Magnetic Particle Inspection is a method of locating surface and subsurface discontinuities in ferromagnetic materials.
The presence of a discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, with some particles, being gathered and held by the leakage field.
This magnetically held connection of particles forms an outline of the discontinuity and generally indicates its location, size, shape and extent.
There are two basics types of yokes that are commonly used for magnetizing purposes:
Permanent Magnet
Are used for application where a source of electric power is not available or where arcing is not permissible (as in an explosive atmosphere).
The limitations of permanent magnet yokes include the following:
- Large areas or masses cannot be magnetized with enough strength to produce satisfactory crack indications.
- Flux density cannot be varied at will
- If the magnet is very strong, it may be difficult to separate from a part
- Particles may cling to the magnet, possibly obscuring indications
Electromagnetic yokes
Consist of a coil wound around a U shaped core of soft iron.
The legs of the yoke can be either fixed or adjustable.
Adjustable legs permit changing the contact spacing and the relative angle of contact to accommodate irregularly shaped parts.
Unlike a permanent magnet yoke, an electromagnetic yoke can readily be switched on or off.
This feature makes it convenient to apply and remove the yoke from the testpiece.
In general, the MT control foresees the following phases:
- Surface preparation
- Magnetization
- Spraying the detector
- Observation
- Demagnetization
- Final cleaning
The testpiece must have a clean surface, free from dust, oxides, grease stains and anything else that could harm the correct formation of the indications or confuse the inspector.
