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ElineH

stress constraint in design variable vs. as constraint topology optimization

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Hi,

 

I know this question is already asked multiple times but I still haven't fully understood it.

 

What is the exact difference when I define my stress constraint in the design variable of topology optimization or when I add it as a response and then a constraint of the optimization.

 

When I add it in the design variable, without any other constraint it doesn't do anything (see attachment). 

Opposite to when I define it as a response and constraint. Then it does give results, although not very good ones.

When I apply them both on one design, it gives a different result then when I only apply one of them 2.

 

I do topology optimization of a hook with a minimum compliance and no other constraints or requirements.

 

thank you!

stressasdesignvariable.PNG

stressasresponse.png

stressasresponseanddesignvariable.PNG

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Hi,

 

Stress constraint is not recommended in the design concept stage: topology, topography and free-size optimization.



The stress constraint definition in a topology optimization is a global constraint and does not target local stress concentrations.  These areas can be addressed subsequently through size, shape, and free shape optimization or a combination thereof.  Artificial stress concentrations are filtered out during topology optimization with stress constraints.  These include regions around rigid connections, concentrations due to hard geometric features such as corners, etc.

 

Stress constraints for a partial domain of the structure are not allowed because they often create an ill-posed optimization problem since the elimination of the partial domain would remove all stress constraints.  Consequently, global stress constraint applies to the entire model when active, including both design and non-design regions. Stress constraints may not work well in a model where there is a large differential in response values between design and non-design spaces. In these cases, it is recommended to modify the problem formulation to say, compliance based for example. It is not recommended to use the global stress constraint along with a mass/volume constraint. The constrained mass/volume may not allow the stress constraint to be satisfied.

 

DRESP1-card stress constraint is used in subsequent design fine-tuning stage such as size, shape and free shape optimization. 

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On 11/7/2019 at 6:49 PM, Hyperman said:

Hi,

 

Stress constraint is not recommended in the design concept stage: topology, topography and free-size optimization.



The stress constraint definition in a topology optimization is a global constraint and does not target local stress concentrations.  These areas can be addressed subsequently through size, shape, and free shape optimization or a combination thereof.  Artificial stress concentrations are filtered out during topology optimization with stress constraints.  These include regions around rigid connections, concentrations due to hard geometric features such as corners, etc.

 

Stress constraints for a partial domain of the structure are not allowed because they often create an ill-posed optimization problem since the elimination of the partial domain would remove all stress constraints.  Consequently, global stress constraint applies to the entire model when active, including both design and non-design regions. Stress constraints may not work well in a model where there is a large differential in response values between design and non-design spaces. In these cases, it is recommended to modify the problem formulation to say, compliance based for example. It is not recommended to use the global stress constraint along with a mass/volume constraint. The constrained mass/volume may not allow the stress constraint to be satisfied.

 

DRESP1-card stress constraint is used in subsequent design fine-tuning stage such as size, shape and free shape optimization. 

 

Hi,

 

Thank you for your answer

 

What I don't really understand is what the global stress constraint exactly does in topology optimization.

In the above definition you mention it ignores local stresses that are too high but what is the point of using the stress constraint then? Your local stresses will still be too high.

What is exactly meant by 'artificial stress concentrations' because as I understood the global stress constraint makes sure they are below the limit (declared in the global constraint)

 but I don't get what they exactly are.

 

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Glad to help.

 

The topology stress constraint acts as a  global stress control. It will filter out only the artificial local stresses caused by point loading or boundary conditions so artificial stresses will not be removed. Refer to Topology Optimization of a Hook with Stress Constraints. Those stress concentrations can be removed in the subsequent design fine-tuning stage such as size, shape and free shape optimization with DRESP1 stress constraint.

 

The artificial stress concentrations are stress singularities due to a point load or constraint, sharp re-entrant corners, corners of bodies in contact,... For more information refer to:

http://www.acin.net/2015/06/02/stress-singularities-stress-concentrations-and-mesh-convergence/

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

https://www.digitalengineering247.com/article/dealing-stress-concentrations-singularities

 

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