For butt welds, how should the test joint be welded?

The correct approach for welding a test joint for butt welds is to use two pipe nipples joined. This method accurately replicates the conditions and configurations found in actual pipeline installations. Butt welding involves joining two pieces of material end to end, which is what taking two pipe nipples and welding them together accomplishes.

This technique ensures that the test is representative of the welding processes that will be employed in real-world applications since it directly matches the geometry and joint design typically encountered in pipelines. The mechanical and metallurgical characteristics of the welded joint formed between two pipes offer a realistic view of the weld's integrity, strength, and performance.

Using test plates or a fillet plate, as mentioned in the other options, does not serve the same purpose as they do not replicate the specific scenario of a butt weld. Test plates may be applicable for other types of welds or configurations, but for the nuances of butt welding, which often deals with stringent quality assurance in pipeline construction, the use of pipe nipples is the most effective and relevant choice.