This method is used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection. The specimen is magnetised either locally or overall, and if the material is sound the magnetic flux is predominantly inside the material. If, however, there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw. This leakage flux is displayed by covering the surface with very fine iron particles applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually even when the crack opening is very narrow. Thus, a crack is indicated as a line of iron powder particles on the surface.

The technique works by inducing a magnetic field in the component tested. If a surface or near-surface flaw is present, the magnetic flux is distorted and ‘leaks’ around the flaw. By dusting the surface of the tested item with fine magnetic particles such as ferrous iron filings, (applied either dry or suspended in liquid), the particles will be attracted to the area of the flux leakage creating a visible indication of the defect. The indication can be evaluated by the operator to determine what it is, what may have caused it, and what action should be taken, if any.

Usually the iron particles – dry powder or suspended in a liquid (magnetic ink) – are applied while the magnetising current is still flowing, but residual magnetisation is sometimes used, when the particles are applied after magnetisation. Some steels retain sufficient magnetisation for this method to be satisfactory, and in this case smaller, more portable, magnetising equipment can be used. Magnetic inks (particles suspended in a liquid) are used more widely than dry powders. They are applied by low-pressure spray, dipping or brushing; it is important to use plenty of ink and to allow time for the particles to flow over the surface and migrate to any cracks. On dark surfaces a very thin layer of white paint can be applied to give a higher contrast indication.

Some of the most common techniques for on-site magnetic testing include:

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