Undercut — Definition & NDT Use
A groove or depression at the toe of a weld where the base metal has been melted away but not properly filled by weld metal. Undercut reduces cross-sectional area and creates stress concentration, making it a critical weld defect. It is readily detected by visual inspection and NDT methods. Acceptance criteria vary by application; critical applications typically reject any undercut, while less critical applications allow limited undercut.
In service, Undercut 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. 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 Undercut 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.
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 undercut 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 "Undercut" mean in NDT?
A groove or depression at the toe of a weld where the base metal has been melted away but not properly filled by weld metal. Undercut reduces cross-sectional area and creates stress concentration, making it a critical weld defect
Is undercut always rejectable?
No. Whether a undercut 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 Undercut?
The most directly related entries in this glossary are "weld defect", "weld toe", "crack initiation"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.