Prediction of Eddy-Current Probe Impedance Due to Surface Cracks in Ferromagnetic Steels

Abstract

Ferromagnetic steels usually have a high value of relative permeability which means that, for frequencies commonly used in eddy-current non-destructive evaluation, the electromagnetic skin-depth is typically much smaller than the depth of a crack. This behavior allows the use of approximate, thin-skin theories to describe the interaction between induced eddy-currents and surface-breaking cracks in steel. Thin-skin approximations are also appropriate for use with non-magnetic materials at sufficiently high frequency. Auld et al [1] treated the high-frequency, surface-crack problem in aluminium using a magnetic scalar potential formulation. The surface impedance boundary condition was applied to the field on the crack faces to obtain an expression for the impedance change in the coil due to the defect in terms of known field components. This approach was followed in a more recent treatment of the long, surface crack by Burke [2].