A groove formed at the toe or root of a weld when the base metal is melted away and left unfilled by weld metal is referred to as:

• A. Underfill
• B. Cold lap
• C. Crack
• D. Undercut

Answer: (D)

The situation describes an undercut. This defect is a groove or notch that forms at the weld toe or root when the base metal has melted away but the weld metal doesn’t fill that area, leaving a groove below the surface. It weakens the joint by reducing the cross-section where stresses concentrate and is commonly caused by excessive heat input, too fast travel, or improper filler metal deposition.

In contrast, underfill would leave a concave surface because the weld metal didn’t fill the joint fully at the top surface, not specifically as a groove at the toe or root. Cold lap is metal that has not properly fused with the base or previous pass, creating a seam of poor fusion, not a groove dug into the base metal. A crack is a fracture path through the weld or base metal, not just a surface groove formed by melting away metal.