WHAT ARE THE PRINCIPLES OF MAGNETIC PARTICLE INSPECTION (MPI) IN NON-DESTRUCTIVE EXAMINATION (NDE)?

The magnetic particle test method of Non-Destructive Examination was developed in the USA, in the 1930s, as a way to check steel components on production lines. 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 visibilty.

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 complete inspection.

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 - as other paramagnetic and diamagnetic materials cannot hold a flux which is strong enough to attract particles.

An MPI inspection comprises several steps:

• Pre-cleaning
• Demagnetisation, if necessary
• Application of a background contrast paint if necessary
• Magnetisation
• Application of magnetic particle powders or inks
• Inspection of surfaces for indications of flaws
• Demagnetisation, and re-magnetisation by another method if necessary
• Recording flaws, if any
• Demagnetisation, if necessary
• Cleaning and protecting

There are several methods of magnetic particle inspection (MPI). Site methods, utilising portable equipment, include:

• Permanent magnet
• Electromagnetic Yoke
• Current flow probes
• Flexible coil
• Adjacent cable

Bench, or fixed, installations in a factory include:

• Current flow
• Threading bar
• Magnetic flow
• Fixed coil
• Induced current (as a specialised option)