Discontinuity
Longitudinal crack
A crack running parallel to the weld axis, either down the centreline of the bead or along the fusion line, driven by shrinkage stress acting across the joint as the deposit cools and contracts.
What it looks like
- A fine dark line following the length of the bead, often on or near the centreline
- May be hairline and nearly invisible until the surface is cleaned and lit from a low angle
- In fillet welds it can run through the throat, the shortest and most highly stressed path across the weld
- Sharp, tight, and irregular in profile rather than smooth-walled like a gas cavity
Causes
- High restraint in the joint so the weld cannot contract freely as it cools
- A deep narrow bead whose depth-to-width ratio drives centreline segregation and solidification cracking
- Excessive current or travel speed producing a bead that is too small for the section it is joining
- Cooling too fast in thick or cold material, so contraction stress builds faster than the metal can yield
- Filler metal with high carbon or sulphur pickup from the base metal, which weakens the last metal to freeze
Prevention
- Balance bead shape so width is at least equal to depth, keeping the last metal to solidify away from a sharp centre plane
- Preheat according to thickness and chemistry, and control interpass temperature to slow the cooling rate
- Sequence welds and use back-stepping to reduce accumulated restraint in the assembly
- Choose a filler metal with adequate tolerance for the base metal's carbon and sulphur content
How it is found
- VT
- Surface-breaking longitudinal cracks are visible after cleaning, especially with oblique lighting that shadows the opening.
- MT
- On ferromagnetic steel a field applied across the weld axis leaks at the crack, and particles build a sharp line directly over it.
- PT
- Penetrant seeps into the tight opening and bleeds back into developer, producing a fine continuous line on non-magnetic material.
- UT
- The planar crack face reflects sound strongly when the beam strikes it, and the signal is sharp with an abrupt drop-off at the crack tips.
How inspectors think about it
- Planar discontinuities are judged far more harshly than rounded ones because a sharp crack tip concentrates stress and can propagate under load rather than simply reducing section.
- Cracks are the one category inspectors treat as unacceptable on sight regardless of size, since a small crack under service loading is a large crack later.
- Finding a longitudinal crack usually raises questions about the whole joint, because the restraint and cooling conditions that cracked one bead were present for every bead beside it.
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
Transverse crackA crack running across the weld axis, at right angles to the direction of travel, caused by longitudinal shrinkage stress in a weld that is stronger or harder than the metal around it.Crater crackA small star-shaped or linear crack in the depression left where the arc was broken, formed because the last shallow pool of metal cooled fast and shrank into a cavity it could not fill.Hot crack (solidification cracking)A crack that forms while the weld is still partly liquid, when low-melting films between growing grains are pulled apart by shrinkage before the metal has any strength to resist it.Toe crackA crack starting at the junction of the weld face and the base metal, where the abrupt change in section concentrates stress and the heat-affected zone is at its hardest and least forgiving.
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.