use of inside coils is restricted by bends or nonuniform
diameters. Encircling coils are used primarily for
inspecting rods, tubes, cylinders, or wire. With the
encircling or inside coils, the entire circumference of
the specimen is evaluated at one time. Consequently,
the exact location of defects cannot be defined. The
surface coil has the ability to better define the exact
location of discontinuities.
Dye Penetrant Inspection
The dye penetrant inspection is a simple,
inexpensive, and reliable nondestructive inspection
method for detecting discontinuities that are open to the
surface of the item to be inspected. It can be used on
metals and other nonporous materials that are not
attacked by penetrant materials. With the proper
technique, it will detect a wide variety of
discontinuities, ranging in size from those readily
visible down to microscopic level, as long as the
Figure 6-17.--Basic coil configurations.
discontinuities are open to the surface and are
sufficiently free of foreign material. Figure 6-19 shows
(bobbin-type) coils. Figure 6-17 shows sketches of the
the basic principles of the penetrant inspection process.
general configuration of each type of coil or probe.
A penetrating liquid, which contains dyes, is applied to
Figure 6-18 shows photographs of typical surface
the surface of a clean part to be inspected. The penetrant
probes used for eddy current testing. Most eddy current
is allowed to remain on the surface of the part for a
testing in the field is concerned with surface coils
period of time to permit it to enter and fill any openings
(probes). The surface probe is used on plates, sheets,
or discontinuities. After a suitable dwell period, the
and irregular-shaped parts. An inside coil may be used
penetrant is removed from the part's surface. You must
on tubes, pipes, or other parts that are accessible to the
exercise care to prevent removal of the penetrant that is
inside. The inside coil should nearly fill the part
contained in the discontinuities. A material called
opening in order to provide good test sensitivity. The
Figure 6-18.--Typical eddy current test probes.