Electromagnetic testing emt-mft chapter 9b

If a slot is cut in the rod, as illustrated in Fig. 1(b), the poles on the surface of
this slot are now also uncompensated and therefore produce a localized
magnetic field near the slot. This additional magnetic field, which is
represented by the extra flux lines in Fig. 1(b), is the leakage field associated
with the slot. Figure 1, although adequate for a qualitative understanding of
the origin of leakage fields, does not provide an exact quantitative description.
The difficulty is the assumption that the magnetization remains uniform when
the flaw is introduced. In general, this does not happen, because the
presence of the flaw changes the magnetic field in the vicinity of the flaw, and
this in turn leads to a change in magnetization near the flaw. With regard to
Fig. 1, this means that the strengths and orientations of the elementary
dipoles (magnets) actually vary from point to point in the vicinity of the flaw,
and this variation also contributes to the flaw leakage field. The end result is
that the accurate description of a flaw leakage field poses a difficult
mathematical problem that usually requires a special-purpose computer code
for its solution.
Charlie Chong/ Fion Zhang