Dear all
First of all, I’d like to tell you all that I’m not a stress expert just a Caesar user and I’ve been trying to be a good one.
Turning back to my doubts, I’m specifically curious about the value of yield stress criterion chosen in the “configuration/set up” between “Von Mises theory” and “Maximum shear theory (Default)” That will give the maximum stress output differently. My questions are as follows
1. What benefit can we use this value for? For the fatigue estimation of pipe material? If I ‘m wrong, please correct or give me some directions.
2. Why is mode “Max3Dshear” yield stress criterion selection, the result of highest stress of “3D max intensity” higher than the “Allowable Stress B31.1” at some nodes whereas “Von Mises” mode will not experience. I do know that these values are derived from different fomulae but I’m concerned that whether it is alright if it's more than allowance or not.
3. Form question 1, In case that we can use this stress for some purposes; for instance, material-lifetime prediction or specific node investigation, which stress value coming from between Max3Dshear and Von Mises should I use?
Perhaps I’m misunderstanding about this value, please make me clear and give me some suggestions. Thank you very much indeed.

Best regards

Hi,
I am trying to reply you. As per the theory of failure, max. shear stress theory is consevative than distortion energy theory(i.e,Von Mises theory). Hence you are finding all this difference. To be in safer side, you should use 3D max intensity obtained by Maximum shear theory for fatigue stress check.
Regards,

Thanks a lot Manoj,
Your suggestions seem to answer my question no.1 and 3 so, my understanding is correct. Max shear stress will be used for either material fatigue or failure estimation as per your anwswer.
How about my second question? Is there any signficant meaning of 3D max stress ? In fact, it is my colleague's concern (who works in the maintenance division). His concern is why 3D max stress is more than the allowable stress.
Still waiting for the answer form other experts making me for clarity.
Thanks and Regards

Different theories of failure have their own allowable limits. If a limit is defined as yield (from a simple tension test), then the maximum shear stress approach would predict failure (of any, more complicated load set) when the maximum shear stress (the radius of the largest "Mohr's Circle") exceeds one half of the material yield stress. When using distortions energy, octahedral shearing stress, or equivalent stress (all three terms are "identical"), failure is predicted when the calculated stress exceeds SQRT(2)/3 times the material yield stress.

The piping codes have their own defined stresses and limits and I do not get uncomfortable when one of these "non-code" stresses exceed the code-defined allowable limits.