Which symbol is used to indicate 100% penetration on one-side welds?

Which symbol is used to indicate 100% penetration on one-side welds?

• A. Melt through
• B. Full penetration
• C. Penetration
• D. Root pass

Answer: (A)

When a weld must fuse everything from one side and actually reach and melt through to the far face, the drawing uses the symbol melt-through. This clearly communicates that the weld has gone completely through the joint thickness on a one‑sided weld, so the backside is fused as well. Full penetration describes complete fusion depth across the joint, but it doesn’t inherently indicate that the far side has melted through—one-sided access can achieve full depth without implying a backside burn-through. Penetration alone is too vague to specify whether the far side has been melted through, and a root pass refers to the initial layer, not to the overall through-thickness condition.

When a weld must fuse everything from one side and actually reach and melt through to the far face, the drawing uses the symbol melt-through. This clearly communicates that the weld has gone completely through the joint thickness on a one‑sided weld, so the backside is fused as well.

Full penetration describes complete fusion depth across the joint, but it doesn’t inherently indicate that the far side has melted through—one-sided access can achieve full depth without implying a backside burn-through. Penetration alone is too vague to specify whether the far side has been melted through, and a root pass refers to the initial layer, not to the overall through-thickness condition.