Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

FIG. 5.28 shows the impedance curves for the two cases involving nonferromagnetic
materials. The coating thickness curves meet the conductivity
curve when the thickness of the coating equals the effective depth of
penetration (EDP) of the eddy currents in the coating.
As with the measurement of the thickness of a conductor, the test frequency
should be f 90 calculated on the thickness of the coating. This gives operating
points at the locations marked on the curves. A further factor that needs to be
taken into account when selecting test frequency is to avoid, if possible, the
eddy currents penetrating through the substrate, as then substrate thickness
will affect the reading. Frequencies higher than f 90 may be required for this
purpose. To determine if the eddy currents penetrate through the substrate,
the material may be laid on a block of copper. If a signal is not obtained, the
eddy currents do not penetrate. The requirements for reference specimens
and the test procedures are essentially the same as for measuring the
thickness of conductors.
Charlie Chong/ Fion Zhang