TM 1-1500-204-23-7
(3)
Evaluating the indication. Lastly, after the indication has been formed and has been interpreted, it must be
evaluated. It is necessary for the operator to decide whether that particular location on that particular part will affect the
usefulness of the part. Evaluation is the determination of whether the part can be used in spite of the indication, whether
the cause of the indication can be remove without affecting the strength of the part, or whether the part must be
scrapped.
4-12. Methods of Recording Indications. The full value of magnetic particle inspection can be realized only if records
are kept of parts inspected and the indications found. The size and shape of the indication and Its location on the part
should be recorded along with other pertinent information, such as rework performed or disposition. The inclusion of
some visual record of the indications on a report makes the report much more complete. Various methods of recording
indications are explained in the following paragraphs.
a.
Fixing Indications with Lacquer. One of the advantages of magnetic particle inspection is that the indication is
formed directly on the part at the exact spot of the magnetic leakage field. This makes it possible to retain the part Itself
for record purposes, but it is necessary to fix or preserve the indication on the part so that the part can be handled and
examined without smudging or smearing the indication. One method of fixing the indication semipermanently on the part
is by using clear lacquer. In order to do this, the part must be dry, if the wet method has been used to develop the
indication, the oil should be allowed to evaporate. Normal evaporation can be accelerated by heating the part; it is also
possible to flow on carbon tetrachloride or other solvent which will evaporate rapidly and leave the indication dry on the
part. It is usually desirable to thin out the clear lacquer by adding lacquer thinner. The lacquer should either be sprayed
on the part or flowed on since brushing would smear the indication.
b.
Applying Transparent Tape. It is also possible to preserve an Indication on a part by covering it with transparent
pressure sensitive tape (such as Scotch brand). This method is not as neat looking as the lacquer method but it is easier
to apply. Before applying the tape, the oil used in the wet method should be removed In the same manner as when using
lacquer.
c.
Dry Particle Indications. If the indication is formed of dry powder particles, excess powder should be removed
from the surface by gentle blowing. Use a piece of tape larger than the indication and gently cover the indication with the
tape, sticky side toward the indication. Gentle pressure should be applied so that the adhesive will pick up the particles;
do not press too or the indication will be flattened too much and the tape may be difficult to remove. Carefully lift the
tape form the part and press it onto the record sheet or report. Tape preserved indications are usually a little broader
than indications on the part because of the flattening effect of the tape. It is easier to remove the tape if a corner of It is
not pressed to the part; this leaves a tab for easy removal.
d.
Wet Particle Indications. If the indication is formed of particles used with the wet method, it is necessary to dry
the surface of the part before applying the tape. Drying the oil can be done by normal evaporation, usually a slow
process, or by accelerating the drying by applying heat and a gentle air steam. The oil can also be removed by carefully
washing the area with carbon tetrachloride or other volatile solvent and then allowing the solvent to dry. The solvent
should be sprayed or flowed on to prevent damage to the indication.
e.
Fluorescent Indications. Tape transfers can be taken of fluorescent particle indications but there are some
disadvantages to the process. Such preserved indications usually must be viewed under black light to properly interpret
them since the number of particles In the suspension is much less than when using visible particles. Some transparent
tape is fluorescent and the fluorescence of the tape may mask the fluorescence of the indication.
f.
Photographing Indications. Photographs of indications can also be taken to be used for record purposes.
Enough of the part should be shown to make it possible to recognize the part and the position of the Indication. It is
helpful to include in the picture some common object to show the size of the part Sometimes this can be done with finger
pointing at the indication or by placing a ruler along the part to show relative size. In photographing indications on highly
polished parts, care must be taken to avoid highlights or reflection which may hide indications. Taking photographs of
fluorescent indications calls for special photographic techniques.
4-13. Demagnetization. Any ferromagnetic material subjected to magnetic particle inspection requires
demagnetization. When performing magnetic particle inspection of aircraft parts, it is essential to demagnetize the parts.
Parts fabricated from ferromagnetic material retain a certain amount of residual magnetism (or remnant field) after
application of a magnetizing force This does not affect the mechanical properties of the part. However, it is necessary to
reduce the residual magnetism retained in a part by demagnetization. Ferromagnetic air frame parts are demagnetized
to prevent magnetic flux from affecting instrumentation.
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