Lack of Penetration (LoP)
Lack of penetration is a weld defect in which the weld root fails to extend completely through the joint thickness, leaving an unfused area at the root face.
Definition
LoP typically occurs when root gap, root face, or amperage are inadequate for full penetration in a single-sided weld. It produces a planar discontinuity at the root that significantly reduces joint strength. RT readily detects LoP as a dark straight line at the root; UT detects it as a strong corner-reflector echo.
How this damage forms
The weld metal has not reached the root of the joint, leaving the intended fusion face unfilled. It is a geometric shortfall at the root, distinct from lack of fusion on a side wall.
How it is detected
Radiography shows it clearly when the beam runs along the unfilled root — a sharp, straight dark line following the joint. Angle-beam ultrasonics gives a strong, consistent corner reflection from the root geometry.
Why it gets missed
It is invisible to surface methods where the root is inaccessible, which describes most single-sided pipe welds. That is precisely why root examination is specified volumetrically.
What causes it, and what prevents it
Cause is nearly always procedural rather than accidental: root gap too tight, land too heavy, current too low, or travel speed too high. Fixing the weld without fixing the joint preparation reproduces it on the next joint.
Codes that govern assessment
ASME Section IX governs the procedure that should have prevented it; acceptance is set by the construction code — ASME Section VIII, B31.3, AWS D1.1 and API 1104 each treat root discontinuities differently, and API 1104 is notably specific about it.
The common reporting error
Confusing it with incomplete fusion in reporting. They have different causes and different code treatments, and a report that uses the terms loosely sends the welding engineer to the wrong parameter.
Frequently asked questions
Is lack of penetration always rejectable?
It depends entirely on the referencing code and the service. Some codes reject any root discontinuity in certain service categories; others permit limited amounts against defined length and depth criteria. The examination standard never sets acceptance — the construction code does.
Where Lack of Penetration fits in an inspection programme
A term is only useful when it connects to a decision. Lack of Penetration 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
- Lack of Fusion (LoF) — Lack of fusion is a weld defect in which the deposited weld metal fails to fuse with the base metal or with the previous weld pass, producing a planar discontinuity along the fusion boundary.
- Crack — A crack is a linear discontinuity in a material with negligible opening width, resulting from stress, fabrication, or in-service damage, and is the most safety-critical defect type in pressure equipment and structures.
- 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.
Further reading
weld defect classification and detection methods
More defect terms
Slag Inclusion · Porosity · Undercut · Overlap · 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.