Magnetic Particle testing
Magnetic particle testing (MT) is a surface method which, like the penetration method (PT), belongs to non-destructive testing (NDT). It is based on the physical phenomenon of magnetism and is used to detect surface and near-surface cracks of up to 0.02 millimetres. In contrast to the penetration method, which is independent of the material, magnetic particle testing can only be carried out on ferromagnetic materials such as iron and steel.
To carry out the test, the material to be tested is first magnetised. The resulting magnetic field lines in the material run parallel to the surface. If these are “interrupted” by a longitudinal surface crack, a magnetic stray field is created. In the next step, a special testing device is used. This is either a black or alternatively fluorescent magnetic powder, which is applied to the workpiece in the form of wet (carrier is water or oil) or dry testing (carrier is air). The powder is attracted by the stray magnetic field and optically highlights the cracks under daylight, artificial or UV light, depending on the medium used.
Cracks running parallel to the field lines do not initially cause the development described above. However, in order to detect both longitudinal and transverse cracks in the material, different magnetisation methods can be used.
While longitudinal cracks become visible with the help of current flow, so-called field flow is used to detect transverse cracks. If it is suspected that cracks in the workpiece do not run in the same direction, a combined procedure using two magnetisation techniques at the same time is recommended, which eliminates the need for lengthy “testing”. It should be noted, however, that in this case at least one of the two magnetisation techniques must be operated with alternating current.
Demagnetisation may be necessary at the end – for example, because the tested workpiece will be welded or ground later.