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ANGLE BEAM.
-Angle-beam
methods are used extensively for
field NDI, and can provide for inspection of
areas with complex geometry or limited access. This is
because angle beams can travel through a material by bouncing
from surface to surface. Useful inspection information
can be obtained at great distances from the search
unit. Angle-beam inspections are particularly applicable
to inspections around fastener holes,
Figure 15-13.-Through-transmission inspection.
Figure 15-14.-Angle-beam inspection.
Figure 15-15.-Surface-wave inspection. inspection of cylindrical components, examination of skins for cracks, and inspection of welds. Figure 15-14 shows typical angle-beam inspections. -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 surfaces, you should exercise caution during set up and interpretation due to the possibility of surface reflection from scratches and breaks in the painted surface. Surface-wave inspections can be used in many field NDI applications
Figure 15-16.-Generation of eddy currents in various part con-figurations. involving surface cracks or slightly subsurface discontinuities. On smooth surfaces, sound energy can travel long distances with little energy loss. Surface waves travel around curved corners, and they reflect at sharp edges. Rough surfaces or liquid on the surface attenuate surface waves so the area in front of the search unit must be kept clear of couplant. Figure 15-15 shows a typical surface-wave inspection.Eddy Current Inspection Eddy currents are electrical currents induced in a conductor of electricity by reaction with a magnetic field. The eddy currents are circular in nature, and their paths are oriented perpendicular to the direction of the applied magnetic field. In general, during eddy current 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 conductor. Figure 15-16 shows eddy currents flowing in various configurations.
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. |
 
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