Which statement best defines lack of penetration?

• A. The weld fills the joint completely from face to face.
• B. The weld penetrates beyond the required thickness.
• C. Weld does not fully penetrate the joint thickness as required by the spec.
• D. The weld has excessive root gap.

Answer: (C)

Penetration is about how deep the weld metal fuses into the joint thickness. Lack of penetration means the weld does not reach the full thickness required by the specification, so the root fusion is incomplete and part of the joint remains un-welded. The statement describing that the weld does not fully penetrate the joint thickness as required by the spec matches this idea exactly, making it the best choice.

Understand that other scenarios describe different conditions: filling from face to face indicates full penetration, penetrating beyond the required thickness is excessive penetration, and a large root gap is a joint-fit issue rather than penetration depth. In practice, avoiding lack of penetration involves proper heat input, correct joint preparation, and sound welding technique to ensure fusion through the entire joint thickness.