The Thin-Skin Electromagnetic Field Near a Surface Crack in a Ferromagnetic Metal
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 . 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. ). 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.
KeywordsCrack Face Uniform Field Surface Field Crack Shape Relative Magnetic Permeability
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