Magnetic
Particle Testing - (MT)
Magnetic
particle inspection (MPI) is used for
the detection of surface and near-surface
flaws in ferromagnetic materials. A
magnetic field is applied to the specimen,
either locally or overall, using a permanent
magnet, electromagnet, flexible cables
or hand-held prods. If the material
is sound, most of the magnetic flux
is concentrated below the material's
surface. However, if a flaw is present,
such that it interacts with the magnetic
field, the flux is distorted locally
and 'leaks' from the surface of the
specimen in the region of the flaw.
Fine magnetic particles, applied to
the surface of the specimen, are attracted
to the area of flux leakage, creating
a visible indication of the flaw.
The materials commonly
used for this purpose are black iron
particles and red or yellow iron oxides.
In some cases, the iron particles are
coated with a fluorescent material enabling
them to be viewed under a UV lamp in
darkened conditions.
