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[GNA] Post-Buckling of arch plate in Optistruct

Altair Forum User

Hello

I want to analyse the post buckling behaviour of an arched plate (SHELL Elements) with GNA (Geometric nonlinear analysis, implicit). The model is attached. In another forum (RADIOSS) a similar case was presented, with an pdf file attached, discribing a similar problem :

I am not able to reproduce the results (should be look like this:

I tried different parameters for shell thickness, force size, starting load increment size and solver options (Arc, RIKS,...). Sometimes I get nothing at all, sometimes only results up to point A (s. picture above). How can I get post buckling results?

Thank you

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Altair Forum User

For thickness = 1mm I got to point A in time step 0,2 but not further

Altair Forum User

...and how can I plot the time steps on the Y-axis to the Displacements on the X-axis?

Altair Forum User

HI,

Are you using OptiStruct as the solver?

Altair Forum User

Yes, I choose Optistruct as the solver (I think he uses a coupling with RADIOSS). I managed to get closer to the benchmarke when using FORCE as convergence criteria and as method for LINE SEARCH. However, I cant get the area between B and D. It would be usefull to know how to plot correctly, than I could send you my outcomes at this moment.

Altair Forum User

Hi,

Please go through the attached model which is a buckling case solved in OptiStruct.

Unable to find an attachment - read this blog

Altair Forum User

Hi,

I have several questions concerning the solver deck you send:

1) No buckling occures? I could increase the load, but anyways, I want to get the deflection of the arched plate from the benchmark (post buckling behaviour including area B-D). The model set up for a general GNA is known.

2) Why using NLOAD in this case? Its static anyways.

3) Why including EIGRL in GNA? It isnt used?

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4) How to plot Load time steps over Displacement (Y-X) in Hypergraph?

Altair Forum User

Hi @ahenkes1

I think there is some confusion. The examples are from RADIOSS. Is there a reason for using OptiStruct instead of RADIOSS?

Also, can you point me to the example from which you got the curve?

Altair Forum User

As I pointed out, it was just a similar problem. I am used to Optistruct and want to use it, as Optistruct should be capable of solving such a problem (it is a non dynamic behaviour I think, because of that no need for RADIOSS?).

There are different sites presenting this, as it is a rather common benchmark:

https://enterfea.com/problems-with-nonlinear-analysis-convergence-read-this/

https://www.comsol.com/blogs/buckling-structures-suddenly-collapse/

https://www.youtube.com/watch?v=JS7dgba2qLc

Similar load path:

https://www.researchgate.net/post/Arc_length_method_for_non_linear_buckling_Nonlin-FEM_in_ANSYS_Is_there_any_way_I_can_continue_the_Simulation_beyond_the_critical_point

And here a different Video with snap-back also:

https://www.youtube.com/watch?v=o5Nu5GF_zZk

----------------------

I want to go through the whole deflection path with the model I attached in the original post. Normaly, the RIKS ARC LENGHT method should be able to get all points .... as pointed out before, with 1mm shell thickness I only get up to point A....

Altair Forum User

I only quoted the post from the RADIOSS forum, because it was also a plate, so you can imagine the problem.

Altair Forum User

I think I got an update, see files attached. I think it could be the solution, but the post processing isnt working as I imagine:

1) also I solved 1700+ time steps, I only get a view (they are actually ordered by deflection size) to e.g. animate the model behaviour (The time steps solved go ++, than --, than ++ again, as supposed, but the time steps in the drope down menu from HyperView are only from 0 to ++, so around 180).

2) again, how to plot time steps (the actual one solved, not the sorted, censored ones) against displacement? (For validation and comparison)

Unable to find an attachment - read this blog

Altair Forum User

Another thing, I was using the geometric nonlinear analysis load step, which indeed uses RADIOSS. I will try the Nonlinear quasi static loadstep with large Displacement and report back.

The post process problem exists anyways, so it would be kind to get help with that.

Altair Forum User

Hi @ahenkes1

Please use NLOUT with NLSTAT and also include OLOAD card which outputs applied load vectors.

This will help you to plot the graph for Time vs load.

Altair Forum User

Thank you for the tip. I would like to plot the load increments over the Displacement, but maybe with oload it works

Altair Forum User

It should work. If you have any problem, do let us know /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'>

Altair Forum User

Hello,

Know, using NLSTAT, I dont get the expected results. I attached the concerning files. I am not able to plot Load vs Displacement (I cant choose Nodes for either Load or Displacement using h3d, for .res I cant choose Loads as Y reference). Therefore I cant realy say, whether my results are right (as in the Benchmark). The solver doesnt converge at a specific increment, therefore I think he cant go through the whole deflection cuve).

Could you look at the results and try to get a Load vs Displacement graph?

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Altair Forum User

Hi,

Let us try with non-linear transient instead of NLSTAT.

Altair Forum User

Ok, I have no idea how to set it up, could you try it or shell I work into this topic

Altair Forum User

'TSTEPNL 7 500 0.0 1 UPW'

*** ERROR # 1000 *** in the input data:
Incorrect data in field # 4.

This error I get, no Idea how to go on

Altair Forum User

DT is required.

I am trying at my end as well. But so far no luck. I think NLGEOM Impl is somehow giving results

I will let you know if I get any good results at my end.

Altair Forum User

Hi, are there any updates concerning our benchmark?

Altair Forum User

Hi,

I tired with NL transient. But convergence issues are seen.

I am checking with NLGEOM at my end today

Altair Forum User

Thank you !

Altair Forum User

Any results with NLGEOM?

Altair Forum User

No Success...

Altair Forum User

I will update my iteration results soon.

Altair Forum User

This works (see attached)!

Unable to find an attachment - read this blog

Altair Forum User

Another question: While using 'Nonlinear transient' I want to load 0.25*25 for 0,1 sec and after that linear progression to 1*25 MPa at 1 sec. Therefore I created a TABLED1 with 3 entries:

0.0 -- 0.25; 0.1 --0.25; 1 -- 1. This table I am using for a TLOAD1 table entry and my force with 25 as well. But now my question: will the TSTEP entry in any kind overwrite this? If I have N=100 and DT=0.01 will the solver still use the table entry?

Altair Forum User

I think I solved it for my own, of course the N,DT combination just divides the given load profile in increments

Altair Forum User

Hi @ahenkes1

Sorry that I couldn't able to reply due to long weekend /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'>

At my end Transient NL run fails with convergence issues at exactly 95% load. I would be interested in looking at your model /emoticons/default_smile.png' srcset='/emoticons/smile@2x.png 2x' title=':)' width='20'>