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Magnetic Particle

MPI is a non-destructive testing technique for detecting surface and shallow subsurface discontinuities (flaws) in ferromagnetic materials which can be strongly magnetised - such as iron, nickel, cobalt and some of their alloys.

It is primarily used for crack detection. The principle of the method is that a 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. Ferrous iron particles are then applied and indications are formed when the iron particles gather in an area of magnetic flux leakage. This indication can then be assessed by the operator under either ultraviolet or white light.

Fluorescent particles, which require UV-A illumination, are widely used and coloured particles are also available. The indications of cracks can be preserved by photography or by the use of peel-off transparent adhesive film. MPI methods can be applied to relatively rough and dirty surfaces, but the flaw sensitivity may be impaired. Magnetic methods for underwater applications have been developed. Only under very special conditions can sub-surface flaws be detected by MPI.

At Testa all of our NDT technicians are qualified to industry recognised PCN (personnel certification in non-destructive testing) in accordance with BS EN ISO 9712.

We can provide PCN-qualified technicians to carry out MPI using either fluorescent or visible techniques. Typically we use the technique to test carbon steel assets. Stainless steels and other non-ferromagnetic materials would be better suited to dye penetrant inspection.

The advantages of MPI include: quick, instant indication of defects, cost-effective, on-site inspection, reliable testing method, batch testing available.

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