Magnetic Particle Inspection (MT) is used to find surface discontinuities on ferromagnetic materials.
During an MT inspection a magnetic field is induced in the part. When the magnetic field encounters a discontinuity, e.g. a crack, there is a leakage of magnetic flux outside of the part. This leakage acts as a small magnet. Ferromagnetic particles of a contrasting color are then flowed over the surface of the part and are attracted to the discontinuity.
The magnetic particles may be a dry powder or in a wet suspension. They may be visible particles or fluorescent UV (Blacklight) reactive. AC, DC, or HWDC current may be used to induce the magnetic field.