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Sherlock Damping Factors

Posted: 28 Mar 2014, 11:31
by Nathan Blattau
Sherlock uses mode based damping computed from the natural frequencies. The damping factor field supplied in the analysis properties (random, harmonic) is a modifier for the computed mode based damping factors.

If you want to look at the computed damping factors for comparison factors you need to look at the generated .inp file for the analysis located in the project folder. For example:
C:\Users\nblattau\AppData\Roaming\Sherlock\projects\ODB++ Tutorial\ODB++ Tutorial\modules\HarmonicVibe\calculix-HV.inp

Open the file with notepad (or a better text editor), scroll to the end and you will see the computed damping factors:

** End of Model
**-------------------------------------------------------
**-------------------------------------------------------
** HARMONIC VIBRATION ANALYSIS STEP
**-------------------------------------------------------
*Step
*Steady State Dynamics
1.0E01,1.0E03,5,3
**
*MODAL DAMPING
1,1,7.3452E-03
2,2,1.4526E-02
3,3,2.1379E-02
4,4,2.5118E-02
5,5,3.1541E-02
6,6,3.6158E-02
7,7,4.0499E-02
8,8,4.3865E-02
9,9,4.7411E-02
**
*Amplitude, name=HarmonicAmpl
1.0E01, 1.0E00
1.0E03, 1.0E00
**
*DLOAD, AMPLITUDE=HarmonicAmpl, OP=NEW
ALL_ELEMENTS,GRAV,9.8066E03,0.0E00,0.0E00,-1.0E00
**
*node file, frequency=1, output=3D
PU
*el file
E
*endstep

You can increase or decrease the damping with the Sherlock damping factor property. Numbers below one reduce the damping (a straight multiplier), 0.1 multiplies all the computed damping factors by 0.1.

Re: Sherlock Damping Factors

Posted: 07 Jul 2015, 22:16
by hakwon
Dear Sir,

If the damping factor is computed based on the Natural Frequency of the board, the preset damping value at the time of vibration simulation seems to be the computed one.
If I need to define a modifier by setting a different valaue on the damping field from 0-1, what would be general guidelines on the specific numbers. For instance, o.1 for rigid mounting up to 1 for chassis mounting and then the intermediate values?

Best Regards,
HA

Re: Sherlock Damping Factors

Posted: 14 Jul 2015, 13:20
by Nathan Blattau
The damping factors are based on the natural frequencies of the pcb. During random or harmonic simulations, Sherlock does a natural frequency simulation first and then computes the damping factor for each mode.

The analysis property in Sherlock is used to scale these already computed factors. I use 0.2 to 0.3 for very rigid mounting conditions (decreases damping) and a value of 1 for a pcb mounted in a chassis.

Best regards,
Nathan

Re: Sherlock Damping Factors

Posted: 11 Jul 2019, 04:57
by long2412
Hello Mr. Nathan,

"During random or harmonic simulations, Sherlock does a natural frequency simulation first and then computes the damping factor for each mode."

Could you please tell how to get MODAL DAMPING from Natural Frequency result.

Because we are doing Random Vibration Analysis in Abaqus. Then import NF.odb, RV.odb results in Sherlock to obtain life prediction.
I got NF.odb but I do not know how to calculate damping factor for each mode.

So I read the Sherlock INP file and set up the FEA same as Sherlock generated, *Modal Damping is from Sherlock INP file. But life prediction after importing is different with Sherlock generated.

Thank you!

Thanh Long
KOREA

Re: Sherlock Damping Factors

Posted: 19 Jul 2019, 08:10
by Nathan Blattau
Hi,

The modal damping factors should are in the HV.inp file. If you make changes to the model in CAE which alters the natural frequencies the results will be different. The damping factors example from an input deck.

*Step
*Steady State Dynamics
2.4E01,2.0E03,5,3
**
*MODAL DAMPING
1,1,4.66E-03
2,2,1.6039E-02
**
You can run a harmonic analysis in Sherlock and copy the damping factors into Abaqus.

Re: Sherlock Damping Factors

Posted: 02 Aug 2019, 19:10
by jcuret
Hello Mr. Blattau,

I was curious on your opinion/suggestion for a value of damping factor before running my simulations. I currently just used the given .2 to start.


I'm simulated shock and vibe and the board is mounted like when it's in operation, to a server, so attached by the gold fingers and bracket. Any thoughts?

I was also curious about when you mentioned that Sherlock will compute damping factor for each mode when NF is ran. I thought I could use this info but I don't see anything about damping in the results. This makes me think I can't used the NF damping to help before running Shock or Vibe.

Thanks for your help,

Cameron