The most common and widely-used method of NDE is magnetic particle inspection (MPI). The technique employs the nature of magnetic fields to identify potential defects. In magnetised materials, the magnetic field tends to travel within the solid material. When an applied field meets a crack or other defect, it will tend to 'jump' across the crack. Inspection engineers use magnetic ink to pick up this 'jumping' mechanism and hence identify a flaw.
Finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene) are used for the inspection; this magnetic fluid is referred to as the carrier. The particles are often coloured and can be coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. This tends to improve the contrast of the inspection surface, and therefore easier to spot defects.
The suspension is sprayed or painted over the magnetised specimen during magnetisation which is provided by a direct current or with an electromagnet, to localise areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build up of particles.