Shipbuilding and marine fabrication
Weld monitoring for shipbuilding
High-volume welding under classification-society rules. The codes that apply, the four monitoring pain points that cost yards most, and which Therness signal addresses each.
ISO 6520-1 group 5 · Imperfect shape and dimension
Continuous undercut (ISO 6520-1 ref. 5011): causes, detection
ISO 6520-1 reference 5011 is a surface groove at the weld toe and a fatigue initiation site. Cause matrix, why visual testing leads here, and the standards that set the acceptance limit.
- ISO 6520-1 reference
- 5011
- Group
- 5 — Imperfect shape and dimension
- Also called
- Undercut, Toe groove
Undercut is unusual among weld imperfections: it is easy to see, easy to measure, and still one of the most common reasons a fabrication is rejected. The reason is that it is judged on depth, and a groove that reads as a shadow to the eye can be over the limit under a gauge.
It also matters more than its size suggests. A continuous toe groove is a stress concentration exactly where the load path turns, which is why codes that deal with cyclic loading treat it far more severely than codes that deal with static loading. On a fatigue- loaded structure it is a crack initiation site, not a cosmetic defect.
Every cause in the matrix is a parameter or a technique, and all of them are present while the weld is being made. That is the practical case for watching current, voltage and travel speed per pass: undercut found by the visual inspector is a rework ticket, and the same condition seen in the parameter record is a setting to correct on the next part.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Welding current above the qualified range | Excess energy melts parent material at the toe faster than filler arrives to fill the groove it leaves. | Record current per pass against the qualified range rather than trusting the panel. |
| Arc voltage or arc length too high | A wide arc washes the toe region and spreads the pool beyond the metal available to fill it. | Log voltage per pass and treat drift as a process signal, not an operator habit. |
| Travel speed too high | The pool has no time to wet and back-fill the melted toe before it solidifies. | Measure travel speed; on mechanised work it is the parameter that silently changes with fixturing. |
| Incorrect electrode angle or excessive weave | The arc dwells on the parent material at the edge of the weave instead of on the joint. | Qualify the technique for the position and verify it in the cell. |
| Welding position and gravity working against the pool | In vertical and overhead work the pool sags away from the toe, leaving the groove open. | Position welds where practical, and qualify separately for positions that cannot be avoided. |
Detection: which route finds it
The column that matters most is the last one. An inspection family marked as not finding this imperfection will not find it however carefully it is applied, and planning around that is cheaper than discovering it during a final examination.
| Family | Method | Capability | What it sees |
|---|---|---|---|
| Visual | Visual testing to ISO 17637, with weld gauges | Primary route | The imperfection is surface-breaking and open, so visual testing is the intended route and the one every code assumes. |
| Thermal | In-process infrared thermography | Supporting signal | Bead geometry and toe wetting show up in the thermal profile, which flags a run before anyone reaches it with a gauge. |
| Camera | Weld pool imaging | Supporting signal | Shows the pool failing to wet the toe while the run is happening, which is the moment the parameter can still be corrected. |
| NDT | Magnetic particle testing to ISO 17638, penetrant testing to ISO 3452-1 | Supporting signal | Confirms and delineates a surface-breaking groove; neither method measures its depth, which is what acceptance turns on. |
Where acceptance is defined
Therness pages never reproduce a standard's acceptance values: those tables are copyrighted and belong in the copy you hold. What follows is the pointer — which document governs, and which part of it to open.
| Standard | Clause topic | Note |
|---|---|---|
| ISO 5817 | Quality levels for surface imperfections, indexed by ISO 6520-1 reference number | The requirement depends on the selected level and on the material thickness. Read it in the standard. |
| AWS D1.1 | Visual inspection acceptance, with different criteria for statically and cyclically loaded connections | Cyclically loaded work is treated more severely because undercut is a fatigue initiation site. |
| API 1104 | Acceptance for pipeline girth welds, assessed on both depth and length | A pipeline criterion and a structural criterion are not interchangeable. |
| ISO 10042 | The equivalent quality levels for aluminium and its alloys | Use this whenever the parent material is aluminium. |
See the condition, not the consequence
Most weld imperfections are produced by conditions that exist while the arc is running. Therness records those conditions on every weld, so a suspect run is known on the shift that made it rather than in a batch report.
