Which aspect cannot be detected using the visual method while a weld is in progress?

The selection of the welding machine current settings as the aspect that cannot be detected visually while a weld is in progress is accurate. This is because the settings of the welding machine, such as the current and voltage applied during the welding process, are determined by the operator before starting the weld and are typically indicated on the machine's control panel. These parameters influence the weld characteristics, but they are not visually observable during the welding process.

In contrast, the other aspects mentioned can be visually assessed. The formation of cracks, for instance, may be visible as the weld cools, allowing for an inspection of the weld's surface for any signs of defects. The width of the root opening can be measured visually against the welding joint specifications. Similarly, the concavity of the weld is also observable during the welding process, allowing the welder to gauge the profile of the bead as it forms. This ability to monitor these attributes visually helps ensure quality and adherence to specifications during welding.