Root penetration defect

If the root doesn't melt through cleanly, you don't have proper penetration and the cross-section won't carry the load. That usually happens when you're using too little heat or moving too fast. Then the weld won't hold and cracks or fractures can occur. So adjust the welding speed and heat so the root melts reliably and bonds cleanly with the base material. If you're unsure, run a test weld and check the root from the back side.