Side-wall lack of fusion

Lack of fusion at the flank is a bonding defect when the weld metal doesn't properly fuse with the joint flank. This usually happens when you don't heat the flank enough or move too fast over the spot. The weld may look okay, but inside there's no real connection, and that's a classic weak point that tears under load. So make sure the flank is clean and hot enough and the arc properly catches it, then you'll weld it cleanly.