Electromagnetic Testing Chapter 3- Electromagnetic Testing

The ratio of test frequency (f) to limit frequency (f g ) provides a useful number
for evaluating the effects of various variables based on their impedance
diagram. The limit frequency, limit frequency equations, and impedance
diagrams are different for solid rods and thin-walled tubing.
A change in f/f g ratio will cause a change in both the phase and magnitude of
voltage developed across the test coil.
The limit frequency (f g ) is the frequency at which additional increases in
frequency do not produce additional increases in eddy current losses. Limit
frequency is defined when the mathematical function describing the
electromagnetic field within a part is set equal to one.
The limit frequency is also known as the “characteristic” frequency of the
material.
If the characteristic frequency is 100Hz, the test frequency that is required for
an f/f g ratio of 10 is 1.0kHz (1000Hz).
Charlie Chong/ Fion Zhang