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About applying fixed constraint in Structural Mechanics

Posted 2010/10/24 14:42 GMT-4 Version 5.2a 4 Replies

Qiulin Ma
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Dear All,

I currently encountered a problem of using fixed constraint. Basically, in my model, fixed constraint is applied on part of a surface. This is how the structure was fixed and held in reality. It is just a single geometry without any contact pair issue. The solution is converged and I get a pretty reasonable stress distribution. However, the maximum principal stress is mesh dependent. It exists right at a corner around the fixed constraint. With my workstation computer, I can compute with very fine mesh. But the meshing dependent problem still exists.

Have anyone encountered this kind of problem before? Any method to solve it? The maximum stress is a very important information for me to see how good the design is. But the mesh dependence really makes me suffer.

Thanks
Qiulin
4 Replies Last Post 2017/02/13 10:56 GMT-5
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Hello Qiulin Ma

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Wei Shao
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Posted: 1 decade ago 2012/11/15 10:50 GMT-5
you are correct, I think the stress distribution is not totally correct, but i can not find a way..........
you are correct, I think the stress distribution is not totally correct, but i can not find a way..........
Aswani Kumar Mogalicherla
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Posted: 9 years ago 2017/02/12 4:51 GMT-5
Hi

I am also having same problem

Is there any solution found for it?
Hi I am also having same problem Is there any solution found for it?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 9 years ago 2017/02/12 10:54 GMT-5
Hi

This is "normal" as a fixed constraint means u=v=w=0 that is NO displacements, but this is not really "physical", your part is fixed to something with a non-infinite Young modulus, and a given Poisson coefficient (the latter links perpendicular swell or contraction to the normal compression/traction) so in reality it will allow some minor displacements at the "fixed" boundary, if not you will get "numerical" stress hotspots for sure.

if you want to study "true stress" when applying theoretical loads toa cube of material (in compression, or in shear etc) then you need to use symmetry conditions, or cut your cube by mid-planes and apply *roller" conditions on these middle surfaces (in 3D you need 3 roller conditions for tensile-compression but only 2 roller conditions for shear forces).

You might also apply more sophisticated weak conditions to your "fixed boundary" to keep it "flat" etc

Two more caveats:
if you solve for non-linear geometrical displacement, then you should NOT use symmetric BC as this will give you further artificial numerical stress values, since the non-linear theory adds an anti-symmetric factor to your displacement calculations, hence in collision with a symmetric BC.
If you solve for thermal expansion, be aware that many of the "fixed, or Rigid Connector" BC have now an optional thermal expansion sub-node, that allows to compensate for the local thermal expansion effect on these "fixed" boundaries

--
Good luck
Ivar
Hi This is "normal" as a fixed constraint means u=v=w=0 that is NO displacements, but this is not really "physical", your part is fixed to something with a non-infinite Young modulus, and a given Poisson coefficient (the latter links perpendicular swell or contraction to the normal compression/traction) so in reality it will allow some minor displacements at the "fixed" boundary, if not you will get "numerical" stress hotspots for sure. if you want to study "true stress" when applying theoretical loads toa cube of material (in compression, or in shear etc) then you need to use symmetry conditions, or cut your cube by mid-planes and apply *roller" conditions on these middle surfaces (in 3D you need 3 roller conditions for tensile-compression but only 2 roller conditions for shear forces). You might also apply more sophisticated weak conditions to your "fixed boundary" to keep it "flat" etc Two more caveats: if you solve for non-linear geometrical displacement, then you should NOT use symmetric BC as this will give you further artificial numerical stress values, since the non-linear theory adds an anti-symmetric factor to your displacement calculations, hence in collision with a symmetric BC. If you solve for thermal expansion, be aware that many of the "fixed, or Rigid Connector" BC have now an optional thermal expansion sub-node, that allows to compensate for the local thermal expansion effect on these "fixed" boundaries -- Good luck Ivar
Henrik Sönnerlind COMSOL Employee

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Posted: 9 years ago 2017/02/13 10:56 GMT-5
Hi,

This is also discussed in

www.comsol.com/blogs/singularities-in-finite-element-models-dealing-with-red-spots/

Regards,
Henrik
Hi, This is also discussed in https://www.comsol.com/blogs/singularities-in-finite-element-models-dealing-with-red-spots/ Regards, Henrik

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