Figure 11-7.--Circular magnetization--prod method.
Figure 11-8.--Longitudinal magnetization--coil method.
three outlets on the front, which makes it easy to change
from alternating to direct current or vice versa. The
outlet on the left is labeled ac, the center is COMMON,
View B of figure 11-8 shows a flexible cable
and the right is dc. One cable will always be plugged
wrapped around the work. This forms a coil
into the COMMON outlet. The other cable is plugged
which, with the passage of current, induces
into the ac or dc outlet, depending upon what type of
longitudinal magnetism in the part of the
workpiece that is surrounded by the coiled
current the test requires. For most work, alternating
cable.
current magnetization will locate fatigue cracks and
similar defects extending through to the surface. Direct
Either of these two methods may be used, but the
current is used when a more sensitive inspection is
prod method is probably the easiest to apply. It will
required to locate defects that are below the surface.
detect surface defects in most instances. With the prods,
however, only a relatively small area of the test piece
The unit can be used to produce alternating or direct
can be magnetized at any one time. This magnetized
current in either of two ways:
area is limited to the distance between prod contact
points and to a few inches on each side of the current
View B of figure 11-7 shows prods attached to
path. To check the entire surface, it is necessary to
the flexible cable and used as contacts through
successively test adjacent areas by changing the
which current is passed into and out of a portion
location of the prod contact points after a given area has
of the test piece. This sets up a circular
been tested. Each area of the test piece must be
magnetization in a local area between the prod
inspected twice; once with the current passing through
contact points.
the metal in one direction and again with the current
11-9