Surface & Near-Surface NDT · MPI
Magnetic Particle Inspection
Flux leakage makes cracks in steel announce themselves.
Magnetic particle inspection (MPI, or MT) detects surface and slightly sub-surface defects in ferromagnetic materials. The component is magnetised; a crack cutting across the field forces magnetic flux to leak out of the surface, and fine iron particles applied to the surface gather at the leakage — drawing a visible line over the defect.
The explainer
Watch how it works.
Schematic · loops automatically
1Magnetise the component
An electromagnetic yoke (or prods/coils) drives magnetic flux through the steel between its poles.
2Flux meets the defect
A crack cutting the field path forces flux to leak out of the surface, creating a local magnetic gradient over the defect.
3Apply the particles
Iron particles in suspension are applied while the field is on — the leakage field pulls them into the crack line.
4The indication builds
Particles bridge the defect, forming a sharp visible line against the contrast background — wider and darker than the crack itself.
5Test both directions, then assess
The field is re-applied at 90°, indications are evaluated against weld acceptance levels, and findings recorded.
The physics
Magnetic flux prefers to travel through steel — it flows through the material like current through a conductor. A crack is a gap of air in that path, and air carries flux poorly: at the defect, some of the field is forced out above the surface as a local leakage field with north and south poles either side of the crack.
That leakage field is a particle trap. Finely divided ferromagnetic particles — black oxide against a white contrast background, or fluorescent particles under UV-A — are applied while the component is magnetised. They migrate to the leakage field and bridge the crack, building an indication substantially wider than the defect itself, in seconds.
Orientation matters: leakage is strongest when the defect lies across the flux, and weakest along it. Practical MPI therefore magnetises in two directions (or uses techniques that rotate the field) so that cracks of any orientation are detected — with field strength and direction verified on the day using flux indicators.
Where it earns its keep
Typical applications
- Structural steel weld inspection to EN 1090 supplementary NDT
- Lifting equipment, hooks and crane components
- In-service fatigue crack surveys on plant and machinery
- Pressure vessel weld toes and nozzles at statutory examination
Standards & schemes
| Designation | Covers |
|---|---|
| EN ISO 17638 | Magnetic particle testing of welds |
| EN ISO 23278 | Acceptance levels for magnetic particle testing of welds |
| EN ISO 9934-1 | Magnetic particle testing — general principles |
MPI is delivered by ISO 9712-certified magnetic particle technicians using calibrated yokes and verified consumables under Metlab’s accredited procedures.
Why is MPI preferred over penetrant on carbon steel?
Three reasons: speed — indications form in seconds without dwell times; tolerance — MPI works through light coatings and slightly rough surfaces that would defeat penetrant; and reach — MPI can reveal defects just below the surface, which PT physically cannot.
Does MPI work through paint?
Thin, well-bonded coatings up to a qualified thickness are generally acceptable with a sensitivity check; thick or flaking coatings are not. Where coatings cannot be removed, ACFM is the crack-detection alternative designed for exactly that situation.
Will magnetising damage my component?
No — the field is temporary and harmless. Some components (bearings, instrumentation, subsequent welding) require demagnetisation afterwards, which is a routine final step when specified.
Related methods
Delivered through
Testing · Inspection · Certification
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