Magnetic-particle inspection is based on the fact that
when a ferromagnetic material is magnetized, discontinuities
that lie in a direction generally transverse
to the direction of the magnetic field cause
a leakage field to form at and above the surface. The presence of
the leakage field, and therefore the presence of the discontinuity,
is detected by finely divided ferromagnetic particles applied
over the surface. Some of the particles are gathered and held by
the leakage field.
The magnetically held particles form an outline of the discontinuity
and generally indicate its location, size, shape, and extent.
Magnetic particles are applied over a surface either as dry
particles, or as wet particles in a liquid carrier such as water or
oil.
The magnetic-particle method is a sensitive means to locate
small, shallow surface cracks. Those large enough to be seen by
the naked eye can produce an indication, but very wide cracks
do not produce a particle pattern if the surface opening is too
wide for the particles to bridge.
Discontinuities that do not actually break through the surface
also are indicated in many instances within certain limitations.
Fine, sharp discontinuities close to the surface (a long stringer
of nonmetallic inclusions, for example) can produce an indication.
However, indications of deeper discontinuities are less
distinct.