Undercut
Undercut is a groove melted into the base metal adjacent to the weld toe that is not filled with weld metal, reducing wall thickness and acting as a stress concentrator.
Definition
Undercut is caused by excessive welding amperage, fast travel speed, or incorrect electrode angle. AWS D1.1 limits depth to 1 mm (sometimes 0.25 mm for cyclic-loaded structures). It is detected by visual inspection with a fillet weld gauge.
How this damage forms
A groove melted into base metal at the weld toe and left unfilled — a geometric stress raiser exactly where fatigue cracks initiate.
How it is detected
Visual with measurement (depth gauges, profile gauges) is the primary detection; RT shows it as a dark line hugging the toe; MT finds the fatigue cracking undercut later nucleates.
What causes it, and what prevents it
Acceptance is depth-and-length based and differs sharply between statically and cyclically loaded work — codes tolerate shallow undercut in static service and clamp down for fatigue. The repair (blend grinding or a wash pass) must itself be checked: grinding below minimum thickness converts one defect into another.
Codes that govern assessment
AWS D1.1 tabulates limits by loading class; ASME VIII and B31.3 carry their own; measurement practice per the visual examination procedure.
The common reporting error
Eyeballing depth. Undercut acceptance hinges on fractions of a millimetre, and shadowed toe geometry deceives — a finding worth writing is a finding worth measuring.
Where Undercut fits in an inspection programme
A term is only useful when it connects to a decision. Undercut appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Overlap — Overlap is the protrusion of weld metal beyond the weld toe without fusion to the base metal, producing a notch-like discontinuity that reduces fatigue performance.
- Porosity — Porosity is a volumetric weld defect consisting of gas-filled cavities trapped in the solidifying weld pool, classified as scattered, clustered, linear, or piping (wormhole) porosity depending on distribution and morphology.
- Visual Testing (VT) — Visual Testing (VT) is the most basic NDT method, using direct or remote optical observation — sometimes aided by mirrors, borescopes, or cameras — to identify surface defects, geometric anomalies, and condition of components.
More defect terms
Crack · Lack of Fusion · Lack of Penetration · Slag Inclusion · Burn-Through · Weld Spatter · Lamination · Lamellar Tearing · Hydrogen-Induced Cracking · Stress Corrosion Cracking
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.