Well, in that case the parentage is pipe -> coupling -> DWA, in other words there is the raw material (the pipe), the derived product (which could still be the pipe, but may be the coupling) and the DWA, which needs to point you back to the original stock, I would presume.
The trick in all this is not necessarily thinking about where something comes from, but how you logically tie things together, rather than physically tying them together.
So, with your work processes, you need to decide "what do I need to know in order for the DWA to make sense". I'd guess that there is a raw stock item, a process, and a product and the result of a test. So the DWA needs to reference a test result, which in turn references a product (either pipe or coupling), which in turn references the process which created it, which in turn references the bill of materials used as feed in to the process. In this I'm presuming the DWA has to be investigated back up the manufacturing chain, but again I'm not sure how you use the DWA or how your processes are set up.
In the end you need to work out what the purpose of the DWA is and decide what the characteristics are, and if you can generalise it.
What exactly is this database supposed to manage? Fault analysis and reporting? Manufacturing from raw stock to final product and sales? Is the DWA central to the database design or just one aspect that you need to manage? These are all questions you will need to resolve for yourself in order to come up with a database design. Try to stop thinking about the physical items and instead think of the processes that you need to support and what data you need to support that.
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