Weld toe

The weld toe is a location, not a defect: it is simply the line, running along each side of a weld, where the bead surface transitions into the surrounding parent material. What makes it worth naming on its own is that this transition is a geometric discontinuity almost by definition — the bead’s convex profile meeting a flat surface at an angle — and a geometric discontinuity concentrates stress under load regardless of how sound the weld metal itself is.

This is why the toe, rather than the weld’s interior, is where fatigue cracks in welded structures most commonly start. A joint can pass every volumetric NDT check on the weld metal and still be fatigue-limited by the stress concentration at its toe profile. The angle the toe makes with the parent surface, and how smoothly it blends rather than undercutting into it, therefore matter for fatigue life in a way they do not for static strength, which is set instead by the throat thickness of the joint. A joint can have generous throat and still have a short fatigue life if its toe geometry is poor.

Undercut is the most common toe-located imperfection: a groove cut into the parent metal at the same line, which turns an already-concentrated location into a sharper notch still. Visual inspection at the toe checks for undercut, overlap (excess metal overhanging past the toe line) and toe cracking together, because all three sit at the same geometric feature and share the same fatigue consequence. Where fatigue life is the governing requirement rather than static strength, improvement treatments — toe grinding to blend the profile, or localized remelting — are applied specifically to reduce the stress concentration at the toe, independent of any repair to the weld metal itself.