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#1 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-03 15:44:08

Apparently I'm one of those users. I think even if someone reads your book few times, will have to go through some of the problems to have this "aha" moment.

...small remark, using C_A in comparison to commercial codes... For example in case of Abaqus, you simply have less insight into all what's under the hood.

#3 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-02 19:41:54

I meant mesh from Case02a not Case02. Jean Pierre's comm file plus mesh from 088-case02-QUAD4.zip (Case02) works without modification. It seems that removing TOUT='OUI', adding 2D mesh groups and LOADD_EL to DKT formulation does the job. I have never added 1D element group to DKT formulation. But I understand that this will create SEG2 elements on LOADD_EL edge. A neat way to create less elements in the study.

From *.mess file you referred to you can find in "Modelisation" column "-", which means that that despite MEBODKT being applied to "hollow 1D mesh" no stiffness is added to SEG2.
I've always used TOUT="OUI" in "Modelisation" and took advantage of overwriting rule. I have never thought that it might make any difference. So I leaned something new also.

#4 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 22:22:48

I attach screenshot with marked node from the error.

#5 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 22:11:41

Stanislav Pavlica wrote:

Hello Krzystof,

088-case02-QUAD4.zip ...
remove elemets 1 2 3 4 next to P2 and elements 1291 1292 1293 1294 next to P1.
And put back DRX=0.0, DRY=0.0, DRZ=0.0, for GROUP_NO=('P1','P2','P3','P4',)


I see what you mean. It's weird, but I did not specify any modelisation to SEG2, so those elements should be "transparent" for the solver. Correct me please if I'm wrong.

The other weird thing is that my last mesh doesn't contain any additional SEG2 elements in those holes and I'm still getting rigid body motion. Well I should have sticked to one mesh and make adjustments where needed.

What is even weirder:
Degré de liberté physique associé au noeud N2212 et à la composante DZ.
This node is in the middle of RHS beam.

#6 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 21:56:25

Mesh 088-case02-QUAD4 plus comm file shared by you works perfectly. Thank you.

Unfortunately stress propagation still is different than in the study from the course. I don't know Femap/Nastran so maybe I'm trying to compare pears to apples? Are there any other fields than SIEQ_ELNO and SIEQ_NOEU for shells from which can we display Von Mises stress?

#7 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 16:46:47

I'm sorry, I didn't fully understand the first time.

Yes, you're right, I picked only 3/4 of hole edge to that HE4 group.

#8 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 15:34:08

Thank you Stanislav, I'll check it in the evening

Jean Pierre,
Sorry for changing mesh all the time.
First mesh was based on model edited in Shaper. Shaper cuts hole arc even if it's not asked to, which is annoying. I had to cut model more to have the same arc length so that I could define one submesh hypothesis. That's why there were more elements in that group than in Case02a.

Since model didn't run and there was a mess in shaper results tree I decided to prepare final model again CAD system and import it to geom module. So after importing it from CAD had to mesh it once more. I run the simulation and had rigid body motion problem again. Case02a is the most recent and I won't be changing it again.

Again sorry for the chaos I introduced.

#9 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 12:21:44

It seems that there's a rigid body motion, but I don't know why. Every connection seems all right.
I attach the most recent files. I changed mesh, changed M_RT_D_N to K_TR_D_N with all 6 stiffness components equal 1. I'll get back to it in the evening.

Line 345 in mess file "à la ligne 1640 qui correspond au degré de liberté" - ligne 1640 reffers to SEG2 element number 1640?

#10 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 11:01:30

No success yet, I obviously overlooked something, but still don't see it. I attach mess file.

I don't know why it run flawlessly when I had TRIA3 mesh and now with QUAD4 there's problem after a problem. Study didn't change that much. So this has to be something with mesh. I've been using Salome 9.7 to mesh it.

#11 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-12-01 10:52:35

...so for LOADD_EL as well, I'll check that. I thought that DKT applies to 2D elements only, not their "border". LOADD_EL is group containing line elements.

Geometrical Group_14 was used to control submesh, it landed in the mesh group automatically, and I didn't notice that it's there. I'll delete it from final mesh.

As for the M_TR_D_N, this is taken from an example I was given during training. Since I'm not using PESANTEUR this, as stiffness, would not be taken into account. I don't know how else I could create LIASON_SOLIDE. I tried using nodal group without POI1 element, but solver reported orphan mesh. Then it was explained to me,  that I need any sort of element in that central node (which was kind of obvious, when it was pointed out). I don't know how to simplify it.

#12 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-30 22:14:29

I changed also the way DKT is assigned to mesh from TOUT=OUI to GROUP_MA=('THK3','THK4'). Then I got the information that there are no elements attached to LOADD_EL group.

I prepared model and mesh again, the result is the same. Please help.

#13 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-30 21:51:54

I changed the mesh to QUAD4 and made it almost exactly the same as the "original" from training (except for 4 triangles). I named the groups as in case of TRIA3 study and run the simulation. Now I'm getting error, that some edge elements were created but they are not attached to any with the stiffness. This information is new for me and I don't know how to deal with it. Please look at line 174 in mess file. The element, and next few to which the error refers to are at the far end of the RHS beam. I attach the mesh, comm, export and mess file. Could you explain what I did wrong?

#14 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-29 15:30:02

OK, so since this discontinuity is ok, why do we see different plot in Paraview? He in the same area has below 60MPa and my results vary from 152 to 251 MPa for elements depending on the layer.

#15 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-29 14:28:24

I'm aware of that. M_TR_D_N is used to apply DDL_IMPO only. Johannes told me during training that I'll get this error in mess file and in this case I may omit that. I don't see any other things in my comm file that could lead to different results. I don't understand why Nastran's and C_A's results differ so much. I'm curious why there's a discontinuity in stress propagation between beam and c channel? Especially if they share the same node where the singularity may occur. I can only guess.

#16 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-29 09:06:55

The study he created is in m, Pa etc. So that beam lenght is 0,7m. The study I created is in mm, MPa etc. Beam is 700mm. They are both the same models. I doublechecked the values. There was a slight difference, he set E=205GPa (205e9) whereas I set E=210GPa (210 000 as is in your book on page 56 for models in mm)
Total displacement at the end of the beam in his case is  0,0019m and in my case with E=205GPa is 2,595mm, so first information is that my model is less stiff in some way.

He applied 1000N to the edge of the beam, I setup FORCE_ARETE = 37.037037 on 27mm edge, which gives us in total  999,999999N. Could you tell me where else I could make an error?

The difference is that he used QUAD4 and I'm not sure how many dofs he set to 0 for "DIS_TR". It might be the case he set all to 0 and I only translations. I'll check that later.

#18 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-28 20:57:41

zoom to connected beam to C channel, why there's a discontinuity in stress propagation between beam and c channel?

#20 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-28 17:36:41

with a node at which arrives 3 elements in different planes
the sum of the values about the X  axis is meaningless
this sum is true  in a 3D model

You were referring to what I have written before about SIXX? Yes, this is meaningless if we take under consideration the results I printed to med file. This thought was rather chaotic, sorry for that. Let's leave that for now.
I understood that SIEQ_NOEU values are meaningless in this particular example.

remember code_aster supposes you are grown boy who knows what he is doing

well, when it comes to Code_Aster I feel rather like a small girl. The problem with FEA is that there are basically little to none useful books about it unless you want to write your own software. I think the only way is to learn it from someone else and/or by checking real structures under real load.

Despite understanding how SIEQ_ELNO and SIEQ_NOEU are calculated one might have no clue how to deal with this information.

#21 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-28 16:41:37

I'll post the view from Femap as soon as I get permission. I asked for it and I'm still waiting. I paid for a general FEM training some time ago. Now I'm back to C_A (this time for real) and I wanted to replicate the study.

I'll change back to TRIA3 or QUAD4 (if I manage to mesh it correctly) and I'll paste results.

Johannes mentioned the difference between SIEQ_ELNO and SIEQ_NOEU several times and I additionally read documentation regarding this matter. I understand the difference how the fields are obtained but still I don't understand why are those two fields present instead of just one. The values represent the same place in space, but since the values are obtained differently they must differ. So which one is "true"? For instance If you're using less advanced software and you choose von Mises stress you are presented with only one field. Why NOEU in this case is meaningless for you? Is it related to singularity in this region?

I changed the model back to what was before.

#22 Re: Code_Aster usage » [SOLVED] Deeper understanding of shell models » 2023-11-28 15:21:32

What I'm trying to do is to get approximately the same results from a study prepared and calculated in Femap/Nastran.  The 3D model is divided in that complex manner simply because I wanted to mesh it with QUAD4 as in the case mentioned above. But from my understanding instead of DKT/DST I should use COQUE_3D, which requires QUAD9 or TRIA7 topology. At first I wanted to focus on comm file to obtain any results. Now I have 5x difference in stress results but the deflection is about the same as in Femap/Nastran. This matter I'd like to park for now and focus on other things. Later I'll change the mesh to QUAD4. I'll share the results from Nastran as soon as I get permission to do that.

So in case of local shell orientation. The goal is to show i.e. SIXX for whole model, by pointing local axes to the same direction.
I plotted how X and Y local axes are located on my model using IMPR_CONCEPT (red - local X, yellow - local Y) I wanted all local x axes pointing in the same direction (in global Z in this case). I understand how to setup local X pointing in required direction by VECTEUR, but I can only do it for that beam section. In case of C section there's no error reported if local X axes point in global Y direction. So I'm not able to show let's say SIXX for the whole model. This will differ if local axes are not pointing in the same direction. Am I right?

and i would clearly split in plane and out of plane bending

could you describe this more in detail?

Thank you for pointing to the examples. Are they described anywhere else than in the book you've mentioned?

#23 Re: Code_Aster usage » [SOLVED sort of]Need help with script running export file fromTerminal » 2023-11-27 21:03:09

well... for part of this conversation I tried to solve already solved problem. By mistake I discovered that I don't have to run singularity inside wsl "VM". Well, this has already been prepared and if you are using what Hassan Berro prepared, after running

wsl -d smeca

you simply run

as_run exampleexportfile.export

and that's it. It works flawlessly...

So to sum up -
all I have written before applies to singularity installed directly on Linux machine. There's no need to change anything in shared "VM".