negative eigenvalues - how to check only posityve ?

cashan

Hi.
I try to check a sample described on:
https://enterfea.com/linear-buckling-explained/

And I found - of course - only negative eigenvalues values ( I use buckling 3D / CCX).
Can we do something in mecway to receive only positive values (as it is described in the mentioned article?)

Victor

I can't find a way to do that, sorry. Other than requesting enough modes to exceed zero.

You can also do linear buckling with modal vibration (frequency / modal) analysis if you have some idea of the critical load. You adjust the loads until the frequency of a mode reaches zero. In that way you can focus on only the buckling mode you want, not any of the opposite-force ones.

Or you can do nonlinear analysis with an initial imperfection.

leebailong

Hi Victor,
Has somethink changed in case of searching only positive eigenvalues?

Victor

I tried implementing the example from that link and it only gives me positive buckling factors. So I'm not sure if it was ever really a problem. Model attached.

There is a bug in CCX (bug 26 here https://mecway.com/product.html#bugs) that causes negative buckling modes but I only know about it affecting beam elements on lattice structures that have members in both tension and compression. There's a patch available if you recompile it, and it should be fixed in CCX 2.16. You may sometimes be able to work around it by specifying a large enough number of modes to get past the negative ones.

Other than that, I also correctly get negative buckling factors when there are no loads that would cause instability. You can see this in the cylinder example by suppressing the axial force.

leebailong

Thanks for reply.
I didn't try that example from enterfea, but I was wondering if the searching only positive eigenvalues is an option.