Magnetic Particle Inspection
Magnetic Particle Inspection (MPI) is used to detect surface and near surface flaws. It is most suitably used for ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The part is magnetised and if there is no surface breaking flaw the magnetic flux remains predominantly inside the material. However, if there is a flaw present the magnetic field is distorted creating flux leakage around the flaw area. The flux leakage is illustrated by covering the surface with a magnetic ink, the particles accumulate around the surface indicating the flaw area. Fluorescent magnetic ink can be applied for more intricate parts to increase flaw visibility.