Linear Buckling

[flyforever85]

Trying to set up a buckling analysis of a flying structure (so no BC). I set the loads and request 5 eigenvalues. I get only one eigenvalue and I'm not sure how to interpret the information since the only eigenvalue I get is 10 power -11 so basically 0. I'm not sure how to read these info and to change the model in case.

 

[Mustaine3]

Your first eigenvalues are probably (close to) zero. Those are the rigid body modes. Request more eigenvalues or set a lower limit of 1.

 

[flyforever85]

No matter how many I request, I get only 1 eigenvalue and it's basically 0.

 

[FEA way]

Can you share some basic information (no need for details) about your model - what kind of structure it is and how it’s loaded ? Maybe you could use inertia relief but that only makes sense if your structure is accelerating.

 

[flyforever85]

It's a flying truss structure with 2 rotors: front and rear. So no boundary conditions. You can imagine it as a simple beam, loaded vertically at both ends with the same load (generating same lift from both rotors).

I didn't know inertia relief could work in these problems, I try now.

Thank you!

EDIT: Inertia relief in not allowed for buckling.

 

[FEA way]

Inertia relief can be used in eigenvalue buckling analyses but apparently it requires BCs and other loads anyway so it won’t help in this case. It looks like you will have to define some boundary conditions for you model to perform this simulation. Also I would definitely switch to beam elements if you use truss ones.

 

[flyforever85]

I have beams only. I have to think something different at this point because haven't been able to find a solution

 

[FEA way]

Apart from boundary conditions, which in this case will be hard to define in such way that they don’t change the solution a lot, you can try with so called soft springs. This technique is often used instead of inertia relief for free floating models. Add springs will low stiffness in the directions of rigid body motions.

 

[flyforever85]

I'll try that, thank you for the hints. As a matter of fact I have a model ready with springs at the wheels for a parametric study of the landing gear stiffness.