# Correlating Exported Cauchy Stress Tensor Values with Von Mises values

#1

Do the exported Cauchy node values from an FEA study directly correlate with the Von Mises values? Or, are the exported von Mises values based on a separate algorithm (eg element averaged)? I ask because my understanding is the von Mises values follow this equation relative to the Cauchy values ((x^2+y^2+z^2-xy-yz-xz+3(Txy^2+Tyz^2+Tzx^2))^.5. However, in the following example I do not get the exported von Mises value. Exported Cauchy values (node viewed in ParaView): 65.9423, 64.9355, 22.4473, 44.7695, -9.65216, -6.16268 (x,y,z,xy,yz,zx). Exported von Mises value 92.8558. Based on the Cauchy values and the equation, I get 90.859. I have done this on more than one study, and find a variable difference.
Any insight would be appreciated.

#2

Hi @felweh!

Will ask about that to be 100% sure Getting back to you as soon as possible!

Jousef

#3

Hi again @felweh!

So both values are calculated at the Gauss points and are extrapolated to the nodes!

There is a difference between SimScale’s workflow and yours:

1. We first calculate v.Mises and extrapolate it to the nodes

2. You first extrapolate the Cauchy Stress to the nodes and then calculate the v.Mises stress

In general, there should not be a big difference. If there is, this is definitely an indicator that your mesh is too coarse.

Does that help you? Let us know if you need any further information. Tagging the @FEA-SQUAD in here in case you have more specific questions.

Best,

Jousef

#4

Hey, thanks for a quick answer. That does help. Some of my analysis process uses the Cauchy values to derive von Mises values, which I compared to the original von Mises. It’s something that could have been questioned in the future, but now we have a reason, thanks.
-Chris