examination of skins for cracks, and inspection of
welds. Figure 6-14 shows typical angle-beam
inspections.
SURFACE WAVE.--The surface-wave method
projects a beam of vibrations that travel along the
surface and just below the surface of the material.
When surface waves are used to inspect painted
Figure 6-15.--Surface-wave inspection.
surfaces, you should exercise caution during set up and
interpretation due to the possibility of surface reflection
conductor. Figure 6-16 shows eddy currents flowing in
from scratches and breaks in the painted surface.
various configurations.
Surface-wave inspections can be used in many field
NDI applications involving surface cracks or slightly
COILS AND PROBES.--Eddy current coils and
subsurface discontinuities. On smooth surfaces, sound
probes consist of one or more coils of wire designed to
energy can travel long distances with little energy loss.
introduce a varying magnetic field into a part to
Surface waves travel around curved corners, and they
determine the effects of test variables on this magnetic
reflect at sharp edges. Rough surfaces or liquid on the
field. Generation of the magnetic field results from an
surface attenuate surface waves so the area in front of
alternating current flowing through the coil. A
the search unit must be kept clear of couplant. Figure
fundamental consideration in selecting an eddy current
6-15 shows a typical surface-wave inspection.
probe or test coil is its intended use. A small diameter
probe or narrow encircling coil will provide increased
Eddy Current Inspection
resolution of small defects. A larger probe or wider
encircling coil will provide better averaging of bulk
Eddy currents are electrical currents induced in a
properties.
conductor of electricity by reaction with a magnetic
field. The eddy currents are circular in nature, and their
TEST COIL CONFIGURATIONS.--Eddy cur-
paths are oriented perpendicular to the direction of the
rent probes and coils can be classified into three
applied magnetic field. In general, during eddy current
types: surface probes, encircling coils, and inside
testing, the varying magnetic field(s) is/are generated
by an alternating electrical current (ac) flowing through
a coil of wire positioned immediately adjacent to the
conductor, around the conductor, or within the
Figure 6-16.--Generation of eddy currents in various part
Figure 6-14.--Angle-beam inspection.
configurations.
6-11
