Learning Forum

[alpivanvik]

Hi everyone!

I have a pretty simple model consisting of 2 stiff beams connected with each other via Bushing joint, also one beam is connected to the Ground by Bushing joint with the same stiffness. There're no inertial loads that's why I don't care about material and chose it to be default structural steel. The only one boundary condition is a Force applied to the second beam (which is not connected to the ground). I want to investigate the behaviour of such beam system under different values of that Force.

The problem is that when I don't turn on Quasi-Static option in Solver Controls, this model converges only when the force is < 145 N. But if I turn on Quasi-Static solution it converges in wider range of force value but it converges to another solution which is not identical to the original one.

So the question primarily is this: why Quasi-Static converges better than usual Static and why it gives another results, while I don't have any inertial loads?

[peteroznewman]

Having no inertial loads is not a valid reason to not care about materials. In a Static Structural analysis, especially one with a force input, material properties have a large influence on the solution.

[alpivanvik]

I have stiff beams, so I only care that it behaves similar to rigid body. Does your note also apply to this occasion? Because in theory for my system I don't have any material terms in equilibrium equations.
Do I understand correctly that in quasi-static there're some influence of material on solution while in usual Static there's not if I use stiff beams?
The reason why I don't like quasi-static solution is that it doesn't correspond to solution obtained from theory.

[peteroznewman]

You could assign the beams to be rigid, then you are right, there is no need for specific material properties. I didn't understand what you meant by stiff beams.
You have springs in the system. Are the rigid bodies undergoing small or large deflection?

[alpivanvik]

For Line Bodies there're 2 options: flexible or stiff beams. There's no "rigid" option.
They are undergoing large deflections.

[peteroznewman]

Ah, I learned something new today. I looked in the Help and found out what a Stiff Beam was, I have never used that before.
The application approximates a rigid beam by making the Young's modulus1e4times higher than defined in the Engineering Data Workspace.
Have you tried looking up Quasi-Static in the Help system? Since I had it open, I copied this:
Quasi-Static
ForQuasiStatic application-based settings, the Backward Euler algorithm is used. In addition, damping energy and the work done by any external loading condition is taken into account. This can be seen through the transient solution optionsTRNOPTcommand.

[alpivanvik]

Yeah, I've seen that. Though I completely don't understand how this "work done by any external loading condition" is taken into account. But it's ok, I've already achieved the convergence for this problem that I wanted so it's only a question of understanding how ansys works inside.