Hello, First off, I am new to mo 1

[DanStro]

Hello,

First off, I am new to modeling vibration/dynamics in Abaqus so most likely there are some basic errors that I am making. I am trying to set up an analysis that includes setting a preload on a series of optics and then look at how that preloaded system reacts to a frequency sweep. This seems like it should be a pretty straightforward set up but I am in this vicious loop. I can create the steps to apply the preload but I can't then add a frequency step after those steps because that has to be the first step in the analysis. But, when I create the frequency model first and then apply the preloads I can't create the dynamic steps(s) that I need do the frequency sweep. I've tried restarts and initial state but I can't figure out the order of the steps to get what I need. For the frequency I don't need the preload but for the dynamics step(s) I will need them.

Thanks,
Dan

 

[FEA way]

It is possible to define static general step first and then frequency extraction one. Just turn NLGEOM on for the static step and then select it as a step after which frequency extraction will be performed. After that you can add modal superposition procedure.
There are some examples in the documentation. For example "Vibration of a cable under tension".

 

[DanStro]

That kills my convergence but I think that might be the only way to do this. Thanks FEA way.

 

[FEA way]

You can try with linear (NLGEOM = OFF) general static step before frequency extraction. It should work too.

 

[DanStro]

I think that because of all of the edge-surface contacts that there will be a relatively large amount of rigid body movement while all of the contacts are being initialized, so I am not sure I can get with that.

 

[FEA way]

Eigenfrequency extraction analysis is linear anyway and it can’t handle nonlinear contact (unless it’s complex eigenfrequency extraction). Thus I suggest turning these contacts to tie constraint. It can be easily done using „Find Contact Pairs” tool with „Show previously created interactions and ties” option on.

 

[DanStro]

That is why I need a step to set the contacts, to keep the convergence well behaved I am using a quasi-static dynamics step with a displacement of the retainer, then I replace the displacement with the desired preload in a (now)general static step. Sometimes if I just jump right into a general static with the desired preload the convergence is really poor because of how 'inaccurately' the CAD model places the parts in contact. As long as I don't go overboard with the amount of displacement, and I don't have plasticity defined, even if I overshoot the amount displacement needed the preload step will bring everything back to where it should be. I don't think Tie constraints will work because after I get all of this done I will be putting the system through shock-vibe and I want to see what happens to the stresses/positions. But, depending on the specific requirements, I need to go through the frequency step to get to the desired goal.

 

[FEA way]

Which dynamic analysis step do you currently use after eigenfrequency extraction ? I suppose it's steady-state dynamics. If yes then you can try its direct-solution equivalent. It doesn't need preceding eigenfrequency extraction step since it's not modal superposition procedure.