Interpretation, Evaluation, and Reporting of Indications
Three words that must not be confused
An indication is the visible particle buildup. Interpretation decides what caused it. Evaluation compares it to acceptance criteria and yields accept or reject. Indications are relevant (caused by a real discontinuity), nonrelevant (caused by geometry or design features - keyways, threads, section changes, shrink fits, weld bead edges, magnetic writing from residual fields), or false (poor technique, contaminated bath, particles shed by the applicator). Only relevant indications that exceed the acceptance criteria are defects.
Recognizing patterns
Leakage at a section change is broad and diffuse; a crack is sharp and linear. Magnetic writing is a distinctive pattern from local magnetization or a magnetized marker; demagnetize and re-examine to confirm. Over-magnetization produces general background that hides everything - reduce current. Surface indications are sharp; subsurface indications are broad and fuzzy, a useful discriminator, though MT is not used to size subsurface flaws.
Reporting
A defensible report identifies the part and drawing, material and heat treatment, the written procedure and revision, equipment and particle type, magnetization directions with currents or ampere-turns, viewing conditions, the acceptance standard, each indication (type, location, size, orientation), the accept/reject decision, and the certified level of the examiner with date. Sketches and photographs must be traceable to the part.
Traps
Failing to re-examine after demagnetization; logging every nonrelevant indication as a flaw; rejecting against a criterion from the wrong code; and ignoring that linear versus rounded classification changes the allowable size. Evaluation cannot be delegated to someone unqualified for the required level.
Rule of thumb
Ask three questions in order: is it real (relevant)? where and how big is it? does the governing criterion allow it? All three answers belong on the report.