Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

The depth of penetration of the eddy currents below the bar surface increases
as the frequency, conductivity, and permeability are reduced, but the penetration
is further limited because of the configuration, which results on the eddy current
density falling to zero at the centre of the bar. FIG. 5.41 shows the eddy current
density at various frequencies. It can be seen that the limiting condition is a
linear decrease in the eddy current density from a maximum at the surface to
zero at the centre of the bar. This means that, however low the frequency, flaws
cannot be detected at the centre of a bar by eddy current testing, and ultrasonic
testing should be used instead. As for all eddy current testing, the test sensitivity
also depends on the degree of magnetic coupling. When testing with encircling
coils, the magnetic coupling is measured as the fill factor (η), as for testing tube
with bobbin coils, and is defined as the ratio of the cross sectional area of the
bar or tube to the average area of the coil.
Charlie Chong/ Fion Zhang