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Modal Damping Table for use in Dynamic Analysis

Experimenter
Experimenter

Hello,

 

Can anyone tell me what the process would be for creating a Modal Damping Table from modal analysis results?  I am running a modal analysis and need help in creating the table to use in a dynamic analysis.

 

Thanks

Mark

 

2 REPLIES

Re: Modal Damping Table for use in Dynamic Analysis

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Dear Mark,

In FEMAP you can define the following damping:

 

  • 6.. Structural Damping vs. Freq.“: this is the structural damping, G
  • 7.. Critical Damping vs. Freq.“: this is the fraction of the critical damping, ζ
  • 8.. Q Damping vs. Freq.“: this is the quality factor or magnification, Q

The relation between the above values is the following:

  • ζ = b/bc (fraction of the critical damping).
  • G = 2ζ
  • Q = 1/G

The typical input in modal dynamic response is to define a constant modal damping in the full frequency range, this is simply done in FEMAP using the function type 7.. critical damping vs. frequency. For instance, the following picture defines a critical damping of 5% in the frequency range between 0 Hz and 2100 Hz. The results is the same if you define the frequency range between 0 Hz and 1 Hz because NX NASTRAN extrapolate the curve to the infinite.

 

critical damping vs. frequency

 

But if you know the specific critical damping value to each frequency then you can enter using the above method, usually the variation of tis data is unknow.

 

Best regards,

Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Modal Damping Table for use in Dynamic Analysis

Legend
Legend

From modal analysis results, you can't really deduct damping.  If you had test data from a sine sweep for instance, you can figure out modal damping, but not simply from a modal analysis.

 

However, there are some tables of recomended critical damping ratios (e.g. 5% for bolted joints).