The Thin-Skin Electromagnetic Field Near a Surface Crack in a Ferromagnetic Metal

  • A. M. Lewis
  • R. Collins
  • D. H. Michael
Part of the Review of Progress in Quantitative Nondestructive Evaluation book series


A problem fundamental to all electromagnetic methods of NDT is that of modeling accurately the changes in an applied field that are produced by a flaw. The structure of such fields depends significantly on two dimensionless numbers. The first of these is the ratio of the skin-depth δ to the global length scale of the perturbed field, which is given by the crack dimension l Many studies relate specifically to thin-skin fields where δ/l « 1 and the present study is also in this context. Within the classification of thin-skin fields, widely different surface distributions can arise depending on the value of the second parameter m=l/µrδ where µr is the relative magnetic permeability of the metal specimen [1]. Values of m vary from very small to very large depending on the material properties and the operating frequency. In the Wolfson Unit at University College London a great deal of work has been done at frequencies around 103 Hz on cracks in ferromagnetic steels where µr is large enough to make m small. The limiting form of surface field obtained as m- 0 is then the surface unfolded field [1,2]. On the other hand, many other investigations have been carried out on non-magnetic materials whereµr=1 so that m is large in thin-skin situations (e.g. Auld et al. [3]). In this case the algorithm for constructing the surface field around a flaw is based on a Born approximation which considers the surface field outside a surface-breaking crack to be unchanged by the presence of the flaw.


Crack Face Uniform Field Surface Field Crack Shape Relative Magnetic Permeability 
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  1. 1.
    A. M. Lewis, D. H. Michael, M. C. Lugg, and R. Collins, ‘Thin-skin electromagnetic fields around surface breaking cracks in metals’, J. Appl. Phys. 64 (8), 1988, 3777–3784.CrossRefGoogle Scholar
  2. 2.
    A. M. Lewis, D. H. Michael, M. C. Lugg, and R. Collins, ‘Thin-skin electromagnetic fields around surface breaking cracks in metals’, Rev. Progress in QNDE, 8A, 1989, 237–244.Google Scholar
  3. 3.
    F. Muennemann, B. A. Auld, C. M. Fortunko, and S. A. Padget, “Inversion of eddy-current signals in a non-uniform field”, Rev. Progress in QNDE 2B, 1983, 1501–1526.Google Scholar
  4. 4.
    R. Collins, D. Mirshekar-Syahkal and D. H. Michael, ‘The mathematical analysis of electromagnetic fields around surface flaws in metals’, Rev. Progress in QNDE, 2B, 1983, 1255–1268.Google Scholar
  5. 5.
    M. McIver, private communication.Google Scholar

Copyright information

© Springer Science+Business Media New York 1990

Authors and Affiliations

  • A. M. Lewis
    • 1
  • R. Collins
    • 1
  • D. H. Michael
    • 1
  1. 1.Wolfson NDE Unit Department of Mechanical EngineeringUniversity College LondonLondonUK

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