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09-18-2012
03:20 AM

09-18-2012
03:20 AM

Modal analysis, displacement and stress

Hi,

At the moment I’m working with mechanica 5.0. I’m looking for modal frequencies. The maximum displacement I found is 1 mm. I think Pro-E impose this displacement to found the frequencies. But the stress I’m founding is not high enough because of this displacement. Do you know how to change this value or how to set Pro-E to find the maximal displacement ?

Many thanks

4 REPLIES 4

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09-18-2012
03:49 PM

09-18-2012
03:49 PM

Re: Modal analysis, displacement and stress

Hopefully I have not misunderstood your question but, I believe you should consider performing a dynamic analysis after the completion of the modal analysis. Modal displacements are "unit normalized" meaning that after the completion of any modal analysis your max displacements and rotations will always be 1 of whatever displacement units you have selected. Stresses and all other measures are "mass normalized" and neither stresses nor displacements should be interpreted directly. The modal frequencies and shapes will be correct but the magnitude of the results are to be utilized in dynamic analyses (time, random, frequency and shock). The dynamic analyses use the results of the modal analysis and perform "modal superposition" to achieve the results.

You are seeking maximum displacements to a modal analysis which is dependent on some input to the system (i.e. some frequency, time or spectral function) and cannot be read directly from a modal analysis.

Hope that helps and good luck,

- J -

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09-19-2012
02:53 AM

09-19-2012
02:53 AM

Re: Modal analysis, displacement and stress

Ok. Modal displacements are "unit normalised" and there is no way to change it.

As you understood, I performed a shock dynamic analyse (seismic). I perform the modal analyse to do it. Are the displacement of the dynamic analyse independent of the modal analyse's displacement ? (The displacements I found are small and makes me wounder)

Regards

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09-19-2012
10:50 AM

09-19-2012
10:50 AM

Re: Modal analysis, displacement and stress

I found why my displacements were so small .... Unit problem.

Thanks for support

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09-19-2012
11:29 AM

09-19-2012
11:29 AM

Re: Modal analysis, displacement and stress

Mark your answer about unit problem as solving your question so if some one searches on the topic, they will see that the question was answered.

Thanks,