Electromagnetic testing emt-mft chapter 9b

The main point illustrated in Fig. 10 is that the mechanically harder
specimens of the same alloy are also harder to magnetize; that is, the flux
density, B, obtained at a large value of H is smaller for mechanically harder
specimens than for softer specimens. For one alloy, AISI 410 stainless steel,
the hysteresis loop intersects the B = 0 axis at larger values of H for the
harder specimen than for the softer specimen; that is, the coercive force is
greater for the harder material. However, for the other material, SAE 4340
steel, the coercive force does not change with hardness. This suggests that,
for the two alloys considered here, the saturation flux density provides a more
reliable measure of hardness than the coercive force. Mayos et al. used two
quite different techniques to measure the depth of surface decarburization of
steels. One method was a variation of a standard eddy current test, with the
difference from standard practice being that eddy current probe response was
measured in the presence of a low-frequency (~0.1 Hz) magnetic field. This
arrangement provides a measure of incremental permeability, that is, the
magnetic permeability corresponding to changes in the applied field about
some quasistatic value. The second method employed was Barkhausen
noise analysis.
Charlie Chong/ Fion Zhang