>>I took a closer look at the original model and noticed that you have a Frictional Contact with some friction between the inner and outer tubes and you have used Adjust to Touch. Was that intentional? I hope you understand that means the contact elements that cover the surface of the small pipe are offset from the pipe surface until they touch the inner diameter of the large tube, therefore the small pipe is already supported by the large tube before any load is applied.

Yes, so I removed the outer tube and any contacts. To count only one inner pipe. I also made the mesh smaller for this pipe. 

I got two values on forms but they are very small:

>>Since you have a nonlinear item in your Static Structural model, the Eigenvalue Buckling is using the Linear Perturbation Analysis procedure.

What is this about? Is it about the material? Maybe remove the bilinear kinematic hardening in the first calculation?

>>Read the ANSYS Help on Eigenvalue Buckling. Eigenvalue Buckling Analysis (

Yes, I read. I did not find answers to my questions.