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Stress Constraint

Hey guys,

I got a question on topology optimization again.

I want to set a Stress Constraint for my optimization, how can I estimate a reasonable value for this constraint to avoid getting an infeasible design?

Besides, how do I know which of my constraints is hurt when this error is occurring?

Kind regards

Simon

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Simon Križnik

Hi,

stress constraint is usually based on the material used and the expected failure condition: fatigue strength, yield strength or ultimate strength.

Performing a baseline analysis run will give you conservative stress values- the stress is expected to be higher after optimization. If you intend to use base thickness (with shells) performing baseline analysis at this thickness will give you the worst case stress.

Regarding stress constraint in topology optimization (from Optistruct help):

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.

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.

You can check the violated constraints under RETAINED RESPONSES TABLE in the run_name.out file.

https://altairuniversity.com/wp-content/uploads/2014/01/Output-File-Structure.pdf

Altair Forum User

Thanks a lot for this answer, this really helped me a lot!

Do you know if it's possible to set a mesh-independency filter?

Simon Križnik

Glad to help.

From Free eBook: Practical Aspects of Structural Optimization (A Study Guide):

• Are There Any Requirements On Mesh Size For Topology Optimization?
The density method used for topology optimization is mesh-dependent. It also causes checkerboarding (density value changing rapidly between elements). Models with second order elements do not show this behavior and use of minimum member size constraint treats the checkerboarding issue.

Altair Forum User

Okay, thanks a lot!

One last question I got:

I run a topology optimization using a stress constraint of 40,000 MPa.

I got the message, that all constraints are satisfied but doing a FE-reanalysis the maximum stress is above 40,000 MPa.

How can this be explained? Did I put the density threshhold too high?

Simon Križnik

I think the answer is already provided in the quote above from Optistruct help:

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.

Also, the stress above 1 GPa could be unrealistic- you should check unit consistency:

UNIT CONSISTENCY.png