Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.2.2.4 Testing ferromagnetic bars
The impedance curves for ferromagnetic bars are identical in shape to those of
non-ferromagnetic bars, but are increased in magnitude by a factor equal to the
permeability of the bar. As the permeability of the bar increases, the amount of
magnetic flux in the bar increases, and so the inductive reactance increases
proportionally. In addition, the increased flux causes an increase in the eddy
current intensity, and so the resistive losses of the eddy currents and the
effective resistance of the coil also increase proportionally. Therefore the
operating point moves down the curve (FIG. 5.42). If the bar diameter increases,
the amount of ferromagnetic material in which the magnetic field can be induced
also increases, so the amount of magnetic flux in the bar increases. This is
exactly the same effect as if the permeability has increased and therefore the
two conditions cannot be separated.
Charlie Chong/ Fion Zhang