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05-17-2017 09:31 AM

Is it possible to define a damping ratio in each material or element property in order to produce a damping ratio graph? I have some discrete damper elements which have higher damping than the surrounding structure, and I want to capture that effect in my harmonic analysis (frequency sweep).

The path I want is:

- Element or property-level definition of damping ratio (which field is the right one for damping ratio z?)
- Natural frequency analysis gives effective damping ratio for each mode (in some log file? or somehow automatically in FEMAP?)
- Graph of modal damping ratios vs. frequency somehow generated (preferably not by hand...)
- Modal damping ratios vs. frequency applied in harmonic analysis

Step 4 I know how to do. At the moment I have a constant value of 5% across all frequencies.

Thanks!

4 REPLIES

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05-19-2017 02:07 PM

Yes, you can introduce a damping value for each material at the material editor.

The modal damping ratio is the damping for each natural mode and depends on the material damping defined for each material, the global damping factor and the way each mode shape affetcs the displacement field of each material.

If you need to introduce a frequency-variable damping ratio (or a structural/ciscoleastic damping) then you need to perform a direct frequency analysis solution and use a tabular input describing the dependence of the damping properties relatively to frequency.

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05-22-2017 04:06 AM

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05-22-2017 03:07 PM

Modal damping ratios are not available from a natural frequencies analysis since it assumes zero damping.

Can you descritbe briefly what are you trying to simulate in order to provide the best hint?

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05-23-2017 12:52 AM

You can calculate modal damping ratios based on natural frequency results, assuming that the material damping ratios are defined. Basically each mode activates a certain portion of the model. If that portion has damping ratio Z1, then the damping ratio for that mode is Z1. The weighted (by "displacement") average of the damping ratios of each element activated by a given mode gives an estimate of the overall modal damping ratio.

It seems like NX-NASTRAN doesn't provide this calculation.

I'm modelling an vibration isolation bracket with four discrete spring-damper elements.

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