Mode Shape interpretation

[nmuoio]

Hello,

The results of a modal frequency analysis typically include a mode shape with each modal frequency. Most FEA packages can plot out displacements values of the mode shape. In a frequency analysis, there is no input load. So my question is:

What do these displacement values mean? and How are they calculated?

Thanks,
Nate.

 

[GregLocock]

They are normalised in one of a variety (I have seen at least three) of ways. So they are scaled by a somewhat arbitrary factor.

Read up in any decent dynamics text to see how they are calculated.



Cheers

Greg Locock

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[elogesh]

Hai,

Mode shapes are relative displacment of the structure when vibrating at the corresponding modal frequency...

Modal frequencies and mode shapes are characteristic of the structure depending upon geometry, material and support conditions.

Regards,
Logesh.E

 

[ThomasH]

If you let us know what software you use somebody might be able to be specific.

Regards

Thomas

 

[corus]

The displacement value means nothing, it is a characteristic shape that is being shown. Actual displacements would depend on an input acceleration, for example.

corus

 

[Drej]

Eigenvectors (the "displacements" you see) are just normalised to a specific parameter. In FE parlance these are just shape functions. In ANSYS for example, you can normalise these "displacements" to either the mass matrix (default - which provides an easy relationship between stiffness and frequency) or to UNITY (where the highest component of the eigenvector is normalised to one). As said previously, they are relative values only, since you have no energy going into the system in free vibration analysis.


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[ThomasH]

Just to add a comment to corus post (hopefully without annoying corus).

I would not say that the displacement value means nothing. It's probably some kind of normalized deformation, typically normalized with respect to modal mass etc.

However, it is no "real" deformation since there is no load applied. In some FE packages you can request "max normalizing" and then you can get max displacement = 1 for all mode shapes.

All this with some reservation since I don't know how "all" FE packages work.

Regards

Thomas

 

[nmuoio]

Thank you for your responses. I have just finished a modal frequency analysis using NEiNastran. I'm trying to convince the individuals, who requested the analysis, that the mode shape "displacements" do not equal the displacements they'll see in their real application.

Thanks again for your help,
Nate.