Using Magnetic Rubber
The magnetic rubber technique was developed for detecting very
fine cracks and is capable of revealing finer cracks than other
magnetic techniques. Additionally, the technique can be use to
examine difficult to reach areas, such as the threads on the inside
diameter of holes, where the molded plugs can be removed and examined
under ideal conditions and magnification if desired. The tradeoff,
of course, is that inspection times are much longer.
The techniques uses a liquid (uncured) rubber containing suspended
magnetic particles. The rubber compound is applied to the area
to be inspected on a magnetized component. Inspections can be
performed using either an applied magnetic field, which is maintained
while the rubber sets (active field), or the residual field from
magnetization of the component prior to pouring the compound.
A dam of modeling clay is often used to contain the compound in
the region of interest. The magnetic particles migrate to the
leakage field caused by a discontinuity. As the rubber cures,
discontinuity indications remain in place on the rubber.
The rubber is allowed to completely set, which takes from 10
to 30 minutes. The rubber cast is removed from the part. The rubber
conforms to the surface contours and provides a reverse replica
of the surface. The rubber cast is examined for evidence of discontinuities,
which appear as dark lines on the surface of the molding. The
molding can be retained as a permanent record of the inspection.
Magnetic rubber methods requires similar magnetizing systems
used for dry method magnetic particle tests. The system may include
yokes, prods, clamps, coils or central conductors. Alternating,
direct current, or permanent magnets may be used to draw the particles
to the leakage fields. The direct current yoke is the most common
magnetization source for magnetic rubber inspection.