shock absorption and phase angle

[sonnenschein987]

Hi, I did a test with a material under 20Hz cyclic loading MTS. To evaluate the force absorption, two load cells were used to simultaneously measure the applied (input) load and transmitted (output) resultant force. For a stiff material, the force applied was the force transmitted to 100% with no phase shift. For another material there was 30% less force measured at the second load cell. I would assume 30% were absorbed/dissipated. But the phase angle was the same (no shift there). How comes? Wouldn’t the phase angle between both signals be an indication of the damper coefficient? How can I have 30% absorbed but 0 phase shift in the signals?
Thanks!!!

 

[Compositepro]

Probably because the applied force travels through the material at the speed of sound.

 

[GregLocock]

" I would /assume/ 30% were absorbed/dissipated. "

/I/ wouldn't.

Cheers

Greg Locock


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[sonnenschein987]

Greg, what would you assume that had happened to the load. Give me us your wisdom or do not leave a comment!
thanks!

 

[zekeman]

Would you please show us the setup so we can understand where and what you are measuring?

Is it a simple one degree of freedom or a distributed system.

From the "description" of the problem you gave, Greg's answer was more than you could expect.

 

[sonnenschein987]

Sure. It is 1 dof system. One block of poly material between 2 metal plates. The piston of the mts applies a cyclic load and measures with external load ell the voltage (previosly calibrated to extract force). Results of this test are as expected. Nearly same load as applied is measured by the mts load cell.
Second test. Similar system but with some hydraulic fluid. Mts measures 70% of the applied load with 0 phase angle shift. My thoughts: hydraulic dissipate energy. But wondering about the phase angle (damping?).
Is that making it any more clear?
Thanks

 

[vpl]

Was this a laboratory (school) experiment?

Patricia Lougheed

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[GregLocock]

The reason i gave an incomplete answer, indicating a lack of rigorous thought on your part, is so that you can do some thinking.

You have correctly identified that energy absorption without a phase shift is unlikely. So....

Cheers

Greg Locock


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[sonnenschein987]

This is not a school project and you are not helping me with my homework. It is part of a larger experiement. I want to exchange part 1 with part 2 because I hope to get better performance (force absorption). Therefore, I did the side test to see if I am right. The results are not quite what I was hoping but I am not really a pro in this area. I am trying right now to find out if it was an error on the testing side or if there is a mechanical explanation for this condition. I would gladly appreciate if you tell me what you know or give me a reference where I can read this up. I do not have too much time and I need to find out pretty quickly what to do or I am holding off an important large experiement.
thanks!
Diana

 

[PNachtwey]

sonnenshein987, what you described makes sense to me.
Compositepro is right. There is some phase shift but the delay is small because speed of sound through oil is about 4ft/ms. You may not be able to detect such a small delay.

MTS systems are not cheap. Teachers or students wouldn't have access to them.

sonnenshein987, how fast does the MTS system sample?



Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[sonnenschein987]

Hi PNachtwey, thanks for the answer. The part is about 1in thick. The cycling was at 20 Hz and we sampled at 100Hz. I would be able to detect any shift if it was there. And I was not quite correct as I said that there was 0 shift. Actually there was 2-5 deg of shift, but I was reading up on this and I found that in generally, the damper coefficient is considered as negligible if the shift between the two signals is <10°. I need to run now and pick up my kids, but will see later today if I can calculate something with the numbers you gave me. It looks like a good explanation to me. Thanks,
Diana

 

[btrueblood]

Well...some alternate explanations could include (a) that a link in the force train slipped or broke, and the force is no longer being transmitted to the opposite side. Presumably a before-and-after static load calibration was run to rule this out? Explanation (b) is that for some reason the input force during cycling was being controlled by displacement rather than by the input side load cell, and the change in stiffness of the system between the two test runs resulted in less force transmitted to the opposite side load cell. Finally (c) the system mass changed significantly, and the force on the input side is being absorbed by having to accelerate the heavier parts at 20Hz. But this is all just guesswork, without a clear diagram and/or drawing and/or photo of what the heck you are testing.