Magnetic Particle Testing (MT) is an NDT method for finding surface and slightly subsurface cracks in ferromagnetic parts. The part is magnetised and fine iron particles are dusted or flowed over it. Wherever there is a crack, the particles gather into a line you can see with the naked eye.
How it works: flux leakage
When a part is magnetised, lines of force run through it. A crack interrupts that path, so some lines leave the part and form a leakage field. That leakage field attracts the particles and forms the indication. The rule to remember: no leakage field, no indication.
Direction matters too. A crack shows best when it is at 90° to the lines of force, and can still be found between 45° and 90°. A crack parallel to the field causes no leakage and stays invisible. That is why every area is tested twice, with the field directions about 90° apart.
Which materials can be tested?
MT works only on ferromagnetic materials, such as carbon steel and iron. Austenitic stainless steel, aluminium, copper and titanium cannot be tested with MT. For those, choose another method such as Penetrant Testing.
Equipment and materials
- Yoke: a U-shaped magnet that gives a longitudinal field between its legs; handy in the field.
- Prods: two electrodes pressed on the surface to pass current; used on large parts such as welds and castings.
- Head shot and central conductor: give a circular field for bars, pipes, rings and nuts.
- Coil: gives a longitudinal field to find transverse cracks.
- Dry or wet (bath) particles: grey, red or black, or fluorescent particles that glow under UV-A light.
The current type sets the reach: AC is best for surface cracks, while DC and HWDC reach deeper for subsurface flaws.
Basic MT steps
- Clean the surface of scale, slag, heavy paint, rust and oil.
- Choose the technique (yoke, prods, coil, head shot) and current type to suit the direction and location of the expected flaws.
- Magnetise the part while applying particles (continuous method), or apply them after the current is off (residual method).
- Repeat with a second field direction about 90° from the first.
- Interpret indications as relevant, non-relevant or false, then evaluate them against the acceptance criteria.
- Demagnetise and check the remaining field with a field indicator.
Flaws MT can find
MT is excellent for surface-breaking weld cracks (including crater and toe cracks), grinding cracks, heat-treat cracks, forging laps, and fatigue cracks in parts already in service. Deeper weld flaws such as slag, porosity and lack of fusion are better found with UT or RT.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| Quick, simple, results visible right on the surface | Ferromagnetic materials only |
| Finds surface and slightly subsurface cracks | Surface and near-surface flaws only; the reach depends on permeability, flaw type and current |
| Usable in production and in maintenance | Must be tested in two directions; cracks parallel to the field are missed |
| Fluorescent particles with AC are very sensitive to fine cracks | Parts usually need demagnetising afterwards |
Learn MT from scratch
In the Magnetic Particle Testing (MT) Level II course you learn through interactive animations, a yoke simulator, a bilingual workbook and a question bank to prepare for the Level II exam. Try free Lesson 1: Principles of Magnetism first.
Fact source: MT Level II workbook, pp. 7–14, 20–25, 40–55, 59–67.
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