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Magnetic Particle
Magnetic particle testing is accomplished by inducing a
magnetic field in a ferromagnetic material and then dusting the surface
with iron particles (either dry or suspended in liquid). Surface and
near-surface imperfections distort the magnetic field and concentrate iron
particles near imperfections, previewing a visual indication of the flaw.
The method is used to inspect a variety of product forms such as castings,
forgings, and weldments. Many different industries use magnetic particle
inspection for determining a component's fitness-for-use. Some examples of
industries that use magnetic particle inspection are the structural steel,
automotive, petrochemical, power generation, and aerospace industries.
Underwater inspection is another area where magnetic particle inspection
may be used to test such things as offshore structures and underwater
pipelines.
In theory,
 magnetic
particle testing 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 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
the at the south pole. The magnetic field spreads out when it encounter
the small air gap created by the crack because the air can not 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, it 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 following pictures are examples of using magnetic testing to
identify flaws.
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