Discontinuity
Root crack
A 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.
What it looks like
- A line following the weld axis at the root, visible from the back side where access allows
- May appear as a split in the root bead itself or in the heat-affected zone just beside it
- On film, a fine dark line at the joint centreline, often with slightly ragged edges
- Frequently accompanied by other root problems such as incomplete penetration or root porosity
Causes
- A root pass too small to carry the shrinkage load imposed by subsequent passes
- Excessive root opening or poor fit-up forcing the first pass to bridge more than it can support
- High joint restraint from heavy sections or from fixturing that prevents any movement
- Insufficient preheat allowing rapid quenching of a thin bead against cold heavy plate
- Hydrogen concentrated at the root from moisture in the gap or a missing purge
Prevention
- Control fit-up so the root opening and land match what the procedure was qualified with
- Deposit a root pass with enough cross-section to resist the contraction of the passes above it
- Preheat heavy or restrained joints and start the second pass before the root cools right down
- Keep the root gap clean and dry, and purge where the material and joint call for it
How it is found
- RT
- The root is directly in the beam path on a groove weld, and the crack's reduced density prints as a fine dark line at the centreline.
- UT
- Angle-beam scanning at the first leg puts the sound directly on the root, and a planar crack there gives a strong, sharply located echo.
- VT
- Where the root side is accessible, direct examination of the root bead finds surface-breaking cracks without any equipment.
- MT
- Applied to an accessible root surface on steel, it reveals tight root cracks that visual inspection alone would pass.
How inspectors think about it
- Root discontinuities are weighed heavily because the root is usually the point of maximum stress in a groove weld and the hardest area to repair once the joint is filled.
- A planar flaw at the root is oriented to open under bending, which is why it is treated more severely than a flaw of the same size buried in the middle of the weld.
- Finding a root crack normally means excavating back to sound metal from one side, so inspectors confirm the finding with a second method before committing to that cost.
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 joint penetrationA 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.Linear porosityGas pores lined up in a row, almost always along the weld root or between passes, because the gas source is itself a line such as an unclean root face or a contaminated interpass surface.Longitudinal crackA 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.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.