Free Vibration test on clamped-clamped nickel nanobeam

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1

Hi all,

I was trying to get natural frequencies from doing vibration tests. I used two different methods but got different reasons for the first natural frequency. Can someone help me out with this?

My first method is to poke the beam in the middle.
Relaxation at the beginning for adequate time.
Then addforce for a few timesteps and then eliminate the force. Then the beam will oscillate and the displacement in the middle part of the beam is computed. And use FFT to get the frequency. I got the first natural frequency as 0.021991 rads/ps.

My second method is that I don’t apply any force and just let it oscillate after the relaxation process. Even the vibration amplitude is small but it is still enough to extract natural frequency.
I got the first natural frequency as 0.084823 rads/ps.

So I am not sure which method is more accurate. I use other things the same in my code (temperature, NPT for relaxation, same running timestep)

Warm regards
Yue

2

Hi

The methodology for calculating natural frequency is well established. I think npt is not the proper ensemble for this study. You can relax your system with nvt and then do nve at the final stage (after giving the initial displacement at the middle). Nve will conserve the total energy of the system. Perform fft on the potential energy to get the natural frequency.

The main reason for fft is to keep the total energy of the system constant.

This paper will help: https://arxiv.org/ftp/arxiv/papers/1209/1209.3091.pdf

Mahmudul Islam
Mechanical Engineering
Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh
Scholars profile : http://scholar.google.com/citations?user=XlEcITYAAAAJ&hl=en