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why is the RBE constraints not working

Description: I am working on a structural analysis for a expansion tank and I created 3 RBE(following the tutorial) to give constraints like fixing points. But the result is that the tank is not properly fixed. What is the possible cause for this? is it because the bad meshing near the fixing points maybe? I don't know.

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Marc_21384

Hi Sandra,

How are you applying the loads and constraints? You said you have 3 RBEs what type and used with what?

Remember RBE 3 are usually used for loads and RBE2 are used for constraints. For your case, I'd recommend using Non Linear analysis, maybe even start with linear analysis.

Can you also share the error in the out file? Once you've tried running either linear or non linear.

In the learning center we have a new section called mastering SimLab. In there you'll find a nice complete tutoring on non linear analysis:
https://web.altair.com/altair-for-simlab-learning-center-trls

Kind regards,

Marc

Marc_21384

It's a warning, not an error. It's hard to tell what the problem could be without access to the model. I see you have boolean them together? Is that right? There are multiple bodies actually there.

Can you unmerge them and check if they are sharing faces correctly? Also, run a normal mode to see the overall behavior

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Guilherme_

Hello @Sandra_Dong

Some checks you could do:

Is there any SPC in the model?
Is the SPC applied to the RBE main node?
Is the SPC assigned to the right load collector?
Is the load collector referenced in the subcase?

Sandra_Dong

sorry but what is SPC?

Guilherme_

Which solver are you using? OptiStruct or Radioss?

SPC is Single Point Constraint, which is the card used to define constraints in OptiStruct.

Sandra_Dong

I am using OptiStruct, I have tried to define the RBE again and it seems like that this time works,

Sandra_Dong

I made the RBE2 like in the picture, there are three flanges like this and I fixed all 6 dofs for them. Is there something wrong here? why the result shows the movement of these flanges? Could you tell me where is the problem,please?@

Guilherme_

Marc_21384

Hello Sandra,

Can you please share some screenshots about the setup you have. Assembly browser, solution browser with the setup, graphic window with your parts. And what you're trying to achieve? Thank you!

Kind regards,

Marc

Sandra_Dong

Hi, Marc

I have noticed that the error 312 which says that the constraints of my part have a problem(so the result is not right), as shown in the screenshots I made 3 RBEs as fully constraints. But it lacks some constraints?

I basically need to simulate a pressure test for the expansion tank with pressure reciprocating between a region for thousands of times. I placed a time dependent pressure table on the inner faces. I need to get a structrual analysis which includes stresses and deformations to figure out the risk part of the tank that needs to be modified to pass the pressure test. Finally I wish to achieve this but at this begining phase I can not even get a simple result from linear static because of the setting. Could you help me to solve this ? Thanks!

Marc_21384

Hi Sandra,

How are you applying the loads and constraints? You said you have 3 RBEs what type and used with what?

Remember RBE 3 are usually used for loads and RBE2 are used for constraints. For your case, I'd recommend using Non Linear analysis, maybe even start with linear analysis.

Can you also share the error in the out file? Once you've tried running either linear or non linear.

Marc

Sandra_Dong

Hi,

I selected all the inner surfaces and placed the pressure table for them as the load(shown in pic)

, and the constraints I did with these 3 points from the cylinder holes(type RBE2) blocking all 6 dofs. I did the same thing learning from the tutorial of static and linear dynamic analysis.

The result animataion shows that the flanges have not been properly fixed(with error 312). Now I try to block the top and bottom surfaces of the flanges(green constraints in pic) and it seems like now they are not moving but the displacement is not realistic. why is this happening?

Thank you!

Sandra_Dong

Hi,

The error 312 is still poping up and is it because the constraints are not correct? I have checked all the values you mentioned below and from my point of view, it seems ok. Now I am entering into a dead end.

Sandra_Dong

Thank you, I have found out where is my problem. I set up the wrong coeficient of thermal expansion. Now the displacement is reasonable.

Sandra_Dong

Hi, I am trying to understand the constaints setting I made from RBEs, you mentioned the SPC in OptiStruct. For example, like the picture I paste in the comment below, I chosen the cylinder surface to make a RBE point and then make a constraint fully 6 dofs blocked, I assume then this flange should be fixed totally, but the result seems like it is not fixed, only this point is fixed but the flange can move. So the fixition is only for the point and has nothing to do with the cylinder surface? (From the tutorial it is properly fixed, I did the same procedure but the results are different I don't know why)