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June 30, 2015, 09:46 |
Stress Analyzes, sigmaEq and Von Mises
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#1 |
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Tobias Holzmann
Join Date: Oct 2010
Location: Bad Wörishofen
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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 |
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June 30, 2015, 14:26 |
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#2 |
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Philip Cardiff
Join Date: Mar 2009
Location: Dublin, Ireland
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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 2-D 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 2-D plane strain and 3-D, everything should be fine. Philip |
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June 30, 2015, 14:55 |
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#3 |
Super Moderator
Tobias Holzmann
Join Date: Oct 2010
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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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Keep foaming, Tobias Holzmann |
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July 1, 2015, 01:50 |
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#4 |
Super Moderator
Tobias Holzmann
Join Date: Oct 2010
Location: Bad Wörishofen
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Rep Power: 51 |
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 |
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July 20, 2015, 22:26 |
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#5 | |
Super Moderator
Philip Cardiff
Join Date: Mar 2009
Location: Dublin, Ireland
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Rep Power: 34 |
Quote:
You could try prepare a simple 3-D block case and apply simple principal stresses to check your calculations. Philip |
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