I have to design a rubber damper for a bike to reduce the road vibration. To select the appropriate elastomer material, I have performed a frequency sweep DMA test of two different elastomer specimen. The test output is, Material A is having more tan delta as well as dynamic stiffness compared to material B. I have attached the images of result. As per my understanding, material having more tan delta has to be selected for isolation application. But I want to know whether dynamic stiffness should be considered for selecting the material or not?
And after selecting a material, at component level, while doing shape optimization of damper, considering isolation perspective, dynamic stiffness of actual component should be high or less?
Dear Tarup, it is difficult to follow 'prescribed rules' like you want - in selecting your material you need to think exactly what you want to achieve. In a damper for a bike, I guess your intention in to select a specific dominant frequency (I don't know - may be related to the wheel rotation rate?) and then achieve the maximum energy dissipation at this frequency. Is this what you want?? In that case, you should learn to calculate the dissipated energy - which you will find is proportional to the {loss modulus}, i.e. the product of {tan delta} and the dynamic stiffness, which I guess is your name for the {storage modulus}. And then you need a material with a peak of {tan delta}*{stiffness} at your frequency.
Or may be you want to maximum total energy dissipated across the whole energy spectrum? Then your result will depend on exactly how the {loss modulus} depends on frequency (i.e. how high or how wide is the peak).
But I am not sure the maximum energy dissipation is what you need. This energy would be coming from your pedaling (or motor): it is remarkably hard to pedal on a bike with solid rubber tyres. Not surprising, in car (and mountain bike) design the shock absorbers are often solid (elastic) springs that work on storing and re-distributing impact energy. You really need to decide what you want out of your damper...
Well, the reason you find elastomers with internal transition (crystallisation, or in fact the glass transition works very well) is because this transition absorbs a lot of energy - there is always a peak of (tan delta) at such transitions. Usually these ate induced by temperature change, but other factors (e.g. shear amplitude promote a transition). But ultimately this is all about finding maximum (tan delta). Same with internal filler: as long as these particles can independently move (usually rotate) in the matrix, the damping will be high. Actually, there are various new 'smart' materials (probably too expensive for your applications) like Liquid Crystalline Elastomers, where the mobile internal microstructure (in LCE case, this is an orientational order) makes this 'maximum' of (tan delta) turn into a broad/flat high value of around 1. Amazing damping.
But there are limitations too. For instance, no rubber material can have a big loss factor at high frequency : certainly a few kHz is as far as the polymeric material can practically go - it becomes glassy at higher frequency.
Thanks for the answers.
Yes Eugene, My interest of isolation is maximum energy dissipation at particular frequency range (lower frequency, approximately 25Hz to 60Hz).
So ideally I should select the material having high loss modulus and tan delta in that particular frequency, irrespective of storage modulus of the material. Correct me, if i am wrong.
Thanks Massoud, for explaining material composition aspect for shock absorbing properties.
Also, can anybody tell, is there any way to further more increase loss modulus and tan delta of a selected material? and what other physical properties of elastomer will effect by it?
Hi Tarup, It seems like your material-B has a fairly constant tandelta across a wide range of frequencies. Can you please let me know what material-B is, I am looking for a material with constant tandelta for one of my project.
Hi Abhishek Gupta
The material with a constant tandelta behavior can be NR, CR or EPDM. You can find more information at https://s3-prod.rubbernews.com/s3fs-public/RN11142784.PDF
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