What is Magnetic Particle?

Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the part and the magnetic field is formed in the part. Indirect magnetization occurs when no electric current is passed through the part but a magnetic field is applied from an outside source. The lines of magnetic force are perpendicular to the direction of the electric current. The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as the metal. Ferrous iron particles are then applied to the part. The particles are attracted to an area of flux leakage and form what is known as an indication, which is visually detectable under proper lighting conditions.