Discontinuity — Definition & NDT Use
Any break in the structural continuity of material, including internal (cracks, porosity, inclusions) and surface discontinuities. Not all discontinuities are defects; some are acceptable per applicable standards. Discontinuities must be detected, characterized, and evaluated to determine acceptability. The terms "discontinuity," "defect," and "flaw" are sometimes used interchangeably, though technically discontinuity is the broadest term.
In service, Discontinuity 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. 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 Discontinuity 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 discontinuity 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).
What does "Discontinuity" mean in NDT?
Any break in the structural continuity of material, including internal (cracks, porosity, inclusions) and surface discontinuities. Not all discontinuities are defects; some are acceptable per applicable standards
Is discontinuity always rejectable?
No. Whether a discontinuity indication is rejectable depends on the acceptance criteria of the governing code (AWS D1.1, ASME Section VIII, API 1104, etc.), the size and orientation of the indication, and any fitness-for-service evaluation the engineer chooses to apply.
What other NDT concepts should I read alongside Discontinuity?
The most directly related entries in this glossary are "defect", "flaw", "indication"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.