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material. Agitating the suspension breaks up the aggregates, but they begin to form again as soon as agitation ceases.
This happens when the suspension has been applied over the surface of the part, since the particles act as agglomerated
units of varying size, and not as individual particles.
3.5.2.2.2.2 Advantages of an Agglomeration of Fine Particles.
This agglomeration of fine particles into larger clumps is advantageous as long as the size of the aggregate does not
become larger than the limit mentioned in paragraph 3.5.2.2.2. Individual particles of exceedingly small size move
very slowly through the liquid of the suspension under the influence of leakage fields at discontinuities. Unless special
techniques are used, exceedingly small size particles are not particularly useful for the location of very fine cracks until
the process of agglomeration into somewhat larger units has taken place. In practical applications this process takes
place while drainage of the suspension from the surface of the part is occurring. As the agglomeration proceeds the
clumps formed will vary in size, and since these clumps act as individual units, the effect is that of a particle size range
from very fine to relatively coarse.
3.5.2.2.3
Fluorescent Wet Method Materials.
Paragraph 3.5.2.2.2 applies primarily to magnetic particles that have not been treated with fluorescent pigments.
Fluorescent particles (or even colored visible particles) must be compounded and structured to produce a pigmented or
colored coating that will not readily separate from the ferromagnetic core.
3.5.2.3
Shape.
The shape of the magnetic particles used for magnetic particle testing has a strong bearing on their behavior in locating
defects. When in a magnetic field the particles tend to align themselves along the lines of force. This tendency is
much stronger with elongated or rod-like particles than with more compact or globular shapes because the long shapes
develop stronger polarity. Due to the attraction exhibited by opposite poles, the north and south poles of these tiny
magnets arrange themselves into strings of particles, north pole to south pole, much more readily than do globular
shapes. The result is the formation of stronger patterns in weak leakage fields, as these magnetically formed strings of
particles bridge the discontinuity. The superior effectiveness of the elongated shapes over the globular shapes is
particularly noticeable in the detection of wide, shallow discontinuities, or of those discontinuities, which lie wholly
below the surface. The leakage fields at such defects are more diffuse, and the formation of strings due to the stronger
polarity of the elongated-shaped magnetic particles makes for more visible indications in such cases.
3.5.2.3.1
Dry Powders.
In the case of the dry powders, there is another effect from the shape of the particles, which must be taken into account.
Dry particles are applied to the surfaces of parts by means of plastic powder bottles, rubber squeeze bulbs, or by the use
of compressed air guns. The ability to flow freely and to form uniformly dispersed clouds of powder that will spread
evenly over a surface is a necessary characteristic for rapid and effective dry powder testing. A powder composed only
of elongated shapes tends to gather together in container, and to be ejected in uneven clumps. When a powder behaves
in this manner, the inspection becomes extremely slow and difficult. On the other hand, globular-shaped particles flow
freely and smoothly under similar conditions. A dry powder must have free-flowing properties for easy application, yet
have optimum shape for the greatest sensitivity for the formation of strong indications. These two opposing needs are
met by blending particles of different shapes. A fair proportion of rod-like particles must be present for a sensitive
blend. A sufficient proportion of more compact shapes must be present in order to have a powder that will flow well for
easy and uniform application
3.5.2.3.2
Wet Method Materials.
In the case of particles for the wet method of inspection, the individual particles are kept dispersed by mechanical
agitation until they are applied to the surface of the magnetized part. Therefore, no need exists to incorporate
unfavorable shapes merely for the purpose of improving the flow of the particles. Long, slender particles, with
otherwise desirable characteristics, could be used exclusively.
3.5.2.3.2.1
Because wet method particles are suspended in a liquid medium, which is much denser and more viscous than air, they
move in the leakage fields much more slowly than the dry powders. Therefore, they accumulate much more slowly at
discontinuities. In the vicinity of leakage fields, they can be seen to line up to form minute elongated aggregates. Even
