Root undercut

Root notch is a lack of fusion at the root of the weld. It happens when you melt too much material in the root area or the weld doesn't run out cleanly. The part then can't handle cyclic loading; it will crack at the notch over time. When welding, aim for a calm, even penetration and clean root formation. If you do the weld properly and get the right welding position, you'll avoid the problem. Check the root after welding, and you'll see immediately if there's a notch.