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Applying material damping

Question asked by Doug Rawlings on May 24, 2012
Latest reply on Jun 19, 2012 by Brad S

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Hello all,

I'm running a Linear Dynamic Harmonic Analysis modelling a vibration test rig I'm using.

The problem I'm having is that when I try to use Modal Damping based on Material Properties I get the same results, every time I run the test.

When I use Modal damping not based on 'Compute from material damping' with a value of 0.005 I get a result that is close (enough) to my experimental results.

But I want to be able to vary the damping for different parts.

So all the parts have custom materials assigned, with a value for Material Damping Ratio. But it seems that no matter what I set that to (and Update All Components) it makes no effect on the results, they look the same as when I ran with a global value of around 0.02.

I expect I'm doing something wrong but I can't work out what it is...

Thanks in advance for any help offered,

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Anthony Botting May 24, 2012 8:55 PM
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Hi Doug: As far as I know, that option to "compute from material damping" is supposed to work. You might need to review each material property data sheet for each component and see if there's an entry at the bottom labeled "material damping ratio". I have noticed, some materials have "N/A" posted and I'm not sure what the software does with an N/A.
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- Doug Rawlings @ Anthony Botting on May 25, 2012 5:04 AM
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  Hi Anthony,
  
  I see an N/A in the units for material damping but I'm reading that as Not Applicable since it's a ratio.
  
  The standard materials have no value entered for the damping ratio, and if you try and run a simulation with one of those materials using material damping it halts and reports "Please define material damping ratio in material properties".
  
  I'm using cusom materials for the parts, basically copied out of the standard library but then each given a value of Damping Ratio. For the first case I tried to give them 0.005 since that was the figure that looked good when run without using "Compute from material damping" but the result doesn't look the same. Then changing the material damping ratio doesn't seem to make any difference, the results still look the same.
  
  I wouldn't be surprised if the option works perfectly when used correctly and that I'm doing something wrong. It's just working out what that is...
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  - Anthony Botting @ Doug Rawlings on May 25, 2012 9:48 AM
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    Hi Doug;
    Yes, I believe it might mean "not available" or "not applicable", but it does not appear that you're doing anything incorrectly. Perhaps, to test the software, you could try entering material damping ratio of 1 (one) for each, which should be the critical damping to cause oscillations to stop. If that does not work, then I would present this fact to your VAR, since there may be a bug in that Service Pack of the software. They are obligated to get it fixed. I hope this helps....
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    - Doug Rawlings @ Anthony Botting on May 25, 2012 10:38 AM
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      Hi Anthony,
      
      I've tried various values of material damping ratio, from the sensible (0.005, 0.02, etc.) to the ridiculuous (1, 12, 0). The results seem to come out the same as when I first ran with the global value of 0.02
      
      When I try using the global setting and enter 0 as the global ratio it halts, asking me to define the damping. Then I switch the "base on material properties" and I get a solution (just the same solution every time). So it's not using the global figure to override the materials figures, I think.
      
      I've put the query in to my VAR support and am waiting for their Simulation specialist to get back to me.
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      - Anthony Botting @ Doug Rawlings on May 25, 2012 3:10 PM
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        Thanks for the update. I am keenly interested in finding out what the VAR says. -Tony
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Doug Rawlings Jun 19, 2012 2:19 PM
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OK, just a quick update for now.

What appears to be happening is that the damping is only updated in the model if the mesh is regenerated.

There was also an issue with SolidWorks crashing if I didn't restart it each time, but I don't know if that was related.

Basically I had to:
- start SolidWorks
- update materials
- generate mesh
- run the simulation
- have a look at the results
- shut down SolidWorks
go back to the beginning.

As far as I can work out the composite damping works by generating a critical damping ratio for the whole system, dependent on the masses and eigenvectors of the model, and then applies that to the whole model as if it were a global value.

http://help.solidworks.com/2012/English/SolidWorks/cworks/Composite_Modal_Damping.htm

So maybe it calculates the damping value as part of the meshing process?
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- Brad S @ Doug Rawlings on Jun 19, 2012 9:11 PM
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  a little titbit you would think should make it into that help page. thanks for finding out for everyone who might use it
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Applying material damping

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