another advantage. The magnetizing effect is 1.41 times that of the current read on the meter. To get equivalent
magnetizing effect from DC more power and heavier equipment is required.
DC on the other hand, magnetizes the entire cross section uniformly in the case of longitudinal magnetization and with
a field that varies linearly from a maximum at the surface to zero at the center of the bar, for direct contact (circular)
magnetization. The types of DC are straight DC from batteries; full wave rectified three phase AC, and full wave and
half-wave rectified single phase AC.
For the inspection of finished parts, such as the machined and ground shafts and gears of precision machinery, DC is
frequently used. Although AC is excellent for the location of fine cracks that actually break the surface, DC is better
for locating very fine nonmetallic stringers lying just under the surface. It is usually important to locate such stringers
in parts of this type, since they can initiate fatigue failures. These comparisons point up the importance of choosing the
right current type to give the best indications possible, and show how the choice will vary, depending upon the nature
and location of the defects sought.
Current / Particle Application Technique.
The use of dry powder with residual inspection has several disadvantages; it is more difficult to apply to interior regions
of a part than is wet media; it is more difficult to completely cover a part in a short time; and removal of powder from a
part can be a problem.
Proper illumination and good eyesight are the principal requirements for observing the presence of indications on the
surface of parts. Selection of the best color powder for contrast against the surface is an aid to visibility. Last, but
certainly not least, the magnetization must be sufficient to generate a useable leakage field at the location of
discontinuities but not excessive to where the background degrades the contrast of any indications formed. On the
large discontinuities, dry powder build-up is often very heavy, making indications stand out clearly from the surface.
For finer cracks the build-up is less, since fewer particles are held by the leakage field. For extremely fine cracks, some
form of the wet method, which is more sensitive to very fine discontinuities, should be used.
The same requirements for proper inspection of surfaces apply for the detection of subsurface discontinuities. The
depth below the surface and the size and shape of the discontinuity determine the strength and spread of the leakage
field. A proficient inspector will observe the surface as the powder is allowed to drift onto it, and will see faint but
significant tendencies of the powder to gather. Often indications are seen under these conditions, but are no longer
visible when more powder has been applied, the excess blown off, and the surface then examined for indications.
Standardized techniques for careful and proper application of the powder can help assure the required sensitivity is
achieved where similar assemblies are repetitively tested.
Indications are held at the defect by the residual field for highly retentive steels. In low carbon steels, the retentivity is
very low. On these steels it is important to perform the inspection while the magnetizing current is on and the powder
is being applied, since indications may not remain in place after the current is turned off. This is particularly true on
vertical and overhead surfaces, where gravity plays a part in causing particles to fall away if lightly held. However,
inspection requirements for the higher retentive steels often require the detection of very small defects. Even though
the residual field may be high in such steel, the leakage fields for small defects will also be small and therefore the
indications are not held at the surface very well.