T.O. 33B-1-1
3-91
3.7.2
Definition of Terms.
The magnetic particle inspector must understand the distinctions between discontinuity, indication and defect.
3.7.2.1
Discontinuity.
A discontinuity is an interruption in the normal physical structure or properties of a part. Discontinuities may be
cracks, laps in the metal, folds, seams, inclusions, porosity, and similar conditions. A discontinuity may be very fine or
it may be quite large. A discontinuity may or may not be a defect; that is it may or may not affect the intended use of
the product or part. A discontinuity, which would be a defect in one part, may be entirely harmless in another part
designed for a different service.
3.7.2.2
Indication.
An indication is an accumulation of magnetic particles being held by a magnetic leakage field to the surface of a part.
The indication may be caused by a discontinuity or it may be caused by some other condition that produces a leakage
field or it may be caused by mechanically held particle accumulation.
3.7.2.3
Defect.
A defect is a discontinuity that interferes with the intended use of a part.
3.7.3
Basic Steps of Inspection.
Magnetic particle inspection can be divided into these three basic steps:
a. Producing an indication on a part.
b. Interpreting the indication.
c. Evaluating the indication.
3.7.3.1
Producing an Indication.
In order to produce a proper indication on a part, it is necessary to have some knowledge of the principles of
magnetism, the materials used in inspection, and the technique employed. Since these subjects have been covered in
previous sections of this manual, observance of the procedural steps therein should insure that a proper indication is
produced.
3.7.3.2
Interpreting the Indication.
After the indication is created, it is necessary to interpret that indication. Interpretation is the determination of what
caused that indication. Knowledge of metal processing is often invaluable in identifying the cause of an indication.
3.7.3.2.1
Indications caused by a discontinuity at the surface of the part are characterized by particles that are tightly held to the
surface by a relatively strong magnetic leakage field. The particle accumulation has well defined edges and there is a
noticeable "build-up" of the particles. This build-up consists of a slight mound or pile of particles, which on deep
surface cracks is sometimes high enough above the surface of the part to cast a shadow. If such an indication is wiped
off the discontinuity can usually be seen.
3.7.3.2.2
Indications caused by a discontinuity below the surface are characterized by a broad and fuzzy looking accumulation of
particles. The particles in such an indication are less tightly held to the surface because the leakage field is weaker.
3.7.3.2.3
The difference in appearance between indications of surface and subsurface discontinuities is clearly shown in Figure 3-
42 and Figure 3-43. Notice the sharpness and definition of the accumulation of magnetic particles in Figure 3-42. The
pattern in Figure 3-43 is much broader than that in Figure 3-43 and is quite typical of the indications formed over
subsurface discontinuities.
