Defect — Definition & NDT Use

In service, Defect starts as a discontinuity that may or may not breach the acceptance criteria of the governing code; the NDT method's job is to detect it, characterise it, and size it so an engineer can decide whether to repair, monitor, or accept. In a composite, the layered structure means impedance changes at every ply boundary; a true delamination registers as a strong reflector at a depth that the C-scan can map across the part to size the affected area for an engineering disposition. Crack sizing is the high-stakes call: amplitude alone is not enough, so techniques such as TOFD, tip-diffraction, or 6dB drop are stacked to bound the height and length used in the engineering critical assessment. The fitness-for-service decision typically pairs the NDT call with material data and stress information; the inspector's job is to give the engineer a clean characterisation rather than to make the keep-or-reject call alone.

The decision tree around Defect runs: detect, characterise, size, and refer to the acceptance table in the governing code; only the last step decides repair, accept-as-is, or fitness-for-service review. Whenever a crack is suspected the inspection plan upgrades from screening to characterisation — TOFD, MT, or tip-diffraction sizing — because the engineering critical assessment needs height and length, not just a yes/no.

Confusing defect with a generic "indication" is a recurring error; the term carries an engineering implication, and the report should distinguish the discontinuity (what was seen) from the disposition (what code says about it).