Figure 15-17.—Basic coil configurations.
COILS AND PROBES.—Eddy current coils and
probes consist of one or more coils of wire designed to
introduce a varying magnetic field into a part to
determine the effects of test variables on this magnetic
field. Generation of the magnetic field results from an
alternating current flowing through the coil. A
fundamental consideration in selecting an eddy current
probe or test coil is its intended use. A small diameter
probe or narrow encircling coil will provide increased
resolution of small defects. A larger probe or wider
encircling coil will provide better averaging of bulk
properties.
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.
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