Magnetic particle testing
according to DIN EN ISO 9934
Magnetic particle testing (also known as flux test, MP test or MT test) is a method of non-destructive material testing. It is suitable for the detection of predominantly gap-like material separations (e.g. cracks) in the surface and near-surface area.
Ferromagnetic materials can be tested. To use the magnetic particle method, the workpiece must be magnetized in the test section. An optimum error indication is obtained when the magnetic field penetrates the material error vertically.
One possible method of magnetization is yoke magnetization. The magnetic flux enters the workpiece via a ferromagnetic yoke. The main field direction is the line connecting the two poles of the yoke. These can be designed as clamping devices. This magnetizes the entire workpiece. Hand magnets can also be placed on the workpiece to magnetize the area between the poles. Two or more types of magnetization can be used simultaneously in a combined process to detect cracks of any orientation.
Coloured or fluorescent magnetic powders are available as detection media. Fluorescent magnetic powders have the highest sensitivity. A distinction must also be made between wet testing (carrier medium: water or oil) and dry testing. The ability to detect defects is greatest in wet testing.
The test area is dusted with a magnetic powder or the test piece is immersed in a container in which the particles are held in suspension by air turbulence (vortex pot method). Dry testing is recommended where wetting of the surface must be avoided or when testing hot parts.
The magnetic powder which is flushed onto the test section is suspended in a carrier liquid. Smaller grain sizes than in the dry test can be used and thus finer cracks can be detected.