Magnetic Particle Inspection (MPI or MT) is the process for detection of surface, and slightly sub-surface cessation using finely divided ferromagnetic particles to indicate defects in magnetic materials. (Ferromagnetic particles are materials that have magnetic properties similar to those of iron such as cobalt, nickel and some alloys. They can become permanently magnetized).
Ferromagnetic particles are applied to the test surface, and magnetic yokes are used to generate a magnetic field to pass through and around the test surface. These particles register abrupt changes in resistance in the path of the magnetic field and will therefore be attracted to and outline the crack or anomaly if there is one.