linear buckling gives a fatal error LNNRIGL

[jlrp]

Hello fellow Femap users

I am trying to model a sailboat rig using a mix of 1D elements
- bars for the shrouds, and they have rotational degrees of freedom released as in real life + offsets to simulate the real positions on the mast tube.
- beams for the mast and spreaders, no release.

constraints are applied on points

I am looking at what we call the docktune stage, that is no loads from the sails, only from pre tensioning of the shrouds, so I have simulated the pre tensioning by applying bolt preloads to the bar elements, and a prescribed displacement at the deck.

The linear static analysis works fine, and I am getting consistent results compared to the software I usually use.

The linear buckling analysis gives a fatal error message LNNRIGL, and is basically telling me that I haven't entered densities in my material properties which is not the case.
I am using g, mm, ms, N, MPa units so for example aluminium with a density of 2700kg/m3 I have entered as 0.0027
I have tried inputing the non structural mass values for each properties, but that didn't work either.

I have attached the modfem file, should anyone have the patience to look at it.

Thanks in advance, Jeremy

 

[karachun]

Open your model in Femap 2019.1, run and have no errors.
Have information message in f06 file but solution run normal.

*** USER INFORMATION MESSAGE 22100 (LNNRIGL)
The mass matrix is relatively dense.
Redmult option may improve performance for Lanczos by setting System Cell 426 to 1.

I note that model was created in Femap version 10.30, try to upgrade Femap and Nastran to newest version, look like newer Nastran versions can handle with such problem without additional options.

 

[jlrp]

thanks for taking the time to have a look.
this is weird, I am running on an old version of Femap 10.3, could that be a reason ?

but I see the Eigenvalues are low, I would be expecting values around 2.5-3.0
it also looks like the rotation is released at the root of the spreaders, weird...

 

[ThomasH]

Hi
Interresting model. A few comments that might help.

The first thing I noticed is that you units are not consistent. mm, N, MPa means that the mass should be in tonnes.
Link with info:

https://femci.gsfc.nasa.gov/units/index.html

However, this does not explain the problems. But I noticed it when I did a frequency analysis to see how the model behaves.

What may be the root to the problem is that you don't seem to be working with the fem-model. A lot (perhaps everything) seems to be based on the geometry. That means that before the model is solved by Nastran it has to be "expanded". In later versions of Femap that happens automatically but that is omething that has evolved over time. It may be so that in your version there was some limitation that is causing you issues.

Perhaps this will help you.

Good Luck

Thomas

 

[jlrp]

hi Thomas
I use this link for unit consistency:

https://www.dynasupport.com/howtos/general/consistent-units

and it seems g or ton depends on the time unit s or ms. but that shouldn't be the a problem when there is no acceleration involved, right ?

you are right, I am not familiar with the femap modeler, so I usually create a geometry on rhino and export it as stp or igs to femap whatever works best.

 

[ThomasH]

Hi
I don't think I have seen a model with ms as time unit before. But you are probably correct. Like I said, I don't think that is the source for the problem. Personally, I always use SI units with no prefix if possible.

Like I said, the model works in v2019.1 and v12.0. Both have been tested. But since your model is based on geometry and not mesh it may be an issue. Try to expand everything before you run the analysis.

Good Luck

Thomas

 

[jlrp]

I'm sorry, but what do you mean by expand ?

 

[ThomasH]

I mean Model-Load-Expand. That moves the loads from the geometry to the mesh. Merge nodes and so on. One reason for that issue can be a singular matrix.

I would actually start with a Eigenfrequency / Modes analysis. Are there any free-free modes?

But before you do any changes, make a copy of the model.

Thomas

 

[jlrp]

hello all

quick update after the holidays:
- Model-Load-Expand didn't work on Femap 10.3
- I managed to try the buckling analysis on a later version of Femap, and it worked, so problem solved here, thanks.

Now another question related to my model: the "initial tension" option in the rod element doesn't work, so I am pre-tensioning the shrouds using "bolt preload".
However, the analysis results show tensions in the elements that are different from the bolt preload I am applying to them, why is this ?

can anyone think of an alternative way to pretension the shrouds ? a geometric pre strain would be ideal.