Yield Criteria

[sushi75]

Hello!

I'm running a FE stress analysis, and checking the maximum Von Mises stresses, the values remain under the yield strength.

However, looking at the maximum principal stress, there are values above the Yield Strength.

So I'm a bit puzzled, according to VM there will be no yielding but there are actual stresses exceeding this value...so the design requirements cannot be met if we consider these peak stresses.

What conclusion should I make ? drama or compliant?

Thanks a lot for any help you can provide me!

 

[desertfox]

Hi

My understanding is there are area's when a component under stress will in small area's yield locally and it then becomes a judgement on whether the area concerned is a problem or not by the person doing the FE.

We cannot see the component in question or what it does only you know that.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[burningDiesel]

From my understanding of your question, the conclusion that you should make is that you do not know what von Mises stress is.
Unless you are performing a uniaxial FE test, why would you be surprised that the von mises yield criteria is not the same as the maximum principal stress theory?
Which yield criteria you want to use depends on your material, loading, loading rate and conservatism you want to build in.

 

[TheTick]

Depends on the material and failure mode. VM stress is good for ductile material. Max strain is better for brittle fracture and fatigue.

 

[metengr]

You can have no yield or permanent deformation based on VM criteron because of constraint associated with plane strain conditions versus plane stress conditions. This is due to component geometry which you provided no information. Under plane strain conditions, the material will not develop gross yielding and in fact can reach the ultimate tensile strength resulting in fracture.

 

[sushi75]

Hello,

Thanks all for your replies! The material is ductile so the VM criterion makes sense. it's still a bit confusing to see that in this analysis (pressure vessel), there are peak hoop stress slightly higher than yield, but that we consider the VM for compliance.

metengr, I don't fully understand why under plane strain conditions, the material will not develop gross yielding?
is it related to the thickness of the structure?

Cheers!

 

[metengr]

sushi75
Yes, thickness is an essential variable. Very simply put, when you look at the the VM equation, you must take into account stresses in principal directions. When you have a uniaxial stress condition in simplest form, yielding occurs when the principal stress equals the tensile YS. When two or more principal stresses are acting, which is typical for most components, yielding will not occur at the tensile YS, it will be at a slightly higher value known as the VM effective stress. Under plane strain conditions, yield happens at 1.157 of the tensile YS. I would suggest you research the VM failure criterion.

 

[sushi75]

Thanks for your reply, it's much clearer now!

Just another quick question, in case I have high compressive stresses for the max principal, should it considered also critical? Is Yield Strength in compression and traction the same?

Cheer!

 

[CoryPad]

For most ductile metals, the yield strength is essentially equal in compression and tension.