Internal stress in a Female NPT 1

[JTraeger]

I'm redesigning a Female NPT fitting and was wondering if anyone had any info on how to figure out internal stress or how it might be distributed throughout the fitting. Is it all hoop stress, or is there a lot of axial stress from the threads pushing against each other? What is a reasonable estimate for the hoop stress? Any help at all would be appreciated, I can't find much info anywhere.

 

[JTraeger]

This fitting can be anywhere in a house, all it does is connect two pipes together. I haven't been able to crack or yield these fittings from over torqueing or thermal cycling. The only way is to induce SCC by loading the fitting and basically dipping it in ammonia repeatedly. It'll cause SCC in a matter of a day or two. In the field about 0.01 percent fail, and all are from SCC. It also seems that multiple fittings fail in the same house and they will fail again of they are replaced. I guess changing materials or chrome plating is the only way around it. Reducing the stress isn't very easy, partly because there are too many variables to consider. Installation, thermal expansion, knocking, they will all contribute to the internal stress on the fitting and are hard to quantify.