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Modal simulation cannot complete when applying RBE3

Hi,

I am running the following model in OS. Due to issues with artificial stress concentrations, I applied RBE3 to distribute the loads more realistically. At the same time, I am also running a modal analysis as subcase 5. When I run, I get the following information message:

*** INFORMATION # 3454
MPC constraints for subcase 5 will be enforced
with elimination of dependent dofs.

The simulation is stuck at 3% for a very long time before finishing. Is there something theoretical I am missing here about RBE3 and its validity in modal analysis?

Thanks

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Robert Lawson

Those undercarriage beams are not connected to the RBE3's in a sensible way. The RBE3 doesn't have any stiffness of it's own, so those undercarriage beams are essentially just floating around. I suspect you ought to extend those beams out another element-ish and connect into the center (dependent) node on the RBE3. That's one problem.

Next, you can't constrain the center node of an RBE3 because it's already dependent on the nodes it connects too. If you still really want to constrain it, you could put something like a stiff CBUSH between the undercarriage beams and the RBE3 to break up the dependency. Then you can constrain the CBUSH grid, just adjacent to the center RBE3 grid.

Crashphys

Hi Robert,

Thanks for the answer. I implemented the suggestions as you recommended. I did not constrain the center node of the RBE3 elements and have attached the undercarriage beams to the dependent node, while the independent nodes are on the shells. The issue however is still persistent. Is there anything else I may have screwed up? It seems to work fine when I perform static analysis, however it can't seem to handle modal...

Looking at the statsub file, it seems the point in the simulation causing problems is from this part of the run:

Total time spent in section :AMLSPRP2; CPU= 23.55; WALL= 224.67
Total time spent in section :AMLSPRP3; CPU= 12.09; WALL= 126.00

Would you be able to explain to me what this step does?

Thanks again!

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Robert Lawson

I ran an analysis w/ just the modal subcase here and it worked fine as you have it.

I tried to run the optimization, but I don't have the latest version on our solvers. I get an error 'acceleration card not supported for optimization' (or something like that). I suspect that is updated/fixed for the current version.

I may be at the end of my usefulness. /emoticons/default_unsure.png' title=':unsure:' />

Good luck!