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Principal stress 2d vs 3d

Posted Nov 12, 2012, 3:29 a.m. EST Structural Mechanics Version 4.3a 6 Replies

Robert Swierszcz
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Hi everyone,

I have the same model made in 2d and 3d. First and second pricipal stresses are similiar in both models. But third principal stress in 2d are unrealistically high, however in 3d have value ​​similar to the expected.

How to explain it?

Best Regards.

6 Replies Last Post Nov 19, 2012, 7:38 a.m. EST
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 12, 2012, 8:16 a.m. EST
Check whether you assumed plane stress or plane strain in 2D, and if that selection is in agreement with the geometry and boundary conditions in the 3D model.

Nagi Elabbasi
Veryst Engineering
Check whether you assumed plane stress or plane strain in 2D, and if that selection is in agreement with the geometry and boundary conditions in the 3D model. Nagi Elabbasi Veryst Engineering
Robert Swierszcz
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Posted: 1 decade ago Nov 14, 2012, 6:50 a.m. EST
I made 2d axisymmetric model and I'm almost sure that boundary and initial condition are the same in 3d model.
I enclose files if you could check them.

Kind regards,
I made 2d axisymmetric model and I'm almost sure that boundary and initial condition are the same in 3d model. I enclose files if you could check them. Kind regards,
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 15, 2012, 9:34 a.m. EST
Hi

I'm not sure they are the same, in 2D axi you have a vertical symmetry axis, that makes cylindrical 3D shapes, your model is a 2D model of a 3D model assumed independent of the depth "Z" (2D out of paper direction)

--
Good luck
Ivar
Hi I'm not sure they are the same, in 2D axi you have a vertical symmetry axis, that makes cylindrical 3D shapes, your model is a 2D model of a 3D model assumed independent of the depth "Z" (2D out of paper direction) -- Good luck Ivar
Robert Swierszcz
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Posted: 1 decade ago Nov 19, 2012, 2:50 a.m. EST
Thank for the raply Ivar but I can't agree with you.

I made two simple models in 2d and 3d (attached below) and as you can see in 2d model third principal stresses are equal about 100MPa, when in 3d they are approximates equal 0MPa. Calculated gradient of temperature is the same in both models.

Does this mean that if I want to get correct results of stresses I have to do model in 3d? This conclusion doesn't satisfy me.

Kint Regard
Thank for the raply Ivar but I can't agree with you. I made two simple models in 2d and 3d (attached below) and as you can see in 2d model third principal stresses are equal about 100MPa, when in 3d they are approximates equal 0MPa. Calculated gradient of temperature is the same in both models. Does this mean that if I want to get correct results of stresses I have to do model in 3d? This conclusion doesn't satisfy me. Kint Regard

Attachments:

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 19, 2012, 3:13 a.m. EST
Hi

first to make your two models really identical, set the 2D depth to 0.05 m (for the thermal flux) not that it changes that much

Then if you plot your third principal stress at three coordinate levels 0 25 50 mm you will see its not constant, and is principally influenced by a singularity at the corners, try increasing the mesh too, its rather mesh dependent too

For me your point fixed constraints are not the best and giving false values

--
Good luck
Ivar
Hi first to make your two models really identical, set the 2D depth to 0.05 m (for the thermal flux) not that it changes that much Then if you plot your third principal stress at three coordinate levels 0 25 50 mm you will see its not constant, and is principally influenced by a singularity at the corners, try increasing the mesh too, its rather mesh dependent too For me your point fixed constraints are not the best and giving false values -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Nov 19, 2012, 7:38 a.m. EST
Hi Robert,

The 2D and 3D models are not identical. If you fix 2 points at the bottom in 2D you should fix 2 edges in the 3D model (not 4 points). If your 3D temperature “slice” plot is closer to the +-Y ends (rather than at the middle) of the block the predicted principal stresses get closer to the 2D results.

Nagi Elabbasi
Veryst Engineering
Hi Robert, The 2D and 3D models are not identical. If you fix 2 points at the bottom in 2D you should fix 2 edges in the 3D model (not 4 points). If your 3D temperature “slice” plot is closer to the +-Y ends (rather than at the middle) of the block the predicted principal stresses get closer to the 2D results. Nagi Elabbasi Veryst Engineering

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