The magnetic particle inspection (MPI) method of non destructive testing is a method for locating surface and sub-surface discontinuities in ferromagnetic materials. When the item under examination is magnetized, discontinuities that lie in a direction generally transverse to the direction of the magnetic field, will cause a magnetic leakage field to be formed at or above the surface of the item. The presence of this magnetic leakage field, and therefore, the presence of the discontinuity, is detected by use of finely divided ferromagnetic particles applied over the surface, some of these particles being gathered and held by the leakage field. This magnetically held collection of particles forms an outline of the discontinuity and indicates its location, size, shape and extent.

The magnetic particle inspection process is ideal for the detection of near surface or surface breaking discontinuities in ferromagnetic materials and is utilized for the examination of crankshafts, connecting rods, gears, forgings and final machined components.