What is a dark crescent shaped spot found in the base metal adjacent to a weld most likely to indicate?

A dark crescent-shaped spot found in the base metal adjacent to a weld is most likely to indicate a film crimp mark. This mark can be a result of the interaction between the welding process and the materials, particularly if a film or other material is involved in the welding area.

These marks typically occur when there is a projection or crimp caused by a film or coating that is on the substrate. The presence of this mark may indicate that the film was not adequately removed or that the weld pool interacted with the film in a way that caused the characteristic dark crescent shape.

In contrast, a crack would generally not have a crescent shape and would more likely appear jagged or linear, while incomplete penetration would manifest as a gap in the weld itself rather than a separate dark area next to it. An undercut typically shows a notch or groove along the edge of a weld bead and would not appear as a crescent-shaped spot either.