Figure 3-28. AC/HWDC Portable Power Pack
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 fields
generated by induced currents in a part, which are produced by rapidly varying longitudinal fields. Another specialized
technique uses magnetic flakes in viscous slurry, taking advantage of the difference in light reflection from flakes that
have become reoriented by leakage fields at discontinuities. Another technique uses a diluted silicone rubber
containing black magnetic particles for the inspection of the interior or otherwise difficult to view surfaces. The liquid
rubber 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, the
rubber material, which has formed a replica of the surface against which it was placed, is viewed under low power
magnification for the indications formed during the inspection. Multidirectional magnetization can be very effective in
detecting randomly oriented discontinuities quickly. The technique energizes two or more magnetizing circuits in
different directions very rapidly (almost simultaneously) resulting in a reduction of testing time and part handling.
Induced Current Magnetization.
A varying magnetic field in any conducting metal generates electrical current in that metal. The amplitude of the
current can be reduced by increasing the length of the current path by a cut, an insulated joint, or a deep surface
indentation. The amplitude will also depend upon the size and shape of the cross section through which the magnetic
field varies the rate of variation in flux lines per second, and the electrical conductivity of the metal. A single pulse of
induced current will flow around in the part, at right angles to the magnetic field, when the magnetic field strength is
changed. When the magnetic field is varying in a continuous manner, as it does in the case of alternating or half-wave
DC fields, a continuing succession of induced current pulses are produced. These induced current pulses are often
referred to as eddy currents.
The process of inducing high amplitude eddy currents in a part to be inspected can also introduce stray eddy currents in
adjacent 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. The
generation of such heat is of little consequence in magnetic particle inspection because of the relatively
short duration of the current flows.