Magnetic Particle Inspection (MPI)

Magnetic Particle Inspection (MPI) is a non-destructive testing technique used on ferromagnetic materials, which typically include carbon and mild steels. MPI uses magnetic fields and small magnetic particles suspended in solution to detect surface and, to a lesser degree, near surface discontinuities. The method is used to inspect a variety of product forms including castings, forgings, and weldments, particularly at fabrication stage.

MPI Theory

If a magnetic field is induced into a test piece, magnetic flux lines will travel through the test piece. If the magnetic flux lines encounter discontinuities, such as a crack, the flux lines are deflected around the discontinuity. If the discontinuity is sufficiently close to the surface to allow the deflected flux lines to deflect beyond the physical surface of the test piece, then magnetic particles will be attracted to this area of ‘flux leakage’. The resulting cluster of magnetic particles is known as an ‘indication’ and is visible to the naked eye.