"force controlled" and "displacement controlled" loadcases

Altair Forum User

Hello all,

I have a question about the equations behind the difference between loadcases that use forces vs loadcases that use prescribed displacements.
I am aware that these two loadcases lead to different results on Optistruct, but have found very few info in the help about it.

I did a simple test watching the behaviour of a part under torsion and bending when I try to optimize it on Optistruct (minimize mass, Von Mises static constraints, gauge optimization).
-When 'enforced displacements' are used, we obtain a lighter part ( decrease of the thickness ) but the stresses decrease as well ! This is totally illogical since a decrease of the thickness should necessarily lead to an increase of the stresses.
-When 'forces' are used, we obtain a logical behaviour ( lighter final part with lower thickness but higher stresses ).

Could anyone explain why the second model is right in this case ? And also is there a way to still use enforced displacements and get logical results.

Thank you very much,

P.S : the only help I found on Optistruct documentation about the matter is the following paragraph
 

What is the difference between using forces and prescribed displacements?

In order to increase stiffness, minimize compliance should be used with forces and maximize compliance with prescribed displacements. The compliance is defined as: Compliance ~ Force · Displacement When prescribed displacements are used, the reaction force must be increased to increase the stiffness. This means that the compliance has to be maximized. In case the forces are given, a stiffer structure means having lower displacements. To achieve this goal, the compliance needs to be minimized.

Altair Forum User

Hello 

• Prakash Pagadala ×

Could you please read this question because it is the complete one. I was not able to delete the other one yesterday.
I used a gauge optimization option where the thickness distribution is even.
Thank you

Altair Forum User

Hi Rimeca,

 

I agree with your comment.

 

Lets us consider case 1:

 

You are trying to displace a set of nodes on your component to a value through enforced displacement. So in order to achieve this, the component should loose its stiffness to move nodes to that displacement. In your case you need to maximize compliance. So in order to achieve this the component may loose material to be flexible to get enforced to your displacement.

 

Is the reaction force from Case 1 is same as force applied in Case 2?

 

 

Altair Forum User

Hello Prakash,

In case 2, I took the force that is induced by the enforced displacement ( through SPC Force ) and then imposed that and removed the enforced displacements ( SPC + SPCD )

I understand what you said about 'having less material' leads to the part being more able to be displaced and that justifies the results found. But in this case does that mean this is correct mathematically but not really correct physically since this is not what we are really asking Optistruct to do ? 

Also, in my optimization problem, my objective function is mass that I want to minimize. Could using the compliance as objective function solve the problem ?

Thank you very much for your help

Altair Forum User

Altair Forum User said:

I understand what you said about 'having less material' leads to the part being more able to be displaced and that justifies the results found. But in this case does that mean this is correct mathematically but not really correct physically since this is not what we are really asking Optistruct to do ? 

I need to study the phenomena. I will try on a simple plate to study the behavior. I can give you some feedback soon on this.

 

Altair Forum User said:

Also, in my optimization problem, my objective function is mass that I want to minimize. Could using the compliance as objective function solve the problem ?

So you want to use Compliance as objective and constraint on mass/mass fraction, correct?

Yes, I think you can use this way.

Altair Forum User

Thank you very much! I will wait for your answer and try the new formulation of the optimization probem you suggested.

I am looking forward to hearing your feedback on the matter.

Altair Forum User

Hi again Prakash,
Any news concerning my question ? Have you tried to test a simple model and see the results?

Thank you,

Altair Forum User

Hi,

 

Not yet. I am occupied with some other work. I shall update to you by Friday.

Altair Forum User

Hi,

 

So I ran a simple topology optimization with force and enforced displacement.

 

I see both the results same.

 

I will check with free size and size optimization as well...

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Altair Forum User

Hi Prakash
My problem happened while using Gauge Optim. So I guess size optimization will answer my question.
Thank you,

Altair Forum User

Thats the next step /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'>

Altair Forum User

Hi,

 

Just to update,  I got the same result for enforeced and force model on the same plate.

 

My Design responses are mass and compliance for Gauge optimization.

Altair Forum User

Hello,
 

Could you please share your simple gauge optimization model ?

Altair Forum User

Hi,

 

Please find the attached model files

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