I have been trying to wrap my head around if a flange will change the maximum stress a material is exposed to to predict fatigue failure. I have not been able to find any information on clamped flange stress analysis for thin walled pressure vessels. I contemplated just using a simple Mohr's circle diagram, but I am still unsure what torsion the material will see, and if a flange makes any difference in calculations. I attached a sketch of what we are working with:
Torque Flange (1).pdf
Basically the vessel is 304 stainless steel cylinder with a 1/8" thickness, inner diameter of 4.022", and a flange that has a soft 90 degree bend with about 1/2" of flange material to clamp on to. The same is repeated for both ends of the vessel, and it is pressurized to about 160 psi 100,000 times. Stress cracking forms at the bend of the flange at around 70k pressure cycles every time. I was hoping to get some pointers for how to find information on how a flange changes things in terms of torsion and load bearing, or any other helpful information.
Your 304 material appears to have be formed to create the flange. The forming stresses and work hardening are not something that can be known to the forum viewers. Was the material annealed after forming? Again we can't know from the sketch. Checking the material in the bend for hardness might tell you something about the work hardening. Are there cracks generated during forming?
We also do not know about the temperature history of the material. Has it been in the sensitizing range for a period of time?
Since stainless is used in harsh environments. These factors can influence both material selection and life expectancy.
The relatively high fatigue life compared to your goal might suggest something other than mechanical stress. A strain gauge applied to the bend might be the best way to determine the local loading.
Pressure vessels should be engineered by a professional engineer with access to all the information and standards because they are a danger to people if not designed correctly. So hire a professional engineer to analyze and fix the problem. Forum viewers don't have sufficient information to correctly analyze the problem.