Ndt – Magneti Particle Testing (mpi)

Description

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 100% coverage.