Magnetic Particle Inspection (MPI) – Effective and low cost non-destructive method for locating surface and sub-surface defects in ferromagnetic materials. The principle of the method is: when the test object is magnetized, discontinuities that lie in a direction perpendicular to the magnetic field will cause a magnetic flux leakage field to be formed at and above the surface of the test piece. The presence of the leakage field (discontinuity) is detected with fine ferromagnetic particles applied over the surface, some of them being gathered and held to form an outline of the discontinuity. This generally indicates the location, size, shape and extent of the discontinuity. The advantages of using an MPI technique are economy, speed, sensitivity, versatility, mobility. The primary limitation of the method is that the test piece must be ferromagnetic.