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Magnetic particle testing is a non-destructive testing method for detection of surface
and near surface discontinuities in ferromagnetic materials, which include most
of iron, nickel & cobalt and their alloys.
PRINCIPLE OF MPT:-
The technique uses the principle, that during the magnetization of a ferromagnetic
material, magnetic lines of force pass through this magnetically conducting medium.
If the magnetic flux lines hit an area of different magnetic permeability such as
a crack near the surface, a portion of these flux lines gets diverted and leak out
above the surface of the material. A magnetic leakage field emerges from the part.
To show this leakage field, colored finely divided iron particles are sprayed to
the area under examination. The leakage field attracts and accumulates some of these
iron powder particles and essentially creates a powder caterpillar worm - like visual
indication for the human eye. The indication is produced directly on the surface
of the part and above the discontinuity.
There are variations in the way the magnetizing field is applied, but they are all
dependant on the above principle.
All surface and near surface crack like defects which produces a leakage field at
the test surface can be detected.
MAGNETIZING TECHNIQUES:-
There are various techniques available for magnetizing a component as given below:-
1. Head shot magnetization - (Circular magnetization by passing current through
the part)
2. Prod method (Circular magnetization of local area of a part)
3. Central conductor (to magnetize the inside bore of hollow parts)
4. Coil magnetization (Longitudinal magnetization by placing the part within the
coil)
5. Permanent magnet (Longitudinal magnetization of local area)
6. Electric yokes (Longitudinal magnetization of local area)
7. Induced current magnetization (for ring shaped parts )
APPLICATION OF MAGNETIC PARTICLES :-
There are two classes of magnetic particles available:-
Dry Particles - applied as dry powder.
Wet particles - fine particles applied as suspended in a liquid. The particles are
suspended in oil or water and are obtained from the manufacturer as a powder or
heavy thick paste.
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