Edge Undercut Groove

The edge notch is a local cross-section reduction at the transition from the weld to the base material. It occurs due to too much heat input or a too flat transition, causing a lack of material there and a notch effect. It's dangerous because it reduces the fatigue strength of the weld – under alternating loads it can crack quickly. To avoid it, ensure a clean, even transition, don't put in too much heat, and fill the weld properly. If you already have the notch, the only fix is re-welding or grinding out, depending on depth.