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 1 · Cracks
Branching crack (ISO 6520-1 ref. 106): causes and detection
ISO 6520-1 reference 106 is a primary crack that has split into connected branches from a shared origin. Cause matrix, why sizing is the real challenge, and where acceptance is set.
- ISO 6520-1 reference
- 106
- Group
- 1 — Cracks
- Also called
- Branching crack, Branched crack
Branching crack describes a crack that has stopped propagating along a single plane and split into two or more connected paths from a shared origin. ISO 6520-1 reference 106 distinguishes it from a group of separate, disconnected cracks by that shared origin.
Branching is a property of an existing crack, not a new initiating mechanism, which is what makes its cause matrix different from every other entry in this group: the question is not what started the crack but what made it split — a locally weaker microstructure at the tip, a multiaxial stress field at a discontinuity, continued cycling, or hydrogen diffusing toward an already-stressed tip.
The practical consequence is for sizing, not for finding it: a branch running out of one method’s favourable plane is exactly the extent a different technique, or probe angle, catches. See transverse crack for a common starting point, and microcrack and lack of fusion for the other entries where sizing is the real inspection question.
Typical causes
| Cause | Mechanism | Control |
|---|---|---|
| Progressive extension of an existing crack under continued strain | Stress concentration at an open crack tip can exceed local toughness in more than one direction, splitting the crack into two or more paths. | Treat any confirmed crack as an active defect and stop rather than continue welding or loading the part. |
| A microstructure with locally variable toughness | A crack tip meeting a weaker or more brittle path — a segregation band, coarse grain, or liquated boundary — deviates and can split. | Control heat input and cooling rate to limit grain coarsening and segregation banding near a likely crack path. |
| Multiaxial residual stress at a geometric discontinuity | At a stop-start, weld intersection, or section change, the stress field is not aligned to one direction, favouring more than one propagation plane. | Reduce geometric discontinuities in the joint design, and treat any crack found there as a higher-consequence finding. |
| Hydrogen continuing to diffuse into an already-cracked region | A crack tip is itself a region of elevated stress, drawing hydrogen preferentially and promoting further branched extension. | The hydrogen controls that prevent the primary crack also limit whether an early crack branches further before it is found. |
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 | Supporting signal | Finds the surface-breaking portion, but the branches may run in directions and depths not evident from the surface trace. |
| Thermal | In-process infrared thermography | Limited | By the time a crack has branched, the causing conditions are usually already flagged by the crack's own initial signature. |
| Camera | High-speed weld pool imaging | Does not find it | Branching happens after the pool has solidified, often after further passes or heat treatment; the pool camera has nothing left to observe. |
| NDT | Ultrasonic or phased array testing to ISO 17640 and ISO 13588, penetrant or magnetic particle testing for the surface trace | Primary route | Sizing each branch's extent and plane is exactly the problem phased array testing suits; a single radiograph can under-report an out-of-plane branch. |
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 imperfections indexed by ISO 6520-1 reference number; cracks, including branching cracks, are excluded from every level. | A branching crack gets no different treatment for having multiple paths — it is still a crack, and still excluded. |
| AWS D1.1 | Structural welding acceptance, treating any crack as a mandatory-repair finding regardless of its geometry. | Repair scope for a branching crack is typically larger, since every branch has to be removed. |
| ASME BPVC Section VIII | Pressure vessel repair requirements following any crack found by the examination methods in Section V. | Full removal and re-examination applies to every branch, not only the one first detected. |
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.
