of Ferromagnetic Materials
There are a variety of methods that can be used to establish
a magnetic field in a component for evaluation using magnetic
particle inspection. It is common to classify the magnetizing
methods as either direct or indirect.
Magnetization Using Direct
Induction (Direct Magnetization)
With direct magnetization, current
is passed directly through the component. Recall that whenever
current flows, a magnetic field is produced. Using the right-hand
rule, which was introduced earlier, it is known that the magnetic
lines of flux form normal to the direction of the current and
form a circular field in and around the conductor. When using
the direct magnetization method, care must be taken to ensure
that good electrical contact is established and maintained between
the test equipment and the test component. Improper contact can
result in arcing that may damage the component. It is also possible
to overheat components in areas of high resistance such as the
contact points and in areas of small cross-sectional area.
are several ways that direct magnetization is commonly accomplished.
One way involves clamping the component between two electrical
contacts in a special piece of equipment. Current is passed through
the component and a circular magnetic field is established in
and around the component. When the magnetizing current is stopped,
a residual magnetic field will remain within the component. The
strength of the induced magnetic field is proportional to the
amount of current passed through the component.
second technique involves using clamps or prods, which are attached
or placed in contact with the component. Electrical current flows
through the component from contact to contact. The current sets
up a circular magnetic field around the path of the current.
Magnetization Using Indirect
Induction (Indirect Magnetization)
Indirect magnetization is accomplished
by using a strong external magnetic field to establish a magnetic
field within the component. As with direct magnetization, there
are several ways that indirect magnetization can be accomplished.
The use of permanent
magnets is a low cost method of establishing a magnetic field.
However, their use is limited due to lack of control of the field
strength and the difficulty of placing and removing strong permanent
magnets from the component.
in the form of an adjustable horseshoe magnet (called a yoke)
eliminate the problems associated with permanent magnets and are
used extensively in industry. Electromagnets only exhibit a magnetic
flux when electric current is flowing around the soft iron core.
When the magnet is placed on the component, a magnetic field is
established between the north and south poles of the magnet.
Another way of indirectly inducting a magnetic field in a material
is by using the magnetic field of a current carrying conductor.
A circular magnetic field
can be established in cylindrical components by
using a central conductor. Typically, one or more cylindrical
components are hung from a solid copper bar running through the
inside diameter. Current is passed through the copper bar and
the resulting circular magnetic field establishes a magnetic field
within the test components.
use of coils and solenoids is a third method of
indirect magnetization. When the length of a component is several
times larger than its diameter, a longitudinal magnetic field
can be established in the component. The component is placed longitudinally
in the concentrated magnetic field that fills the center of a
coil or solenoid. This magnetization technique is often referred
to as a "coil shot."