How does eddy current testing work?

The variable of ferromagnetic materials decisive for eddy current testing is permeability. Permeability actually means relative permeability, a number without any dimension, which indicates how much better a certain material can conduct electric flux lines in comparison to air. Air has the value of 1.

A steel which can be magnetized has a value between 10 to a few 1,000. The correlation between permeability and field strength is not at all linear. In case of a very small field strength, permeability is low (starting permeability).

With an increasing field strength permeability increases to a maximum value, whereupon it decreases subsequently to a smaller value. The reason can be seen in the dislocation of the Bloch wall as well as in the direction of the "Magnetic Weiss's Domain." Every structure and every material has a characteristic permeability.

This means that parts with different heat treatments and different microstructures exhibit different permeability. ... more.

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