repair Conference Society of Automotive Engineers, Inc.
  3. Spencer F W, (1998) Identifying Sources of Variation for Reliability Analysis of Field Inspections, RTO Workshop - Airframe Inspection Reliability Under Field/Depot Conditions, Brussels, RTO-MP-10, RTO/NATO.
  4. Spencer F W, (1998) Fitting Probability of Detection Curves to Single Inspector Hit/Miss Data, ATA NDT Forum, Albuquerque, New Mexico, USA

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