Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

A signal from a ferromagnetic condition at the inside surface can be
distinguished from a flaw signal by retesting at a lower frequency, for example,
¼ f 90 or lower. As the frequency is reduced, the angle between the shallow
inside surface flaw and the shallow outside surface flaw signals decreases, as
shown in the upper operating point in FIG. 5.34. That is, the outside surface flaw
signal rotates anticlockwise towards the inside surface flaw signal. However, a
signal from a ferromagnetic condition at the inside surface will rotate slightly
clockwise.
Phase lag β = x/δ radian
δ = (πfσμ) -½
Charlie Chong/ Fion Zhang