1st mode frequency has almost zero... 1

[jbs0572]

Hi! guys.

I have a question about modal analysis

I didn't build a free-free condition but, 1st mode frequency has almost 0.
When I checked a strain energy about its rigid mode, It looked good.

Anyone who knows why this happened, please let me know.

out file came like this

Subcase Mode Frequency Eigenvalue Stiffness Mass
1 1 6.542622E-03 1.689909E-03 1.689909E-03 1.000000E+00
1 2 4.407759E+01 7.670001E+04 7.670001E+04 1.000000E+00
1 3 4.731641E+01 8.838597E+04 8.838597E+04 1.000000E+00
1 4 7.236782E+01 2.067525E+05 2.067525E+05 1.000000E+00
1 5 9.648626E+01 3.675282E+05 3.675282E+05 1.000000E+00

 

[ThomasH]

Hi
I think it is almost impossible to guess without more information, element type(s) that you have used etc.

But zoeo frequency indicates rigid-body modes, that is correct.

Best Regards

Thomas

 

[GregLocock]

One possibility is that you have a mechanism. Usually the animations will reveal where.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[jbs0572]

Hi, ThomasH

I guess the reason this happened is 1d mesh(bush etc).

Hey, Greg

That's why I checked strain energy, but it was fine.

 

[ukbridge]

Probably stating the obvious but...

1. Try a quick static analysis under gravity to check all the mass has been incorporated into the model.

2. What are the (sum of) the mass participation factors in the direction of the desired mode. If these are very small these can be localised modes that arent of interest to you. This is analogous to say local buckling of web panels when doing an LTB analysis for a plate girder with shell elements - the buckling mode of interest might not be the first eigenvalue).

3. Have you done a quick check? For example for a cantilever you might estimate the frequency based on a 1/4 of the mass applied at the tip with a stiffness of 3EI/L

4. As stated above have a look for any mechanisms/breaks in the mesh.

 

[stressebookllc]

This may also be useful to you:

http://www.stressebook.com/rigid-body-mode/

Aerospace Stress Analysis and FEA Courses

http://www.stressebook.com

Stressing Stresslessly!

 

[rob768]

are you sure you have no disconnected nodes/elements flying around? certainly sounds like something isn't properly restrained.