THEOREM 4
If the course of magnetic field lines in ferromagnetic material is disturbed, a stray field is generated at the point of disturbance!
It is of crucial importance, therefore, that a defect actually means a disturbance for the magnetic field lines.
Figs. 5.1 to 5.3 show three principal possibilities of a defect. As in the previous example with the iron ring, in fig. 5.1 the defect is at right angles to the field line direction and thus causes a stray field. Thus, after application of iron powder, this defect could be detected.
However, if the field line direction has the same orientation as the defect (fig. 5.2), this results in only a very insignificant influence on the field lines, which does not cause a stray field. A defect of this type cannot be detected. In the case of defects which are now below the surface, such a clear decision is not possible, because in this case further factors have to be taken into account.
Basically, however, it can be seen that the field line direction and the fault direction must be transverse to each other in order to generate a stray field. The angle between the two directions should ideally be 90 degrees, although angular positions of up to a minimum of 30 degrees are also sufficient.