Magnetic particle testing (MT) is used to locate surface and slight subsurface discontinuities or defects in ferromagnetic materials. Such flaws present in a magnetized part will cause a magnetic field, i.e. flux, to leave the part. If magnetic particles are applied to this surface, they will be held in place by the flux leakage to give a visual indication.
While several different methods of magnetic particle tests can be used, they all rely on this same general principle.
Therefore, any magnetic particle test will be conducted by creating a magnetic field in a part and applying the magnetic particles to the test surface. To understand the principle of MT, it is necessary to have some basic knowledge of magnetism.
Therefore, consider some of the important characteristics of magnetism in and around typical bar magnet. First, there are magnetic lines of force, or magnetic flux lines, travelling from one end (or pole) of the magnet to the opposite end (pole). These poles are generally designated as the north and south. The magnetic flux lines form continuous loops that travel from one pole to the other. These lines generally remain parallel to one another and will not cross each other. Also, the force of these flux lines (and therefore the flux density of the resulting magnetic field) is greatest when they are fully contained within a ferromagnetic material. Although they will travel through air gaps, their intensity is reduced as the length of the air gap is increased.