Modal Analysis 4

[yusf]

Hi all;

My question is about the interpretation of modal analysis results.You know the most important thing is having an engineering sense over the results rather than getting colorful screen shots..

while determination of dynamic response of the body we find natural frequencies and design the structres such that there will not be resonance but how about the modal shapes how do we use and interpret the modal shape datas?

So can anyone explain the practical usage of modal shapes.

All explanations will be appreciated..

thanks in advance

i will look forward hearing from you

 

[cbrn]

Hi,
if you perform modal analysis, isn't it also important to know where the mode's nodes are, and "what" is swinging "more" or "less"?

 

[GBor]

I suggest you get a good vibrations book, but cbrn hit on at least one important point. IF you have a vibration at a resonance and you need to stiffen the structure (to increase the natural frequency), you don't want to do so at the node of the vibratory mode...you want to do it at the locations of greater displacement. While mode shapes don't give true displacement (need additional analysis such as frequency response to get actual displacements), they do provide the "shape" of the displacement.

Mode shapes are also helpful in determining if the vibration is a local response or a global response. If it is a local response, you may be able to easily tie down the specific location, but if it is a global mode, you may just need to thicken primary members.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[corus]

The other point is that the frequency is as important as the shape. If the part is in a vibrating machinery then you don't want resonance to occur and the modes to be at the same as the frequency of the input.

corus

 

[GregLocock]

The practical interpretation of them can be used to identify the best place to add a dynamic absorber, at an antinode, or the best place to mount the vibrating structure to another structure, at a node.

We'd also look at them to determine which component in an assembly is vibrating at a given frequency.

An experienced analyst may be able to identify weak points in complex structures by observing mode shapes, but this can be misleading.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Qshake]

For me, the modal shapes give an idea of how the structure reacts to natural or forced vibration.

Generally speaking structural dynamics probems can be reduced to three or four important levels:

Mass,
Stiffness,
connections,
damping

While many might lump connections in with stiffness I separate on the basis of element stiffness vs joint behavior.

When looking at mode shapes you can discern wether or not the structure is behaving at it was hypothesized. If not you need to re-evaluate one of the four main factors i've listed.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[outpost11]

Modal Analysis - What to look for!
As mentioned previously, fundamental natural frequencies of a structure should differ from those of the forcing function to avoid being excited and causing excessive motion. It is important to distinquish between local and global modes. Modal effective mass (MEFF) is a good indication of the percentage of total mass and in which global direction the structure is being excited for the frequency in question. Participation factors (squared) are a good indication of base shear values associated with each mode.
Control of modal frequencies is best carried out by examining both strain energies (for improving stiffness) and kinetic energy (for re-positioning of mass closer to supports). Strain energy density plots show preferred locations for increasing stiffness (greater thickness) that would result in a higher natural frequency.
Hope this helps.

 

[gurmeet2003]

Ysuf,

I have found another use for mode shapes. In general compressor structures that I deal with have lot of modes of vibrations. I am refering to assemblies. This is probably true of most 3-D problems. So generally modal analysis produces a lot of natural frequencies. However generally I know the loads and their directions. Therefore knowing the load directions, their point of application and the mode shapes, I can make a judgement whether a particular mode would be excited or not.

For example there might be a torsional mode of vibration. But my load is acting in the bending direction. I assume that this load even if it has a frequency that is close, will not excite the mode. This may have some thing to do with what is called modal participation factors. I think that these factors are also calcualated by ABAQUS.

I would like to hear opinion of other members on my comments above.

Thanks,

Gurmeet

 

[GregLocock]

In a linear structure you can't excite a mode by applying a force at a point and direction where it is nodal, so yes I agree. However, with typical complex structures you are rarely so lucky.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Spirit]

Hi all, my two cents: in general I consider a Modal Analysis as one of the steps useful in evaluating the representativity and correctness of my model (no mechanisms; realistic masses and stiffnesses distribution; correct unit system use). Once this is verified, you can venture yourself in the full field of dynamic analysis, that is how your structure responds to a dynamic excitation (displacements vs. frequency or acceleration vs. frequency, or displacement/accelerations/forces/stresses time histories once the excitation time history is known). Other conclusions based only on mode shapes are, in my view, roughly approximated. I think that every analysis package capable of performing a Modal Analysis can help you go much deeper into dynamics.

Regards

SD



'Ability is 10% inspiration and 90% perspiration.'

 

[torpen]

Hello,

You can calculate the Modal Assurance Criteria (MAC) between two sets of modes. This can be used to compare the vibration modes of a condensed model (superelement)
with those of the original whole model.

Regards,

Torpen.

 

[GregLocock]

In my experience MAC is USELESS for the modes of interest in entire vehicles.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[torpen]

Hello Greg,

The initial question of this post was to explain the practical usage of mode shapes.
That's why I propose the Modal Assurance Criteria which uses mode shapes to compare two models.
MAC is an possible and useful (in my experience too) usage of the mode shapes.

Regards,

Torpen.

 

[Spirit]

Greg,

in my experience MAC is useful for all modes in the sense that it helps prevent geometric aliasing and allows for comparison, for example, of predicted vs. measured modes, and more.
I would like to know why your experience is so different.

Regards

SD



'Ability is 10% inspiration and 90% perspiration.'

 

[GregLocock]

In my field, where we are working on complete vehicles, we see a lot of non linearities, and have rather noisy data. The modes also tend to be heavily damped, and close coupled.

As such if we run MACs against the modes in a given frequency range (say 5-60 Hz) we'll find that adjacent modes have strong MACs. Yet any attempt to reduce the number of modes to eliminate these close coupled pairs will degrade the fit significantly.

There is little more than hand waving agreement between the mode shapes we measure, and those we model, for complete vehicles in this frequency range. The reasons for this are reasonably well known, to the extent that we now strip out the powertrain and so on, in order to linearise the response.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.