What does a straight, dark line in the center of a weld bead image on film suggest?

A straight, dark line in the center of a weld bead image on film typically indicates a lack of material in the weld area, which is consistent with incomplete penetration. This visual characteristic occurs when the weld bead does not fully fuse with the base material or permeate through the entire thickness of the joint. As a result, the straight line suggests that there is a gap or void between the base material and the weld metal, which can compromise the integrity of the weld joint.

In terms of understanding the context, overheating during welding would generally produce a different set of indications, such as distortion or burn-through in the material rather than a linear artifact. Similarly, excessive welding speed would usually result in a lack of fusion but could present differently on film. Lastly, excessive filler material would lead to a different visual appearance, such as a more pronounced bulge or uneven surface, rather than a straight line through the center. Thus, recognizing the signs of incomplete penetration through a straight dark line is essential for evaluating the quality and soundness of the weld.