Improving rigid body mode magnitude to 1e-6 or better

[Azeemsha]

Dear experts,
I am trying FE Analysis of a complicated composite system in nx nastran. When I do a free free run my first 6 rigid body modes are of order 1e-4. But my client is insisting on modes better than 1e-6. In order to understand better I modelled a simple plate with isotropic material property and all quad mesh with good quality, and given a free free run. But it too provided me with rigid body modes of order 1e-4. I modelled the same using patran and run using msc nastran, the answer was same.
Both the models passed ground check with strain energy of order 1e-6.
So, how to improve this right body modes magnitude?

 

[rb1957]

by passing the ground check, your results show that the actual structure is very flexible.

How do displaced shapes plots look ?

Was your isotropic material similar to your composite ? What if you increase E by a factor of 1000 ? If you get the same then it is the shape that's causing the problem, not the material. And this should be visible in a displaced shape plot.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Azeemsha]

rb1957 : isotropic material is titanium and composite is CFRP.
By shape you ment element quality, the plate is all CQUAD4 square, good quality

 

[SWComposites]

Huh? A rigid body mode is just that, rigid body moving in one piece without any relative displacement. There is no "mode shape" for a rigid body mode. And by "order 1e-4" what specifically are you referring to? units??

 

[Azeemsha]

SWComposites: 1e-4Hz, I was referring to the first 6 rigid body modes that we get in free free analysis. And How can it be improved

 

[rb1957]

SWC, yes, confused me too. I imagine the model has finite stiffness CBUSH elements, which were grounded in another check run (which passed).
Maybe the OP can explain what he changed between the two runs ?

By "shape" I mean the topology of your structure. Can you post deformed shape plots ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

there is no "improvement" possible for a rigid body mode, as there is no mode shape or modal frequency. the "1e-4Hz" is likely some fake meaningless numerical value programmed into Nastran.

 

[Azeemsha]

The shape is just a rectangular plate with 100x70 square CQUAD4 elements with pshell property and isotropic material. And first free free run is with another complicated model, Second run is with above said model. both provided first 6 rigid body modes of order 1e-4Hz, can you please guide me on how to improve this number,
Edit
Swc: I didn't get these rigid body modes as exactly 1e-4Hz, the mode got was of this order. The femci book quoted that the first 6 rigid body modes in free free run to be less than 1e-4Hz. That's why I am trying to improve the model

 

[rb1957]

sorry, I'm confused.

You started with a complicated model and got 1E-4.

You then ran a simple plate model, and got 1E-4 ?

You then did "something" (to this simple plate model ?) and got 1E-6 ?

I think you're running frequency checks on the model, and the first 6 modes (the rigid body mods) are showing a frequency that is too high (compared to some text standards).



"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

The "femci book"? what is this?

Why are you wasting time on the fictitious "frequency" for a rigid body mode? Did you read and understand what we posted above?

 

[Azeemsha]

Really sorry, please excuse my poor articulation.
this is the femci book I told
I started with a complicated model and got 1e-4Hz.

Then I ran for simple model and got 1e-4Hz, that's all

I want to improve this value to an order of 1e-6 Hz

I was informed that for a free free analysis the first 6 rigid body modes has to be zero(ie very small or order 1e-6Hz or lower). A higher value (of order 1e-4Hz or higher) shows an ill conditioned problem and needs to be improved.

Hope I am clear now

 

[SWComposites]

The femci doc says rigid mode freq should be less than 1e-4. Still don’t understand why you want anything lower.

 

[SWComposites]

And if freq not exactly 1e-4 then I still don’t see the problem.

There isn’t much simpler than a flat, rectangular plate. Maybe post a picture of that model. Are you using shell elements?

 

[rb1957]

what version of NASTRAN are you running ?

I think most people (including NASTRAN) would say that your 1e-4 reaults pass ... looking at the FEMCI example I think they are comparing model results to a value of 9.2 (no idea what that value means.)

But if you're trying to reproduce these FEMCI results .. then you have to run their model, their version of NASTRAN.

You might try a model intended to fail ... to see what failure looks like.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[rb1957]

this is I think a key paragraph ...

The model should obtain a minimum of six rigid body modes. These modes should have frequencies less than or equal to 1.0E-04 Hz. The GROUNDCHECK case control card should be used as a stiffness equilibrium. Strain energy filters are set by the THRESH parameter in the card. By default, the threshold is calculated by dividing the largest term in the stiffness matrix by 1.0e+10. The output will indicate whether the check is passed or failed.

The important thing is the threshold ("9.2") is calculated by dividing the largest term in the stiffness matric by 1E10. Now I don't know what the values for rigid body freedoms are ... I don't think they are modal frequencies ... I have a hard time visualising a free-free run (a model without constraints to ground). I don't know that 1E-6 is any "better" than 1E-4. The important thing is that the model PASS th e6 rigid dof (as these examples do).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Azeemsha]

Swc: I am using CQUAD4 elements
The client is insisting on rigid body modes of order 1e-6Hz or less. They will integrate my model to their very large model. They say that if they integrate my model to theirs, it might reduce the accuracy. I am not able to find a technical reason to reply to them that it's not the case.

 

[SWComposites]

I don't think the client understands rigid body modes, or there is something else going on with their large model that they are not communicating.

If you have a simple plate, and you run a free-free modal analysis, the rigid body "frequencies" are just whatever numerical roundoff value that Nastran spits out. There is nothing that I know of that can be done to change those values. Think of it this way, a rigid body mode is just a simple translation displacement, there is no vibration frequency associated with the mode, so nothing to calculate for a frequency.

Give the simple plate model to your client and ask them to "fix" it by whatever magic they want to get their desired 1e-6 value and then send it back to you.

 

[Azeemsha]

Yeah, I too wasted my time chasing this.
Thanks a lot swc and rb1957 for your guidance and suggestions.
I am going to tell them that if the model pass groundcheck you are good to go.

 

[rb1957]

the problem is, though you may be technically right, the customer is always right. Presumably his FE guy is telling this (but maybe it's just some PM flexing his (tiny) muscle. Try to have a word with the customer's FE guy. If he is pushing this, why ? Our opinion seems to be that rigid body modes are PASS/FAIL.

What is you model like ? we've been talking about some flat plate test model.

What's your contract with the customer like ? Can he hold payment ? Can you afford to burn this customer ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.