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**Cinzia_L****Member**- Registered: 2018-02-08
- Posts: 5

Hello everyone,

I tried to perform a beam nonlinear buckling analysis, created with tria quadratic shell elements with COQUE3D modelization, fixed on one and.

Material is linear elastic.

I’ve done modal analysis first, to find the first buckling mode and respective critical load, that is 16652 N such as for theory.

Then I pre-formed my model about 1.0 mm and I perform non linear analysis imposing a displacement, for node on other and, about 5 mm in 500 steps.

I’ve search the sum of nodal reactions for node in the joint, that are non linear, as I expect, but critical load is 65 kN, not 16 kN. Why?

I tryed also a 3 mm pre-deformation but critical load is 64 kN.

How can I solve this problem?

I attach .comm and .rmed (1.0 deformed) file about non linear analysis.

Thanks a lot for any suggestion.

Cinzia

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**jeanpierreaubry****Guru**- From: nantes (france)
- Registered: 2009-03-12
- Posts: 3,103
- Website

hello

a file with the input mesh would help in understanding

EDIT

using the mesh 'traveI_shell_bucklingnl.med' from your previous post

and altering the present 'Stage3.comm' file as required

i find a very non linear behavior with a maximum reaction of 2.17054E+05

jean pierre aubry

*Last edited by jeanpierreaubry (2018-04-16 14:21:59)*

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**Cinzia_L****Member**- Registered: 2018-02-08
- Posts: 5

Hello,

thank you for your reply,

now I attach all my file.

In folder number 1 there is my input mesh and stage 1-2. After perfoming analysis with these files, I get deformed mesh (defo1.rmed), that becomes the input mesh of Stage 3, that is in folder 2.

I obtain non linear relation, but from stage 3 results (reazioni) , I see that structure have critical load approximately at 64 kN, while for Euler's critical load should be 16,65 kN (Fcr = π2 E I / L2 where L=2l, so Fcr = π2 E I / (2l)2

E = modulus of elasticity = 70000 Pa (N/mm2)

L = length of column (mm) -->l=1500

I = minimum area moment of inertia of the cross section of the column = 216979 (mm4)

I don’t understand why reaction on encastre results wrong.

PS: From stage 1 I find a critical load value that is equal to -1.28093E+03, that multiplied to 13 (that are node where I applied force nodale) is 16652 N, the right value by Euler’s formula.

Thanks a lot for any suggestion.

Cinzia

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**jeanpierreaubry****Guru**- From: nantes (france)
- Registered: 2009-03-12
- Posts: 3,103
- Website

why should the non linear analysis gives the same results as the critical Euler analysis ?

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**Cinzia_L****Member**- Registered: 2018-02-08
- Posts: 5

jeanpierreaubry wrote:

why should the non linear analysis gives the same results as the critical Euler analysis ?

Because I want to study the structure's behavior in pre and post buckling phase, by imposing a shift to one end, and encastre reactions sum between linear and non linear behaviour should be equal to Euler's critical load. I want to study a beam's non linear geometry, modeled with shell elements.

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