Error in Anbaqus: Linear buckling analysis, defining contact interaction and boundary condition.

[Tinni1]

Hi,
I am performing linear buckling analysis of cold-formed steel channel section and want to find out the buckling load.
I have modeled the cold-formed steel column supported on a hinge bearing assembly using Abaqus. I have provided a snapshot of the end connection(Both top and bottom) in the attachment.
The modeling assumptions are:
1. The channel section is supported on the rectangular plate. I have defined the contact interaction as Node to surface contact, master surface- rectangular plate top surface, slave surface- Channel end surface(channel perimeter). Contact property for stud to plate: normal-hard, tangential-rough
2. The rectangular plate is placed over the semi-circular roller. I have defined the contact interaction as a surface to surface contact, master surface- the semicircular surface of the roller, slave surface- rectangular plate bottom surface. Contact property: Normal: hard contact, tangential: penalty :friction coff=0.2 (based on existing data)

I want my stud and endplate assembly to rotate about the longitudinal axis of the semicircular roller, and they are not allowed to slide over the roller(No translation). Hence I have provided a displacement rotation boundary condition at the bottom surface of the rectangular end plate with constraint degreed of freedom as U1, U2, U3, and UR1.

At the top, the channel section has the same boundary condition only difference is U2 free.

Load application point: concentrated load at the top center of the semi-circular roller guide. (Shown with a black arrow in the attachment)

Apart from the contact interaction: I have not defined any constraint in my model.
When I submitted the job I got the below error:
The eigenvalues cannot be found. This could be caused due to instabilities in the base state (check for negative eigenvalues in the general analysis step)


Could anyone provide any direction here? What could be the possible cause of the error? Is the way of defining my boundary condition correct? Do I need to define any additional constraint?

Thanks in advance and looking forward!

 

[FEA way]

Nonlinear contact can’t be used in linear buckling analyses, you should change it to tie constraints and make sure that all parts are properly connected (modal analysis can help you find disconnected regions).

 

[Tinni1]

Hi,

Thanks for your response.

I have further questions:
1. Do you mean to say, If I want to use non-linear contact, I need to perform a general static analysis?
2. For linear buckling analysis, which are the locations that I need to use, tie contravariant? Is that at all contact locations?

 

[FEA way]

You can perform nonlinear buckling simulation using general static, Riks or dynamic (implicit/explicit) step. Just keep in mind to turn Nlgeom on.

 

[Tinni1]

Many thanks again!