Hi, anyone can help me with this problem I encountered?
Brief explanation on my research.
So I have this cubic sample made of ABS material formed into specific structure/configuration. I want to investigate if this structure would make a good isolator or not. Therefore, in ansys design modeller, I made solid plates made of mild steel and place it one on top of the ABS structure and another one at the bottom of the structure. I select the lower face of the bottom plate as fixed support. A force of 150N exerted on the upper face of the top plate.
after running modal analysis and harmonic response (for both top and bottom plates), using the responses from bottom and and top plates. I plotted a transmissibility graph. But my problem here is that the peaks I got from the graphs are different from the modal analysis frequencies. From my understanding, the peaks should be at the same frequencies.
your help much appreciated.
Hi Muhamad Syafwan Bin Azmi. Resonant peaks do not necessarily occur at all natural frequencies and depending upon the load, some of the modes would be strongly activated and some would be weakly activated or not activated at all ( if mode participation factor is zero for that load). You may find a simple worked out problem in the following paper:
https://m.box.com/shared_item/https%3A%2F%2Fapp.box.com%2Fs%2F0e3zkk9e6dedhexf89byq9ci5w7zeoxm
Hope this helps.
Regards,
Abbas
Hi
Mir Aamir Abbas is on the right track.
The reason why not all natural frequencies show in a Frequency Response Function is that it physically depends where both response (output) and excitation (input) is applied and oriented with respect to the mode shape associated with a natural frequency. Excitation and/or response at a position on the mode shape with a low response simply generates a low response in the FRF.
Figure 4.2 in HP Application Note 243-3 The Fundamentals of Modal Testing clearly demonstrate why this is so. http://hpmemoryproject.org/an/pdf/an_243-3.pdf
AN 243-3 is a classic. Other high quality self study oriented material can be found here. https://qringtech.com/learnmore/recommended-reading/
/Claes
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