## General Mechanical

#### Critical buckling load (MODE N=1) of a beam on ANSYS APDL

• JTWD
Subscriber

Good morning/evening everybody !

My main goal is to have the critical buckling load on a composite airfoil on ANSYS APDL.

To be sure I understand the methodology, I used a simple case : a beam with a cross section of 10mm (H=B=10mm) and a height of 100mm (L=100mm) with a Young modulus of 2e9 and a Poisson ration of 0.3 and an axial load that will "provide" the buckling.
According to the theory,  Pcr= (E*L*pi^2)/(4*L^2) for n=1 so my critical buckling load will be 4.11233e11 N.

How, on ANSYS APDL, can I have access to this value (calculate by the computer) ? And the values for the other modes ?
I will join my case commands lines attached here.

• jj77
Subscriber
When you run the linear buckling solver, the results are the eigenvalues that are to be multiplied wit the applied load, which will give the buckling load. N is one then use the first eigenvalues reported by ansys. See the Internet for tutorials say uni. Of Alberta
https://sites.ualberta.ca/~wmoussa/AnsysTutorial/IT/Buckling/Buckling.html

As you will see there after static antype, 0, then one needs to run a linear buckling analysis.
• JTWD
Subscriber

Unfortunately, in the example you give, the user apply a -250 N load in the FX direction on Keypoint 2. And I don't need that because my problem doesn't require an other force than the axial one.
So when I do the Time History Postprocessing in order to view the results I have nothing except error or invalid regarding the stress.

So if you have any other solution that don't apply a force on the FX direction ?

• jj77
Subscriber

Well you can apply the force in any direction you want. This example is for you to learn how to do a buckling analysis.

Just go through and do it and then do your own model in what ever direction you want the force in.

That is it from me - I think the tutorial is very clear and shows you what you need to do so study it !

• JTWD
Subscriber

I already done that and as I said when I do that and then "do the Time History Postprocessing in order to view the results I have nothing except error or invalid regarding the stress."

I put my code here,but I follow this tutorial for non linear buckling (exepect for FX part) but it's still not working.

• jj77
Subscriber

The first post is mentioning Pcr and the Euler buckling load eq. In order to compare that and obtain this, one needs to run a linear buckling analsyis as we said and as done in the first part of the tutorial, so linear buckling analysis. So the work flow is linear static and buckling after. ANTYPE,BUCK see tutorial for details (they have also the commands)

Thus add that your code after the static analysis using only 1 N load and then run a buckling analysis. Then the Pcr from Ansys will be 1 N * the eigenvalue reported by the solver. Just follow the tutorial simple.

You are doing a nonlinear in your file that you attached in your previous post, and again your are not following the tutorial. In order to run (nonlinear buck.) that one needs to have an imperfection or as in this case they use a perturbation (small transverse load, perpendicular to the axial force). This is to make it buckle, otherwise without it (small force) it will just be compressed, and never buckle. So one needs this small force in order to get buckling, it is all explained there, just follow it!.

• JTWD
Subscriber

Hello ! I found my mistakes.
First I input the wrong initial condition so the solution was incorrect compare to my analytical solution. Then, I thought with a disturbance will change my solution, but no influence.
Thanks you for your help !