Discontinuity
Incomplete joint penetration
A groove weld that did not reach through the joint as the design required, leaving unfused root faces and an effective throat smaller than the drawing calls for.
What it looks like
- An unfused gap at the root, with the original prepared faces still visible in cross-section
- From the back side, an unwelded slot along the joint centreline where the root should have fused
- On film, a straight dark line at the joint centreline with unusually sharp, parallel edges
- The cap can be entirely normal, so the deficiency is invisible from the welding side
Causes
- Root opening too tight or root face too thick for the current and electrode size being used
- Insufficient current or excessive travel speed on the root pass
- Electrode too large to reach the bottom of the groove, or held at an angle that misses the root
- Included groove angle too narrow for the process, keeping the arc off the root faces
- Misalignment that hides part of the root face behind the opposite member
Prevention
- Detail and control fit-up so root opening and root face match the qualified procedure
- Use an electrode small enough to reach the root and enough current to fuse it
- Back gouge and back weld from the second side where the design requires full penetration
- Check the first root passes on a production joint before the groove is filled and access is lost
How it is found
- RT
- The unfused root gives a continuous straight-sided density change at the centreline, one of the most recognisable indications on film.
- UT
- The flat root face reflects the angle beam strongly at a predictable depth, and scanning from both sides confirms the root position.
- VT
- Where the root side is open, direct examination shows unwelded root faces without any further equipment.
How inspectors think about it
- This is often a design conformance issue rather than a workmanship flaw: the drawing called for a complete joint penetration weld and the joint does not have one.
- The reasoning turns on whether penetration was required at all, since a partial joint penetration weld with a specified depth is a legitimate detail, not a defect.
- Where full penetration was required, there is normally no size at which the shortfall becomes tolerable, because the design assumed the whole thickness was carrying load.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
Incomplete fusionWeld metal that lay against the joint face or a previous bead without melting into it, leaving a tight unbonded plane that carries no load even though the joint looks completely filled.Root crackA crack in or immediately beside the root pass, the thinnest and most highly restrained part of a groove weld, where shrinkage stress from every later pass concentrates on the least metal.Misalignment (high-low)Two members offset from one another across the joint so their surfaces do not line up, forcing load to change path through the weld and often leaving part of one root face unfused.UnderfillA groove weld whose face or root sits below the surface of the adjoining base metal, so the joint has less cross-section than the surrounding material it is supposed to match.
Can you name it on sight?
The gauge trainer's identification mode shows you a weld and four names. Reading about porosity and picking it out of a line-up are not the same skill.