Magnetic Particle Testing

When magnetizing a ferromagnetic material (magnetic permeability of µ>100µH/m), 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 (air inside the defect), higher magnetic resistance causes a change of the flux.

In this connection the magnetic field lines to jump over the area with the higher resistance. This causes a so called stray field on the surface and this finally causes an accumulation of ferromagnetic particles at this point. Thus surface defects become visible (see illustration).

The particles inside the test liquid are covered with a fluorescent or mainly black color. Thereby the due to the magnetic stray field caused magnetic particle accumulation can be made visible under UV- or Day light conditions.

 The minimum extent of a evaluable surface defect have to be 1µm width, 10µm depth, 100µm length.

 To make sure , that every defect in every orientation on the surface can be detected, it is necessary to magnetize the testing area successively in two directions. In the ideal case those should be in an angle of 90° to each other.