Among different methods to calculate the damping ratio of structures, Logarithmic Decrement, and Half-power bandwidth methods are probably the most used approaches. The damping ratio values derived from Logarithmic Decrement method are obtained from a global damping measure of multiple modes that may have been excited. On the other hand, Half-power Bandwidth method produces individual modal damping values. Sometimes there is a considerable difference between the calculated values of damping ratio using the mentioned methods. I would like to distinguish which method is more reliable in terms of accuracy. Additionally, I need your expert opinion regarding the possible reasons affecting a structure to show different damping ratios for various natural frequencies calculated through Half-power Bandwidth method. I shall be grateful if you could give me some advice in connection with the asked questions.
Regards,
Saeed
Hi,
based on your application, you have to select damping measuring technique.
Suppose, an oscillator is undergoing transient response involving amplitude decay or gradual growth. Then amplitude decay method is best for measuring damping.
If the oscillator response is within the half-power frequency band like when excited by harmonic forces near its resonance, then half-power bandwidth method gives accurate value of damping.
second query: if you are referring to getting different damping ratios for different modes of structure. that is okay. damping can be different for modes. It depends on response of structure.
good day
Hi Deepesh, and thank you for the answer.
Your explanation about the selection of the best method to calculate damping is interesting and seems persuasive. I need to discuss it in my report by reference. Could you please tell me if there is any reference which discusses this topic?
About the second query, for the tests I have performed, generally, the damping increased by raising the natural frequency. I would like to know which parameters affect the damping ratio in different modal shapes and is there any relation between the modal shapes and increase or decrease in the damping ratio in a specific structure? Again, it would be much better if I have a reference to refer.
Thanks
Hi,
1. I do lot of experiments. So I tried different damping estimation techniques and I explained to you from my observations. don't have any references.
2. There is no particular relation between damping and mode shapes as far as i know. damping is ad hoc and system specific. But damping generally increases with excitation force when the structure is excited at particular mode shape. Hope this helps.
Thanks.
Thank you Deepesh,
Your explanation was very helpful.
Hi
As stated above, different methods provide differences in answer, in essence, because you are solving an inverse problem, i.e. a problem of the type where you know the answer (your FRF) to be, say, 4.0, and your task is to solve what is the question.
For obvious reasons, the solution space is infinite, so differences in assumptions regarding the system that you try to fit to your answer, will and must provide outputs that vary.
On a more physical note - one should expect damping values to vary from one mode to the next for a long list of mundane reasons. You find some ramblings of mine on this topic here if you are interested. http://qringtech.com/2014/06/22/designed-damping-types-mechanisms-application-limitation/
You might want to take a look here as well, where you find links to self study material on FRF measurement, experimental modal analysis and FE calculation of damping
http://qringtech.com/learnmore/recommended-reading/
Sincerely
Claes
Hello Saeed Eyvazinejad Firouzsalari : Refer to my answer to a similar question at:
https://www.researchgate.net/post/How_Can_I_define_Damping_Ratio_for_Concrete_or_another_Material_in_ABAQUS#view=552bf2b8d039b1e4788b45e6
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