T.O. 33B-1-1
3-46
b. The second effect of stray eddy currents is important in magnetic inspection. The magnetic fields
resulting from the stray eddy currents is in opposition to the magnetic fields which produce them,
resulting in either a reduction of the amplitude of inducing alternating magnetic fields or a decrease in
decay rate for an inducing field generated by a collapsing DC current. Either condition results in a
reduction in amplitude of the induced current in the part to be inspected. Precautions must be taken to
minimize the generation of any induced stray eddy currents in metals in contact with or in the
immediate vicinity of the part to be inspected. Any pole pieces should be made of laminated silicon
transformer steel or low carbon steel with a low magnetic retentivity. Any part supports or contact
plates should be split or cut partially through in such a manner as to produce as long a current path as
practical. In some cases part supports, in addition to being split, are made of nonmagnetic metals such
as brass or stainless steel, which are also poor electrical conductors. This also reduces the stray eddy
currents generated in them.
3.3.14.2.1.2
Induced current magnetization is used for the detection of circumferential defects in rings, discs, and cylinders. The
advantages of using the induced current method are:
a. No current contact need be made on a part.
b. Strong fields are generated in a part by the induced currents.
c. Parts with L/D ratios of less than one can be inspected without the need for extremely high coil
currents.
3.3.14.2.1.3
Induced current techniques require that the part be circular in shape and have no deep radial cuts or slits which would
prevent the generation of an induced current around in the part. It is the circular field produced by such an induced
current that generates the leakage fields at circumferential discontinuities. Circumferential discontinuities, in order to
be detected using the induced current method, must be at or very near the surface of a part. The circular magnetic
fields generated by induced currents tend to be crowded toward an outer surface. Circular, disc, or cylindrically-shaped
parts, which are retentive, may be inspected residually using a single pulse of induced current such as obtained when
DC current in a coil is suddenly interrupted allowing the coil field to rapidly collapse to zero. Parts having a low
retentivity must be inspected using the continuous method and AC or half-wave DC current in the coil. The repeated
induced current pulse generated by each cycle of these currents is responsible for the formation of the indications at
discontinuities. For parts with smooth surfaces, care is required when handling the parts after inspection to prevent
mechanical loss of the indications. Washing action is much less of a problem with parts having rougher surfaces, as
indications are held by both mechanical and magnetic bonds.
3.3.14.2.2
Technique.
Parts to be inspected using the induced current method must be positioned with their axis parallel with that of the coil,
or coils. Two coils, one on each side of a part, may be used when the part's diameter is larger than that of the coils.
The coils in this case must be connected electrically; assuring that the coil fields will be in the same direction through
the central region of the part. If the part is retentive and is to be inspected residually, DC current is used in the coil.
The power pack supplying the DC to the coil must have quick break electrical circuitry so as to obtain a rapid collapse
of the coil field. Alternating or half-wave DC current must be used in the coil with the continuous method when a part
is made of steel having a low retentivity.
3.3.14.2.2.1
The longitudinal flux density in a part and the rate of decay or collapse of this flux determines the magnitude of the
induced current that will be generated in the part. The higher the coil amperage, the higher will be the coil field
strength and the higher the flux density in a part up to a coil amperage which produces magnetic saturation in the part.
The flux density and thus the induced currents in short cylinders having an L/D ratio of less than 3 or 4, can be
increased by placing the part between two laminated pole pieces while being magnetized. Induced currents in ring-
shaped parts, such as bearing races, can be increased by placing a laminated core or pole piece in the ring while it is
