Incomplete Root Penetration

Undercut means the weld doesn't properly penetrate the base material at the root, leaving a void or notch. This usually happens when you're going too fast, the arc is too long, or the weld is too narrow. That spot is the weak point and will crack quickly under load. Adjust travel speed and arc length to the material thickness and make sure the weld pool properly wets the edges. If unsure, rather make a slightly wider weld or an extra root pass to ensure clean penetration.