Acfm above a Hemispherical Pit in an Aluminum Block

Much of the recent research at the NDE Centre at UCL has revolved round the study of the scattering of thin skin electromagnetic fields by a fatigue crack. The early work concentrated on the perturbations to the surface electric field (ACPD), while the more recent work involved the scattered magnetic field (ACFM). The Centre has recently become involved in a collaborative project, a part of which involves the detection and sizing of pits in mild steels using ACFM and other inspection techniques, and the purpose of this paper is to study the complementary problem of pit detection in non-magnetic conductors. The case of a hemispherical pit was initially chosen on grounds of simplicity, since the modeling work can be done in spherical polar coordinates, whereas the more general case of a spherical cap would require the use of bipolar coordinates. As was the case with most of the fatigue crack modeling, it was assumed initially that the unperturbed magnetic field was uniform.