Currently going through impact hammer test of cantilever beam(200mmx20mmx2mm) which is made of particulate composite(Epoxy/PZT/CB). Hammer sensitivity is 2.25mV/N and accelerometer sensitivity is 100mV/g. I'm trying my best to hit the same spot without skewing to get the average value in order to find driving point. Here are some problems I've been facing.
1. Shrinking magnitude at FRF
Let's say I attached the sensor at point A and hit the opposite side. I got pretty neat 2 sharp peaks with antiresonance placed between them. When I hit the same spot again, the 2nd mode peak of FRF dropped down to 0 which become barely visible peak. Same thing happend when I hit several more time. Can I say it's because of nonlinear result due to micro cracks and voids inside the beam? (I checked many voids by SEM image)
2. Phase
If I check the degree change from 60 to -120, then is it ok to assume resonance there? And does 360 degree changing and density of phase graph doesn't have any meaning at all? Lastly, does the 180 degree phase decrement going through long range of frequency mean high damping?
3. Coherence
I know the best result of coherence is mostly at 1 with slight drops at antiresonance points. In my case, the coherence is showing a noisy wave shape or like a thick dense phase graph. I've been trying to hit the same spot as uniformly I can. What is the cause of this result?
Thank you.
Adding picture of your setup including a place of the point A and hammer/accelerometer plots can help for better understanding. Do the beam rigidly fixed at one end?
Some things to check:
How does coherence look like? If close to 1, then there is a nearly linear relation between input an output, if closer to 0 then maybe something wrong with your measurement setup (bad SNR?) since
SNR = gamma²/(1-gamma²), with gamma being the coherence.
Bad SNR can be due to the fact that the eigenmode (at a certain eigenfrequency) is not "excitable", i.e. you hit it at a node, not at a point of maximum amplitude, or your acceleration sensor has bad "observation", i.e. it likewise sits at a node of this mode and not at a point of maximum amplitude.
You might also want to check the APS (Autopowerspectrum) of the hammer pulse. Depending on the hammer tip you are using, you will be able to excite a different distribution of energy, i.e. more energy at lower frequencies for low stiffness tips (like rubber) or higher energy at higher frequencies for higher stiffness tips (like hard resin or metal). The shape of the APS of the hammer pulse will show you the details.
Regarding windowing you should also be careful to choose rectangular or exponential for the hammer pulse (and the response). Do not use Hanning or Hamming windows, since they will "fade-in and out" the time signal and you loose too much energy of the hammer pulse (which sits at the start of the window).
There are some good commercial programs available like Head Acoustics Artemis, LMS, etc. that support you in doing it right.
Hope this helps (or is it all known?)
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