Electromagnetic Testing Chapter 3- Electromagnetic Testing

The frequency of the alternating field is about 1 to 30kHz, so that penetration
of the magnetic flux is only a few tenths of a millimeter or few hundredths of
an inch. With very-high-intensity alternating fields, requiring exciting yokes
using kilowatts of power, the area of the rod near the surface and near the
sides of the crack is magnetically saturated. Increases in intensity increase
the depth of saturation. The permeability of the saturated areas approaches
the permeability of air (one) while the inner areas of the bar, identified by the
“x’s”, have no magnetic flux and remain unchanged.
From a magnetic point of view, both the crack width and depth has been
increased by the amount of saturation. In effect, this magnifies the effect of
the defect and results in a very high signal-to-noise ratio that is easily
detected by the pickup coil even on relatively rough bar surfaces.The
probability of detecting a 0.01-in.-deep defect is 95% with both the Rotoflux
and magnetic particle methods, but the magnetic particle test cannot be
adapted for automatic, high-speed, in-line testing.
Charlie Chong/ Fion Zhang