being magnetized. The laminated core in this case increases the total flux threading the ring. It should be remembered
when using the induced current method, that any means used to increase the flux in the direction of the coil field
through the part, will increase the magnitude of the induced currents, up to the point of magnetic saturation.
Magnetic flux through the center region of disc-shaped parts which have a small bore hole, or none at all, can be
increased by placing a laminated core centered against each side of the disc. Another variation for the use of a
laminated core is in the inspection of holes in large parts suspected of having circumferential discontinuities. In this
case the magnetizing coil is placed around one end of the core and the other end is used as a probe for placement in the
hole. Alternating current is used to energize the coil. In operation the core is placed in a hole and, while the coil is
energized, liquid magnetic particle media is sprayed around the inside surfaces of the hole. Before withdrawing the
core from the hole, the coil is de-energized so as not to demagnetize the area around the hole. When demagnetization
of the area is wanted, the core is simply removed from the hole while the AC current is flowing.
Selection of Current Level.
No rule-of-thumb formulas have been developed for the induced current method of magnetization. Lacking any other
information upon which to select a current level, the rule-of-thumb formulas given in paragraph 18.104.22.168.3 may be used
to obtain trial amperages for parts having L/D ratios up to 15. Part diameters, which approach, or are greater than that
of the coil and are very short in length, for example, disc-shaped parts will usually require laminated cores to be used,
so the rule-of-thumb coil formulas are not applicable. The formulas were developed for the determination of coil
amperages, which will produce a longitudinal flux density of 70,000 lines per square inch in a part. The rate of change
or rate of collapse of this longitudinal flux produces an induced current in the part, which in turn results in leakage
fields at the discontinuities.
A magnetic particle testing material is available that supplements both wet and dry magnetic particle testing materials.
This material formulation uses selected magnetic particles dispersed in a viscous, oily vehicle which results in slurry
having the consistency of paint. The material is brushed on a surface to be inspected until the magnetic particles are
evenly and thoroughly distributed. A magnetic field is generated in the test part through conventional AC or half-wave
DC magnetizing techniques. Any discontinuities show as contrasting black indications on a gray background.
Alternating current fields using a yoke or probe are capable of revealing very fine surface discontinuities using this
The slurry, being a viscous liquid applied by brush, has the advantage over dry powder of eliminating any hazard to
adjacent equipment by airborne magnetic particles. Another advantage is that the slurry can be applied and used
successfully on vertical or overhead surfaces, on wet (even underwater) or dry surfaces and over scaly, plated or painted
surfaces if the coatings are not too thick.
The slurry concentration can be varied to suit particular inspection requirements. The material is brushed evenly on a
part, much as paint would be, prior to magnetization of the part. If needed the material can be brushed repeatedly
permitting magnetization in various directions. The oily vehicle used in the slurry mixture is nondrying, and the slurry
can be removed using dry rags, paper towels, or prepared cleaning solvents.
Magnetic rubber formulations using finely divided magnetic particles in a silicone rubber base are used for the
inspection of holes and other surfaces which are not easily accessible. The liquid silicone rubber mixture is poured into
holes or against the surface of the magnetic parts to be inspected. Curing time for silicone rubbers varies from about 10
to 30 minutes, depending upon the particular silicone rubber, the catalyst, and the amount of catalyst used to produce
the curing reaction.