When magnetizing a ferromagnetic material, the magnetic lines of force (flux) will follow the best magnetic conductive media, the line of least resistance. If the magnetic field lines meet a poorly conductive area, such as a surface crack, high magnetic resistance causes a change of the flux. This provokes a disturbance of the flux on the surface, which accumulates ferromagnetic particles.

Magnetic particle testing can be distinguished into two material specific main groups:

• soft magnetic materials with poor remanence (after cutting-off the external magnetic field there is no magnetic flux remarkable any more),

• hard magnetic materials with high remanence (after cutting-off the yoke a high residual field remains).

Magnetic flux becomes visible
For testing, during the magnetization of the material, magnetisable particles will be applied on the surface of the work piece. They are geared to the created magnetic field over the flux fields along the geometry of the crack. The magnetisable particles will be optically brought out by (fluorescent) colour dyes enclosing them and are therefore clearly visible.

The magnetization has to take place successively in two directions to ensure that the defect crosses the magnetic fields at least in one direction.

With magnetic particle inspection any ferromagnetic materials with a magnetic permeability of µ>100 can be tested. This includes all sorts of cast steel and cast iron (exception: material with austenitic structure).
All material defects near the surface that have, due to their position, a sufficient component to the created magnetic field can be found. A crack must have a minimum extension of 1µm width, 10µm depth and 100µm longitude to be detected.

When looking for defects the main advantages of magnetic particle testing compared to penetration testing is the widely insensitivity regarding surface geometry, surface roughness and microstructure.