What is a sign of a bad fillet weld?

• A. Convexity
• B. Concavity
• C. Even toe
• D. Smooth surface

Answer: (B)

A fillet weld’s strength comes from the throat—the minimum thickness of weld metal that actually resists the load. The toes should be smooth and the weld should fill consistently so the throat isn’t reduced. When the weld surface is concave, the throat becomes thinner than intended, weakening the joint and making it more prone to fatigue and cracking. This underfill often points to issues like insufficient heat or deposition during welding. So concavity signals a weaker, defective weld. In contrast, a smooth surface with even toes and appropriate reinforcement indicates proper fusion and a sound weld, while a pronounced convexity can be acceptable if not excessive but isn’t as clear a defect as a concave throat.