Magnetic particle inspection
Magnetic particle inspection (Short: MT – magnetic testing) is a surface applied produce which, like the liquid penetrant inspection (short: PT – penetrant testing), belongs to the field of nondestructive material testing (known as NDT – non-destructive testing). It is based on the physical phenomenon of magnetism and serves to detect and analyze cracks on or near the surface up to 0,02 millimeters. Other than the penetrant testing, which is independent of the working materials used, magnetic testing may only be applied to ferromagnetic materials such as iron and steel.
In order to execute this procedure the material that is to be tested itself will be magnetized. The field lines within the material that are generated this way run parallel to the surface. If they are “broken” by longitudinal cracks a magnetic stray field will be created. A special testing agent is applied during the next step. This agent is either a black or, alternatively, a fluorescent magnetic powder, which is applied to the test piece in either wet testing (base is water or oil) or dry testing (base is air). The powder is attracted by the magnetic stray field and optically highlights the cracks under daylight, artificial light or UV-light, depending on the agent that has been used.
Cracks that run parallel to the field lines do not cause the prior described development. In order to detect longitudinal as well as diagonal cracks various magnetization methods can be used.
While longitudinal cracks become visible with the help of the current flow method, diagonal cracks are made visible with the field flow method. If there are any assumptions that the cracks in the work piece are not aligned into the same direction, it is recommended to apply a method that combines two magnetization techniques, which erases the need of long-winded testing. It should be noted, however, that in this case at least one magnetization method must be powered by alternating current.
Should the situation arise, demagnetization might be necessary in the end, for example if the work piece has to be welded or polished later on.