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June 30, 2015, 10:46 
Stress Analyzes, sigmaEq and Von Mises

#1 
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Tobias Holzmann
Join Date: Oct 2010
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Hi all,
I am a bit confused at the moment. Using the solver solidDisplacementFoam and a simple test case (beam with tension; fixed pressure) leads to a sigmaEq which should be the von Mises stress. I have to check it in my book tomorrow but up to know I checked the "wikipedia" where I get the equation for calculating von Mises like: Code:
sigma_vonMises = sigmaEq = sqrt(3*I) where I is the invariant of the stress tensor deviation: I = 1/2 * s_ij * s_ij Code:
magSqr(dev(sigma)) Code:
sigmaEq = sqrt((3.0/2.0)*magSqr(dev(sigma))) If I compute the stresses in all directions with Code:
foamCalc components sigma If I do this in paraview I get a complete different result from the equivalent von Mises calculated by FOAM and by the general eqn. Both should be the same if I understand it correct. All hints are welcomed.
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Keep foaming, Tobias Holzmann 

June 30, 2015, 15:26 

#2 
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Philip Cardiff
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Hi Tobias,
I tried this using the plateHole tutorial case and the results were almost the same using both methods (I assume the small difference was due to precision). If you were looking at a 2D case, you need to be careful for the plane stress cases as sigma_ZZ is not directly forced: it is instead implied through manipulation of mu and lambda lame parameters: this means sigmaEq will not be correct. For 2D plane strain and 3D, everything should be fine. Philip 

June 30, 2015, 15:55 

#3 
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Tobias Holzmann
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Hi Philip,
thanks for your replay. I already saw today, that you are using stress solver within openfoam. I will check it out tomorrow but in my case it was a 3d cylinder with a tension force. Maybe I will upload the case tomorrow. Are you using extend for stress analyzes or the standard version? I should simulate the stresses in a disc with a borehole. Imagine that there is a screw in the borehole with tension force. Then I wanna know how the strain is. It should be possible with FOAM (maybe with Extend). Can you confirm that?
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July 1, 2015, 02:50 

#4 
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Tobias Holzmann
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Hi,
I also checked the von Mises at the plateHole but did not get same results. Using that formula: I get different values for the von Mises stresses. I assumed tau as zero here. Any suggestion would be appreciated. Thanks in advance.
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Keep foaming, Tobias Holzmann 

July 20, 2015, 23:26 

#5  
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Philip Cardiff
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Quote:
You could try prepare a simple 3D block case and apply simple principal stresses to check your calculations. Philip 

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