Surface breaking or very near surface discontinuities in ferromagnetic materials give rise to leakage fields when high levels of magnetic flux are applied. These leakage fields attract magnetic particles (finely divided magnetite) to themselves leading to the formation of an indication.

The magnetic particles may be visibly or fluorescently pigmented to provide contrast with the substrate or conversely the substrate may be lightly coated with a white background lacquer to contrast with the particles.

Fluorescent magnetic particles normally provide the greatest sensitivity in a liquid suspension, usually applied by spraying. In certain cases dry particles may be applied by a gentle jet of air. The technique is applicable only to ferromagnetic materials at a temperature below the Curie point (about 650°C). The leakage field will be greatest for linear discontinuities at right angles to the magnetic field so for a comprehensive test the magnetic field must normally be applied in two directions, mutually perpendicular.

The test is economical to carry out in terms of equipment cost and rapidity of inspection and the level of operator training required is relatively low.

Advantages

Limitations

Inexpensive equipment

Only magnetic materials

Direct location of defect

May need to demagnetise components

Surface conditions not critical

Access may be a problem for the yoke

Can be applied without power

Need power if using a yoke

Low skill level

No permanent record

Sub-surface defects found 1-2mm

Calibration of equipment

Quick, instant results

Testing in two directions required

Hot testing (using dry powder)

Need good lighting – 500 lux minimum

Can be used in the dark (UV light)