Karamba Shell View : why two mesh outputs for Utilization?


I ran a simple Mesh analysis on a shell in Karamba 1.3.1 (most recent version).
When I wanted to visualize the Shell View outputs I had only one mesh for all the outputs (displacement, sig1, sig2, Von Mises) except for the utilization which showed two different outputs which were really hard to interpret. They were very different from each from each other and I couldn’t see any logical link with the Von Mises stress distribution.
Has anyone ever met that issue ? Shouldn’t the utilization be just the ratio between the yield stress of the material and the Von Mises stress ?
For information, the cross-section dimension and the material are the same in the entire model.

Thanks in advance for your help !

Hello Sobo,
the two different results are the utilization values at the top- and bottommost layers of the shell.
It is the ratio between the Von Mises Stress and the material strength. The sign comes out negative if the negative value of the second principal stress is larger than the first principal stress.
In case you look at the Von Mises Stress do not forget to set the ‘Position of Results’-slider of the ShellView-component to ‘+1’ or ‘-1’ in order to display the values for the top and bottommost layer.

Hello Clemens,

Thank you for the clarification. Do you have by any chance a reference for the analytical expression of the Von Mises stress that you’re using in the Utillization factor? I was expecting the Von Mises stress to be positive as the square root of a function of the principal stresses :
Or maybe are you using the maximum shear stress (Tresca criterion) in the Utilization factor?

Hello Soraya,
the Von Mises stress in Karamba3D is calculated according to the formula above. It is just the sign which is switched in case that the absolute value of the second principal stress is larger than the first principal stress. The idea behind this is to signal whether a region is under compression or rather in tension.