T.O. 33B-1-13-45Figure 3-28.AC/HWDC Portable Power Pack3.3.14 SpecialMethods.3.3.14.1 Summary.Many parts, because of their small L/D ratio, shape, complicated geometry, or the location and kind of discontinuities,require specialized techniques to obtain a good magnetic particle inspection. One of the techniques uses the fieldsgenerated by induced currents in a part, which are produced by rapidly varying longitudinal fields. Another specializedtechnique uses magnetic flakes in viscous slurry, taking advantage of the difference in light reflection from flakes thathave become reoriented by leakage fields at discontinuities. Another technique uses a diluted silicone rubbercontaining black magnetic particles for the inspection of the interior or otherwise difficult to view surfaces. The liquidrubber is catalyzed, placed against the surface to be inspected and held in place with the appropriate dams and fixtures.Applied magnetic fields cause the particles to migrate to defect locations while the rubber is cured. After cure, therubber material, which has formed a replica of the surface against which it was placed, is viewed under low powermagnification for the indications formed during the inspection. Multidirectional magnetization can be very effective indetecting randomly oriented discontinuities quickly. The technique energizes two or more magnetizing circuits indifferent directions very rapidly (almost simultaneously) resulting in a reduction of testing time and part handling.3.3.14.2 InducedCurrentMagnetization.3.3.14.2.1General.A varying magnetic field in any conducting metal generates electrical current in that metal. The amplitude of thecurrent can be reduced by increasing the length of the current path by a cut, an insulated joint, or a deep surfaceindentation. The amplitude will also depend upon the size and shape of the cross section through which the magneticfield varies therate of variation in flux lines per second, and the electrical conductivity of the metal.A single pulse ofinduced current will flow around in the part, at right angles to the magnetic field, when the magnetic field strength ischanged. When the magnetic field is varying in a continuous manner, as it does in the case of alternating or half-waveDC fields, a continuing succession of induced current pulses are produced. These induced current pulses are oftenreferred to as eddy currents.3.3.14.2.1.1The process of inducing high amplitude eddy currents in a part to be inspected can also introduce stray eddy currents inadjacent metallic components. The effect of stray eddy currents in a metal is twofold.a. Heat is generated whenever an electric current flows in a conductor because of resistance. Thegeneration of such heat is of little consequence in magnetic particle inspection because of the relativelyshort duration of the current flows.
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