Interpretation and Elimination of Non-Relevant Indications.
It may at first appear to the inspector that some types of non-relevant indications discussed and illustrated in the
preceding material would be difficult to recognize and interpret. For example, the non-relevant indications shown in
Figure 3-73 and Figure 3-74 may look like indications of subsurface discontinuities. However, there are several
characteristics of non-relevant indications, which will enable the inspector to recognize them in the example cited and
under most other conditions. These characteristics of non-relevant indications are:
a. On all similar parts, given the same magnetizing technique, the indications will occur in the same
location and will have identical patterns. This condition is not usually encountered when dealing with
real subsurface defects.
b. The indications are usually uniform in direction and size.
c. The indications are usually "fuzzy" rather than sharp and well defined.
d. Non-relevant indications can always be related to some feature of construction or cross section, which
accounts for the leakage field creating the indication.
Elimination of Non-Relevant Indications.
Although non-relevant indications can be recognized in most cases, they do tend to increase the inspection time, and
under certain conditions may mask or cover up indications of actual defects. Therefore, it is desirable to eliminate
them whenever possible.
In most cases non-relevant indications occur when the magnetizing current is higher than necessary for a given part.
Consequently, these indications will disappear if the part is demagnetized and re-inspected using a sufficiently low
magnetizing current. Under most conditions the value of magnetizing current which is low enough to eliminate non-
relevant indications will still be sufficient to produce indications at actual discontinuities. This will be true where the
non-relevant indication is magnetic writing, and for several other types, but may not hold where there are abrupt
changes of section. It is therefore desirable to determine whether the non-relevant indication was caused by an abrupt
change of section before re-inspecting.
The proper procedure is to demagnetize and reinspect using a lower value of magnetizing current, repeating the
operation with still lower current if necessary until the non-relevant indications disappear. Care must be taken not to
reduce the current below the value required to produce indications of all actual discontinuities. Where there are abrupt
changes of section two inspections may be required: one at fairly low amperage to inspect only the areas at the change
in section; the other at a higher current value to inspect the remainder of the part. Another solution is to use AC
magnetization for inspection. AC magnetization responds less to changes in cross section than DC magnetization and
is acceptable when it is not necessary to inspect for subsurface defects.
METHODS OF RECORDING MPI INDICATIONS
METHODS OF RECORDING MPI 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