FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 5. Nondestructive Inspection (NDI) | 4. Magnetic Particle Inspection | 5-50. Special Examination Techniques
AC 43.13-1B
9/8/98
Curing time for silicone rubbers varies from
about 30 minutes and up depending upon the
particular silicone rubber, the catalyst, and the
amount of catalyst used to produce the curing
reaction.
b. Curing. While curing is taking place,
the insides of the hole must be maintained in
the required magnetized state. This can be ac-
complished using a permanent magnet, a DC
yoke, an electromagnet, or some other suitable
means. Whatever method of magnetization is
used, the leakage fields at any discontinuities
inside the holes must be maintained long
enough to attract and hold in position the mag-
netic particles until a partial cure takes place.
A two-step magnetizing procedure has been
developed.
(1) The first magnetization is accom-
plished for a short time in one direction fol-
lowed by a second at 90 degrees to the first for
the same length of time. This procedure must
be repeated for whatever period of time is
needed until the cure prevents particle mobil-
ity. Magnetization in two directions
90 degrees apart ensures formation of indica-
tions at discontinuities in all directions inside
the holes.
(2) After curing, the rubber plugs,
which are exact replicas of the holes, are re-
moved and visibly examined for indications
which will appear as colored lines against the
lighter colored background of the silicone rub-
ber. Location of any discontinuities or other
surface imperfections in the holes can be de-
termined from the location of the indications
on the plugs. The magnetic rubber inspection
method is covered in detail in Air Force Tech-
nical Order 33B-1-1, section XI.
sections, that cause obscuring and nonrelevant
indications during normal examination prac-
tices. The procedure provided herein is the
most sensitive method for detecting the early
beginnings of in-service fatigue cracks in the
sharp, internal radii of ferro-magnetic parts.
Magnetic particle examination equipment may
be used; however, alternating fields are not re-
liable to provide the necessary high level of re-
sidual magnetism. Optical aids are necessary
to realize the maximum sensitivity provided by
this magnetic particle procedure. Low-power
(10x-30x) binocular microscopes are recom-
mended. As a minimum, pocket magnifiers of
7 to 10 power may be used with the following
procedure.
(1) Thoroughly clean the part at the
sharp radii and fillets where soils, greases, and
other contaminants tend to accumulate and at
other places where they might be overlooked
during a casual or hasty examination.
(2) The residual method should be used
as an aid in particular problem areas, even
though it is not considered the best practice in
most of the instances. The conventional wet
continuous methods should be used initially
for overall examination and the residual tech-
nique should be applied only for supplemental,
local examination of the sharp radii. It should
not be applied except in those cases where
nonrelevant indications have proven to be a
problem in the initial examination.
(3) Methods of magnetization should be
done according to standard procedures; how-
ever, alternating fields should not be used, and
the level of magnetizing force imposed should
usually be increased above the normal levels to
ensure a higher residual field within the part.
c. Critical Examination for Sharp Radii
Parts. A critical examination is required for
cracks in sharp radii; such as threaded parts,
splines, gear teeth roots, and abrupt changes in
(4) Following magnetization, apply
particles in liquid suspension. The application
should be liberal and in a manner to cause
maximum particle buildup. Immersion of
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