Yield stress in PE

[Grill]

I have ran a non linear analysis on a plastic rotomoulded tank where the tank is pressurised up to 0.3 bar for 30 mins. The tank is made out of polyethylene which we are assuming has a yield stress of 17 Mpa.

The analysis is telling me that one section of the tank will see a stress of 20 Mpa for this test. This value is not a hot spot and is a constant along the section of the tank. This area is also a radiused corner so the stress is bot due to a rightangle either.

My question is will be the tank be permantly weakened due to it seeing this stress for this period of time??? We have an actual tank and can perform this test on it. What measurements if any should i take off the physical tank to put my mind at ease???

Any other suggestions would be gratefully appreciated

Thanks in advance

Grill.

 

[jetmaker]

As for the test, the placement of strain gauges on the area of the hotspot is desireable. Suggest that you load the system up to 25%, then 50% of max pressure. Compare results against the FEM to make sure they are in agreement. If they are, suggest you hold on further testing till you get some more info on your question about exceeding yield.

Regards,

jetmaker

 

[sdm919]

I'm not familiar with the behavior of PE, but I would ask the following questions. Is the yield stress of PE defined as for metals? How does it behave after yielding? Is creep an issue?

Did your nonlinear analysis include material nonlinearity or large displacements or both? If you did not use material nonlinearity, then the stresses are inaccurate anyway, since they do not account for post-yield redistribution. If you did use material nonlinearity, then the stresses should not exceed the ultimate allowable stress.

Another issue to consider is the model details in the area of high stress. Is it a beam, shell, or solid model, or a combination? Is the stress caused by poorly shaped elements? Does the high stress occur at a boundary, and how is that boundary modeled? If the boundary is assumed rigid, but the actual structure is flexible, you might get artificially high stresses there.

Finally, what is your design criteria? Are you using a factor of safety? You might have to show that there is no yielding at a certain load level and no failure at a higher load level. I assume there are specific design requirements for pressure vessels, but I don't have them.

Good Luck!