Measurement of Material Constants for the Surface Impedance Boundary Condition
The alternating current field measurement (acfm) technique of crack detection and measurement in metals is described elsewhere in these proceedings by Lugg  and by Dover . In common with many eddy-current crack inspection probes, acfm probes are usually operated at a frequency f such that the electromagnetic skin-depth (1) δ=(μ,μ0σπf)−1/2 is much smaller than the depth of a significant crack. In this formula μr is the relative magnetic permeability of the test piece and σ its electric conductivity. Such thin-skin fields near a surface-breaking fatigue crack, with a uniform incident field, have been calculated by using a modified surface impedance boundary condition which includes a line source of magnetic flux to model the effect of the crack , .