It is a sketch I drew to help you visualize how the incomplete fusion is transverse to the tensile forces (residual stress) transverse to the longitudinal axis of the weld. The incomplete fusion is a significant stress riser. The larger it is, the grater the influence. It is a stress riser, but it also represents a reduction is cross section. That increases the unit stress in that localized area,
If you isolate the area where the incomplete fusion is located, it is easy to visualize how it reduces the cross-sectional area. The stress, S, divided by the cross section equals the unit stress. When the unit stress exceeds the tensile stress of the base metal/ weld metal, the crack is going to increase in size, and increase the unit stress even more. The incomplete fusion is going to "open “and the unit stress increases and shed some of the load to adjacent areas.
I'm trying to help you envision the mechanics of what happen and how if the incomplete fusion is small, it may be "insignificant. However, as the size of the incomplete fusion increases in size, it can represent a significant reduction in cross section. At this point the loading is static.
If the loading is cyclic, the repercussions are more significant.
Best regards - Al