Magnetic Particle Inspection (MPI) can be used for the detection of surface and near-surface flaws in ferromagnetic materials. To conduct such inspection, a permanent magnet, electromagnets, or electromagnetic coils are used to build up a magnetic field to the item being tested. If there is a flaw present within the item, the magnetic flux will be distorted and will 'leak'. Fine magnetic particles (normally used in spray form in carrier fluid) are applied to the surface of the specimen. They are then attracted to the area of flux leakage, creating a visible flaw indication.
:
