Field Orientation and Flaw Detectability
To properly inspect a component for cracks or other defects,
it is important to understand that the orientation between the magnetic
lines of force and the flaw is very important. There are two general
types of magnetic fields that can be established within a component.
|A longitudinal magnetic field has magnetic lines
of force that run parallel to the long axis of the part. Longitudinal
magnetization of a component can be accomplished using the
longitudinal field set up by a coil or solenoid. It can also
be accomplished using permanent magnets or electromagnets.
|A circular magnetic field has magnetic lines
of force that run circumferentially around the perimeter of
a part. A circular magnetic field is induced in an article
by either passing current through the component or by passing
current through a conductor surrounded by the component.
The type of magnetic field established is determined by the method
used to magnetize the specimen. Being able to magnetize the part
in two directions is important because the best detection of defects
occurs when the lines of magnetic force are established at right
angles to the longest dimension of the defect. This orientation
creates the largest disruption of the magnetic field within the
part and the greatest flux leakage at the surface of the part.
As can be seen in the image below, if the magnetic field is parallel
to the defect, the field will see little disruption and no flux
leakage field will be produced.
An orientation of 45 to 90 degrees between the magnetic
field and the defect is necessary to form an indication. Since
defects may occur in various and unknown directions, each part
is normally magnetized in two directions at right angles to each
other. If the component below is considered, it is known that
passing current through the part from end to end will establish
a circular magnetic field that will be 90 degrees to the direction
of the current. Therefore, defects that have a significant dimension
in the direction of the current (longitudinal defects) should
be detectable. Alternately, transverse-type defects will not be
detectable with circular magnetization.
Watch this short movie showing the effect of field direction on indication visibility. (775 KB mov)