Fillet Weld
A fillet weld is a weld of triangular cross-section that joins two surfaces approximately at right angles to each other (typically T-joints, lap joints, or corner joints), characterized by leg length and throat thickness rather than by groove preparation.
Definition
Fillet welds are inspected primarily by visual and surface methods. Leg and throat dimensions are checked with fillet weld gauges. Volumetric inspection is generally not practical due to the geometric reflectors at the weld toes.
What it is
A weld of approximately triangular cross-section joining two surfaces at an angle, typically a tee, lap or corner joint. Strength derives from throat dimension rather than from leg length, though leg is what gets measured.
Correct use
It is largely outside the reach of useful volumetric examination — geometry makes radiography and conventional angle-beam ultrasonics difficult to interpret — so acceptance leans heavily on visual and surface methods, with profile, leg size, undercut and root fusion being the practical criteria.
Governing standards
AWS D1.1 for structural fillet weld acceptance and sizing; ASME Section V Article 9 for the visual examination; ASME Section IX for procedure qualification.
The usual mistake
Accepting leg size as evidence of throat. A convex weld can show correct legs with an inadequate effective throat, and a concave one can meet throat with legs that look undersized.
Where Fillet Weld fits in an inspection programme
A term is only useful when it connects to a decision. Fillet Weld 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
- Butt Weld — A butt weld is a weld in a butt joint, joining two components edge-to-edge in approximately the same plane, with groove preparation (V, U, J) that allows full-penetration welding from one or both sides.
- Socket Weld — A socket weld is a fillet weld used in small-bore (typically up to 2") socket-welding pipe fittings, in which the pipe is inserted into a socket and the fillet weld is made around the resulting circumferential gap.
- Welded Joint — A welded joint is the connection produced when two metal components are fused together by welding, classified by joint type (butt, fillet, lap, corner, edge) and groove geometry (V, U, J, square), with NDT scope driven by joint type and code.
More equipment terms
Calibration Block · Transducer · Dual-Element Probe · Angle-Beam Probe · Normal-Beam Probe · Couplant · Reference Reflector · Side-Drilled Hole · Flat-Bottom Hole · Notch Reference
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.