Lack of Fusion — Definition & NDT Use
A weld defect where fusion did not occur between the weld and base metal (at the sides of the weld) or between weld passes. Lack of fusion creates a linear discontinuity parallel to the weld surface. It significantly reduces weld strength and is a critical rejectable defect. Lack of fusion is best detected by angle-beam ultrasonic testing from the side. Proper arc voltage and technique prevent lack of fusion.
In service, Lack of Fusion 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 Lack of Fusion 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.
- Etymology / Origin
- Welding-engineering term from the 1940s arc-welding handbooks; abbreviated LOF in modern weld inspection reports.
- Formula
- Detected as planar reflector at the bevel face or between weld passes; UT angle-beam amplitude can rival a back-wall echo at favourable orientation.
- Units
- Length and height in mm; orientation relative to the weld axis.
- Typical Range
- Sidewall LOF most common in narrow-gap GMAW; inter-pass LOF in multi-pass SMAW/FCAW; detection threshold ~3 mm for UT, ~2% wall for RT.
- Measured / Produced By
- RT (favourable orientation), UT angle-beam (best for sidewall), PAUT (encoded sizing), MT/PT (only if surface-breaking).
- Code References
- AWS D1.1 Cl. 6.12 (LOF rejectable in static and cyclic); ASME B31.3 Table 341.3.2 (LOF acceptance); API 1104 Sec. 9.3.6
- Worked Example
- Sidewall LOF at 22 mm depth in a 30 mm weld, UT 60° angle-beam: indication at 110% DAC, length 18 mm by 6 dB drop — rejectable under AWS D1.1 cyclic-load criteria.
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.
ASME Section V Article 4
Ultrasonic examination methods for welds and components.
ASTM E114 / E164 / E2375
ASTM straight-beam, contact, and wrought-product UT practices.
Confusing lack of fusion 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 "Lack of Fusion" mean in NDT?
A weld defect where fusion did not occur between the weld and base metal (at the sides of the weld) or between weld passes. Lack of fusion creates a linear discontinuity parallel to the weld surface
Is lack of fusion always rejectable?
No. Whether a lack of fusion 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 Lack of Fusion?
The most directly related entries in this glossary are "weld defect", "fusion boundary", "incomplete penetration"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.