Surface electromagnetic fields around surface flaws in metals

Abstract

In an earlier paper⁽³⁾ it was shown mathematically and confirmed by experiment that when a uniform surface alternating conduction current is incident on a spherical cap indentation at the surface of a metal there is no perturbation to the current flow on the plane surface outside the cap. It is shown here that this result is true of all axisymmetric surface cavities which are interrogated by any form of surface Laplacian current flow. The result applies, for example, to surface cavities in the form of right cylindrical stopped holes or to conical pits. The paper describes experiments confirming the result in those two cases for uniform incident currents. It also describes the case of an annular cylindrical hole where the theory predicts that a uniform surface current of lower strength than the incident uniform current is established on the central part of the surface plane. This gives rise to a linear variation in surface probe readings in a center line traverse which is also confirmed by experiment. These results are in marked contrast to the perturbations to the surface currents produced by surface cracks, which have been described in earlier work⁽²⁾ by the use of unfolding algorithms.