Magnetic Particle Testing (MPI)
Magnetic Particle Testing is the method used to detect surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys.
The principle of the method is that the specimen is magnetised to produce magnetic lines of force, or flux, in the material. If these lines of force meet a discontinuity, such as a crack, secondary magnetic poles are created at the faces of the crack. Where these secondary magnetic fields appear at the surface of the metal, they can be revealed by applying magnetic particles, as a powder, or in a liquid suspension, to the surface. The particles are attracted to the flux leakage and clump round the flaw, making it visible.
The particles may be black, or coated with a fluorescent dye to increase their visibility. The magnetic flux lines should be at right angles to a flaw to give the best indication, as this creates maximum flux leakage.
This governs the choice of a suitable magnetising technique. Often, more than one technique must be used to give a 100% coverage A flaw attracts more particles if it cuts more magnetic lines of force, so the ability to show a flaw depends on the depth of the flaw, the angle of the flaw to the lines of force, and the magnetic field strength induced during magnetisation.
The method is limited to ferromagnetic materials - iron, cobalt and nickel and their alloys - as other paramagnetic and diamagnetic materials cannot hold a flux which is strong enough to attract magnetic particles
