Exceeded yield stress

[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.

 

[stanier]

Pe as is common with thermoplastic materials will exhibit creep. When the portion of tank deflects due to load the stress dissipates. The same occurs in plastic strain of thin wall stainless tanks.

In addition the properties of PE are strain rate dependent. Thus it is impossible to define yield stress as you have.

Suggested further reading may be by Hertzog on failure mechanisms.

http://www.knovel.com

has excellent books on plastic failure mechanisms.

You could always attach strain gauges to the tank test it and compare the strain to the acceptable strain for the material.