Weld Defect — Definition & NDT Use
Any discontinuity in a weld that reduces its strength or fitness for service. Common weld defects include cracks, porosity, slag inclusions, lack of fusion, incomplete penetration, and undercut. Detection and evaluation of weld defects are critical aspects of quality assurance in fabrication and in-service inspection. Standards like AWS D1.1 and ASME Section V define acceptance criteria for various weld defects.
In service, Weld 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. 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 Weld 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. 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 weld 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).
What does "Weld Defect" mean in NDT?
Any discontinuity in a weld that reduces its strength or fitness for service. Common weld defects include cracks, porosity, slag inclusions, lack of fusion, incomplete penetration, and undercut
Is weld defect always rejectable?
No. Whether a weld defect 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 Weld Defect?
The most directly related entries in this glossary are "weld", "crack", "porosity"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.