In theory, magnetic particle inspection (MPI) is
a relatively simple concept. It can be considered as a combination
of two nondestructive testing methods: magnetic flux leakage testing
and visual testing. Consider
the case of a bar magnet. It has a magnetic field in and around the magnet.
Any place that a magnetic line of force exits or enters the magnet
is called a pole. A pole where a magnetic line of force exits
the magnet is called a north pole and a pole where a line of force
enters the magnet is called a south pole.
When a bar magnet is broken in the center of its
length, two complete bar magnets with magnetic poles on each end
of each piece will result. If the magnet is just cracked but not
broken completely in two, a north and south pole will form at
each edge of the crack. The magnetic field exits the north pole
and reenters at the south pole. The
magnetic field spreads out when it encounters the small air gap
created by the crack because the air cannot support as much magnetic
field per unit volume as the magnet can. When the field spreads
out, it appears to leak out of the material and, thus is called
a flux leakage field.
If iron particles are sprinkled on a cracked magnet, the particles
will be attracted to and cluster not only at the poles at the
ends of the magnet, but also at the poles at the edges of the crack.
This cluster of particles is much easier to see than the actual
crack and this is the basis for magnetic particle inspection.
The first step in a magnetic particle inspection
is to magnetize the component that is to be inspected. If any
defects on or near the surface are present, the defects will create
a leakage field. After the component has been magnetized, iron
particles, either in a dry or wet suspended form, are applied
to the surface of the magnetized part. The particles will be attracted
and cluster at the flux leakage fields, thus forming a visible
indication that the inspector can detect.