Indications are any particle accumulations that can be seen on the specimen under test. Just as not all flux leakage fields are defects, not all indications are caused by flux leakage fields.
Indications can be further subdivided into:
Relevant indications are discontinuities or flaws, which in turn are un-designed imperfections.
When it is considered that a relevant indication will affect the fitness for purpose of a test specimen, it is classified as a defect, but not all defects are cracks.
Product and process knowledge (a knowledge of product technology and the processes that a test specimen has been through) is necessary to define and interpret defects more closely. It is perhaps safer, without that knowledge, to categorise indications by their:
Indications which are not held on the surface by a flux leakage are called spurious. Lint, scale, dirt, hairs, drainage lines, etc, are examples.
However, one spurious indication, called magnetic writing, is a little different. If two pieces of steel touch when one of them is in a magnetised condition, local poles are created at the areas of contact. If magnetic particles are then sprinkled on the surface the local poles become visible as fuzzy lines.
Non-relevant indications are true magnetic particle patterns actually formed and held in place by leakage fields. However, they are caused by design features and the structure of the specimen and only exceptionally will they affect the fitness for purpose of the specimen.
Below is a non-exhaustive list:
- Scores and scratches
- Key ways
- Internal splines and drillings
- Abrupt changes of section
- Fine threads
- Force fits
- Dissimilar magnetic material (HAZ or heat treated material)
- Forging flow lines
- Grain boundaries
- Brazed joints
- Cold working
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