If a magnetic flux is generated in a ferromagnetic material, this will lead to cracks on the surface as well as to inner cracks near to the surface, a magnetic stray flux on the surface of the material.
If you add ferromagnetic particles in the part of the stray flux, they will be attracted and held. As the magnetic stray flux is bigger than the crack, also the accumulation of the magnetic particles is shown in a bigger way, thus it is better visible for humans.
A requirement for developing the stray flux is a magnetic flux going perpendicular to the crack or a sufficiently strong component in this direction.
The material to be inspected forms a way for the magnetic flux from one pole to the other. Thus the part of the material surface lying between both magnetic poles is strongly magnetized. This discloses all cracks that go almost perpendicular to the field direction. Cracks lying parallel to the field do not generate stray flux and are therefore not shown.
Different procedures can be used to magnetize the components.
Magnetizing with hand magnets
The hand magnet also called yoke magnet works either with a DC field or a AC field magnetizing. For most inspection jobs a magnetic AC field is the ideal solution. A magnetic AC field put in the component primarily includes the areas near the surface (skin effect).
Magnetizing with magnet coils
With magnetizing equipment the component is fixed and exposed to a magnet field with magnetomotive force. Also these two magnetizing procedures can be combined.