Abstract
The ACFM technique has been proved to be an extremely practical NDE method for crack detection and sizing. Its original mathematical model is based on a uniform interrogating surface current field. In this paper, the theoretical aspects of the ACFM technique are re-examined. The thin-skin theory of electromagnetic perturbation due to a surface-breaking crack has now been generalised to allow non-uniform interrogating current fields and has then been combined with the theory of electromagnetic induction in a half-space to produce a non-uniform ACFM model. More realistic simulations of many types of ACFM probes are obtained from the new theoretical model.