Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Permeability variations in ferromagnetic material are quite common because of
small variations in composition, heat treatment, amount of cold work, and
residual stress, for example from straightening the bar, and, if the material has
been magnetized, because of different degrees of magnetization. This leads to a
noisy signal. These two limitations, the inability to measure bar diameter and
noisy signals, and a third limitation, the very restricted depth of penetration
because of the high permeability, can be overcome by magnetizing the material
to or close to saturation. In this condition, the material has an incremental
permeability of, or close to, one, and therefore shows the same behaviour to
eddy current testing as non-ferromagnetic material. In this case, the analysis
used for non-ferromagnetic bars applies. Ferromagnetic bars and tubes are
therefore often tested with two large coils carrying a large direct current, one
either side of the eddy current coil(s). The current required is usually large
enough to cause heating problems in the magnetizing coils, and pulsing the
current and the use of water cooled jackets are often employed to overcome
these problems.
Charlie Chong/ Fion Zhang