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TEST COIL CONFIGURATIONS.
-Eddy
current probes and coils can be classified
into three types: surface probes,
encircling coils, and inside (bobbin-type)
coils. Figure 15-17 shows sketches of the general
configuration of each type of coil or probe. Figure
15-18 shows photographs of typical surface probes
used for eddy current testing. Most eddy current testing
in the field is concerned with surface coils (probes).
The surface probe is used on plates, sheets, and irregular-shaped
parts. An inside coil may be used on tubes,
pipes, or other parts that are accessible to the inside.
The inside coil should nearly fill the part opening in
order to provide good test sensitivity. The use of inside
coils is restricted by bends or nonuniform diameters.
Encircling coils are used primarily for
Figure 15-18.-Typical eddy current test probes.
Figure 15-19.-Basic penetrant process. 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 discontinuities are open to the surface and are sufficiently free of foreign material. Figure 15-19 shows the basic principles of the penetrant inspection process. A penetrating liquid, which contains dyes, is applied to the surface of a clean part to be inspected. The penetrant is allowed to remain on the surface of the part for a period of time to permit it to enter and fill any openings or discontinuities. After a suitable dwell period, the penetrant is removed from the parts surface. You must exercise care to prevent removal of the penetrant that is contained in the discontinuities. A material called "developer" is then applied. The developer aids in drawing any trapped penetrant from the discontinuities and improves the visibility of any indications. For more information concerning the dye penetrant inspection, consult the Nondestructive Inspection Methods Manual, NAVAIR 01-lA-16. |
 
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