Magnetic Particle Testing (MT)

Magnetic particle testing is a non-destructive testing method for ferromagnetic materials in which a magnetic field is applied and fine iron particles gather at surface and near-surface discontinuities, forming visible indications of cracks and other flaws.

When a ferromagnetic part such as carbon steel is magnetised, a crack or other discontinuity that interrupts the magnetic field creates flux leakage at the surface. Ferromagnetic particles, applied as a dry powder or suspended in liquid, are attracted to the leakage field and accumulate along the flaw. Visible particles, often used over a contrasting white background paint, are inspected under white light, while fluorescent particles are inspected under ultraviolet light in darkened conditions for higher sensitivity. Magnetisation methods include yokes, permanent magnets, current passed through the part, coils and central conductors.

MT is widely used to detect surface-breaking and slightly subsurface cracks in welds, forgings, castings, shafts, gears, crane hooks, rail components and aircraft parts, both after manufacture and during in-service inspection for fatigue cracks. It is fast, relatively inexpensive and sensitive to fine cracks, and portable yokes allow inspection in the field.

MT works only on ferromagnetic materials, so it cannot be used on austenitic stainless steel, aluminium, copper or titanium. Indications are strongest when the flaw lies across the magnetic field, so parts are usually magnetised in two directions roughly at right angles. Coatings, surface condition and residual magnetism need attention, and parts may require demagnetisation afterwards. ISO 9934-1 specifies general principles for magnetic particle testing.

Key points

Detects surface and near-surface flaws in ferromagnetic materials.
Particles gather at magnetic flux leakage caused by discontinuities.
Fluorescent particles under ultraviolet light give higher sensitivity.
Not usable on austenitic stainless steel, aluminium, copper or titanium.

Where AiVibe comes in

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Related terms

Terms that refer to Magnetic Particle Testing (MT)

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