How to apply constrains to empty mass center of model

Altair Forum User

Hello, everyone 

   As shown in the picture, I add some forces to the model to do the linear static anlysis. I don't want to it move or rotation under that conditions, but the node of the cube can't be fixed in my problem. So I want to fixed the six DOFs in mass center which is the red point in the picture. But the mass center is empty i.e. don't have material and don't have connection with the model. So, the problem is : 

  Is there any method to constrain the six DOFs of mass center and won't effect the result?

  Can the reb2 element use for solve this problem?(I am a newbie of FEA, I don't familiar with this element)

  

  Thank you so much for your time. And any advice will be appreciate. (^_^)

 

Best wishes.

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Altair Forum User

Hi,

 

You can create a RBE2 with mass center as independent node and connect that to model and constrain the same

tinh

I wonder, if you fix moving and rotation then its displacement is zero and dont need to analysis

So do you mean you want the box will not fly away but still see its deformation?

If it is what you expecting, maybe you need inertial relief support

Altair Forum User

Hi, @Prakash Pagadala

  Thank you so much for your reply. (^_^)

  I have tried the RBE2 element. I think I didn't make the problem clear enough. I can't constrain the node of the cubic, since every node may have force in x, y and z direction, because I add random force to this cubic. The direction and the magnitude are both random. So I can't fix any node on the cubic. 

  I am not very sure if this problem is solvable. Thank you so much for your any advice. Waiting for your reply.(^_^)

Altair Forum User

Hi, @tinh

  Yes, yes. I mean this. May I get more detail about inertial relief support from you?  Thank you so much for your time and this advice.. Thanks a lot~~(^_^)

tinh

Optistruc seems to apply IR automatically. Could you try running it without constraint?

Altair Forum User

Hi, @tinh

  I have try to runing it without constraint. But It seems I get the wrong answer, I wonder if I miss something or make some mistakes...The result is shown in the following picture. 

<?xml version="1.0" encoding="UTF-8"?>

 

As I search something from website, I set the inertial relief by using anlysis --> control cards --> param --> inrel set it to -2. When I solve this problem I get different stress distribution and strain energy(calculate by using derived result) from using spc constrain as shown in following picture. I have no idea about how to judge whether the result is correct or not?  

 

<?xml version="1.0" encoding="UTF-8"?>

 

Thank you so much for your help~~(^_^)

tinh

Hi

Check displacement first.

Can you calculate it by theory expression?

Offcourse two results are different /emoticons/default_rolleyes.gif' title=':rolleyes:' />

Because BC of them are different.

Altair Forum User

Hi, @tinh

   Thank you so much for your patient.

   But I don't learn a lot of FEA, I am a student of computer science. So... I don't know how to calculate it by theory expression.

   I really don't want to trouble you more. But  en~~ may you give me some detail material about this problem?

  Thank you so much ~~(^_^)

 

  I don't know if the problem I want to solve has other method. I just want to add some random forces to model, and get the correspond strain energy. The directions and magtitude of force both random, so the model may fly away. Since this, I want to add a constraint to the mass center.

tinh

Hi,

If your real structure is unconstrained, I think you should model it with IR, do not use spc

You can search some fea expression in text book, internet,... to verify results