Quantcast Interpretation and Elimination of Non-Relevant Indications

T.O. 33B-1-1 3-112 Interpretation and Elimination of Non-Relevant Indications. Interpretation. 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. SECTION VIII METHODS OF RECORDING MPI INDICATIONS 3.8 METHODS OF RECORDING MPI INDICATIONS. 3.8.1 General. 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


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