Linear Indication — Definition & NDT Use
A defect or indication whose length is significantly greater than its width, such as a crack or lack of fusion in a weld. Linear indications are potentially more serious than rounded indications of equivalent area because stress concentrations are more severe. Linear indications in fatigue-critical applications require more stringent acceptance criteria. Proper measurement and characterization of linear indication length is essential for flaw evaluation.
In service, Linear Indication 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 Linear Indication 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. On welded fabrication it is most often paired with VT and one volumetric method (RT or UT) so surface and internal defects are both addressed. 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.
AWS D1.1
Structural Welding Code — Steel; defines visual and NDE acceptance for static and dynamically loaded welds.
ASME Section IX
Welding, brazing, and fusing qualifications referenced by every U.S. pressure-equipment code.
Confusing linear indication 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 "Linear Indication" mean in NDT?
A defect or indication whose length is significantly greater than its width, such as a crack or lack of fusion in a weld. Linear indications are potentially more serious than rounded indications of equivalent area because stress concentrations are more severe
Is linear indication always rejectable?
No. Whether a linear indication 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 Linear Indication?
The most directly related entries in this glossary are "indication", "planar indication", "crack"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.