Magnetic Particle Testing -Techniques
Magnetic Particle Testing (MT)
Magnetic Particle Testing uses
one or more magnetic fields to locate surface and near-surface discontinuities
in ferromagnetic materials. The magnetic field can be applied with a
permanent magnet or an electromagnet. When using an electromagnet, the
field is present only when the current is being applied. When the
magnetic field encounters a discontinuity transverse to the direction of the
magnetic field, the flux lines produce a magnetic flux leakage field of their
own as shown in Figure 1. Because magnetic flux lines don't travel well
in air, when very fine colored ferromagnetic particles ("magnetic
particles") are applied to the surface of the part the particles will be
drawn into the discontinuity, reducing the air gap and producing a visible
indication on the surface of the part. The magnetic particles may be a
dry powder or suspended in a liquid solution, and they may be colored with a
visible dye or a fluorescent dye that fluoresces under an ultraviolet
("black") light.
MT Techniques
Yokes:
Most
field inspections are performed using a Yoke, as shown at the right. As
shown in Figure 2(a), an electric coil is wrapped around a central core, and
when the current is applied, a magnetic field is generated that extends from
the core down through the articulated legs into the part. This is known
as longitudinal magnetization because the magnetic flux lines run from one leg
to the other.
When
the legs are placed on a ferromagnetic part and the yoke is energized, a
magnetic field is introduced into the part as shown in (b). Because the
flux lines do run from one leg to the other, discontinuities oriented
perpendicular to a line drawn between the legs can be found. To ensure no
indications are missed, the yoke is used once in the position shown then used
again with the yoke turned 90o so
no indications are missed. Because all of the electric current is
contained in the yoke and only the magnetic field penetrates the part, this
type of application is known as indirect induction.
Prods:
Prod units use direct induction,
where the current runs through the part and a circular magnetic field is
generated around the legs as shown in Figure 3. Because the magnetic
field between the prods is travelling perpendicular to a line drawn between the
prods, indications oriented parallel to a line drawn between the prods can be
found. As with the yoke, two inspections are done, the second with the
prods oriented 90o to the
first application.
Coils
Electric coils are used to
generate a longitudinal magnetic field. When energized, the current
creates a magnetic field around the wires making up the coil so that the
resulting flux lines are oriented through the coil as shown at the right.
Because of the longitudinal field, indications in parts placed in a coil
are oriented transverse to the longitudinal field.
Heads:
Most horizontal wet bath machines
("bench units") have both a coil and a set of heads through which
electric current can be passed, generating a magnetic field. Most use
fluorescent magnetic particles in a liquid solution, hence the name "wet
bath." A typical bench unit is shown at the right. When
testing a part between the heads, the part is placed between the heads, the
moveable head is moved up so that the part being tested is held tightly between
the heads, the part is wetted down with the bath solution containing the
magnetic particles and the current is applied while the particle are flowing
over the part. Since the current flow is from head to head and the
magnetic field is oriented 90o to
the current, indications oriented parallel to a line between the heads will be
visible. This type of inspection is commonly called a "head
shot."
Central Conductor:
When testing hollow parts such as
pipes, tubes and fittings, a conductive circular bar can be placed between the
heads with the part suspended on the bar (the "central conductor") as
shown in Figure 6. The part is then wetted down with the bath solution
and the current is applied, travelling through the central conductor rather
than through the part. The ID and OD of the part can then be
inspected. As with a head shot, the magnetic field is perpendicular to the
current flow, wrapping around the test piece, so indications running axially
down the length of the part can be found using this technique.
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