Fillet weld penetration is typically specified as a percentage of base metal thickness.

• A. 60-75 percent
• B. 70-85 percent
• C. 75-85 percent
• D. 25-50 percent

Answer: (D)

Fillet welds fuse into the base metal along the edge but are not meant to penetrate the full thickness of the plate. The depth of fusion, or penetration, is controlled by heat input, weld size, and joint geometry, and for typical fillet welds it’s a partial penetration rather than a through-thickness weld. In practice, this fusion depth is usually about a quarter to half of the base metal thickness. That range (roughly 25% to 50%) provides adequate strength for many fillet-welded joints while avoiding burn-through and excessive distortion. Higher penetration depths (60–85%) are more characteristic of groove or full-penetration joints, not standard fillet welds, and would be unnecessary and riskier in many cases. Lower penetration than about 25% could compromise the weld’s throat strength, which is what ultimately governs the joint’s load-carrying capacity. In many codes and specs, throat size or leg length is emphasized, but when penetration is given as a percentage, 25–50% is the typical expectation for fillets.