Preliminary test on how much does TT rubber and TT balls compress

[brokenball]

Today I put a rubber and TT ball in our hydraulic system to watch how the force increases as a function of compression.
Why bother? Because I want to know if rubbers really bottom out.

I can calculate the energy that the ball has when it hits the paddle. Eventually the ball stops relative to the paddle. At that point the rubber AND ball have absorbed all the energy. That is equal to 1/2K*distance^2. This test allows me to calculate the spring constant K. Actually the "spring constant" isn't really constant. It is non-linear.

Some of the results were as expected. At first the force increases slowly and builds as the more and more of the ball gets pushed into the rubber. I was using Rakza 7 2mm scraps for the rubber and a old Stiga 3 star ball. What surprised me is how far the rubber seemed to be compressed. We compressed the ball and rubber about 2mm or 0.0.80 in. I didn't expect the rubber alone to compress that much without "bottoming" out. Later we squeezed just the ball and again we squeezed just the ball about 2mm or 0,080 in. The ball compressed linearly until the ball was compressed close to 0.080in or 2mm and then the force didn't increase linearly any more. The ball didn't break and it seems to be OK. I need to test a plastic ball as they don't seem to compress as much as a celluloid ball.
So now the question is what was compressing during the first test. Obviously it is the rubber AND the ball. I need to test the rubber by itself without the ball. The top sheet compresses too so there are 3 springs in series. The top sheet the sponge and the ball.

I have video and recorded data. It is crude. The load cell I was using when from 0 to 5000 lb and I was using only a small fraction of the load cell rage. We need to obtain a load cell that goes up to about 100 lb or 500N. 1lbf = 4.45 N. My position sensor was calibrated in inches. What is old is the sensor naturally returns position is microns but the people here are used to inches. Also, I need to get a better valve. We have them but it was not on the machine at the time.

I wanted to test just the rubber but it got accidentally smashed. The machine can apply forces to 4500 psi.

What I found encouraging is that the models I have been using were in the ball park given the assumptions I had to make.
pictures and videos to come.

 

[hclnnkhg]

Looking forward to the final, complete report. I'm assuming there will be some stress-strain/displacement curves? Figures are a lot more easier to understand than long texts.

How would "bottom-out" looks like on such a figure (or how would you define it)? Also, is hysteresis taking into account? Last but not least, how do measurements of slow compression test translate to fast collision in actual table tennis rallies?

 

[brokenball]

This video shows one of our first tries. We compressed the ball 0.040 inches or about 1 mm.
To the right of the ball is the load cell. It has a range of 0 to 5000 lbf and we only needed about 0 to 100 lbf. The system can apply 4500 lbf so we were only using a small fraction of the range. We need a load cell that goes from 0 to 500 lbf or maybe 1000 lbf to get better resolution.
The read line is the position. The position changed at a constant rate of 0.001 in/s
The black line is the force. The line is flat until contact is made. The force does not increase linearly with the position at first because the ball is round and isn't making much contact at first.

https://deltamotion.com/peter/TT/testing/compress%20040.mp4

The second video show compressing the ball and rubber by 0.080 in at 0.002 in/s
I was surprised the change in force vs the change in position was relatively linear. I want to stop at 0.080 in.

https://deltamotion.com/peter/TT/testing/compress%20080.mp4

I need a load cell that is scaled for the range we need to increase resolution. I also need to compress until the ball breaks. I didn't get a video of compressing the ball alone but it would compress 0.080 in easily and it was relatively linear at first and then at about 0.070 in the force didn't increase much.

The rubber AND the ball compress much more than I thought.

I have recorded data that I need to look at. More coming.

 

[WingTT]

This is very interesting. Brokenball, you may want to host your videos on some other dedicated video hosting site as it seems slow and the folks trying to get to your videos may internet hug your site to death.

 

[brokenball]

This is very interesting. Brokenball, you may want to host your videos on some other dedicated video hosting site as it seems slow and the folks trying to get to your videos may internet hug your site to death.

Yes, download the video then play it.
I am not that proud of the video because I was holding the stick that kept the ball in place before contact was made and my cell phone. I plan to replace these video. I just wanted the forum to see how it is done. I hope to have some pictures of the captured data tonight. BTW, I have one of the best hydraulic testing labs in the country. Major hydraulic valve manufacturers send their valves to us to test. The valve we were using in this test is not that good. The main purpose of the video is just to answer my intuition about "bottoming" out. Like the rest of you, I believe it can happen but NO ONE HAS EVER DONE A TEST to check if we are just repeating fairy tales. What bothers me now is that the ball compressed 2 mm. So know I wonder if the ball doesn't deform enough before the rubber bottoms out. I plan to find out. I plan to update my models. I do have a hydraulic YouTube channel where I can post the good videos.

 

[brokenball]

I am posting the two best plots of force vs compression or stress vs strain. The first one is of the rubber and ball compressing. The rubber and ball were compressed 0.080 in or about 2mm. I should have compressed them more because the force still looks relatively linear with respect to compression.
The red line is position and has its scale on the left. The units are inches. The black line is force and has its scale on the right. The units are pounds force.

The second plot shows the compression of just the ball alone. I have always wondered how much the ball compresses.

Next time I will have the system calibrated in mm. The positions can be displayed to 3 digits so the position will be microns. I need a load cell with a smaller range so the force will have more resolution. Finally, I need to compress until something breaks. I didn't want to do it this time because I know my setup was not optimal. Finally, I need to test other rubbers. I have a drawer full.

I should have tested the how the Rakza 7 compress without the TT ball. This way I can get a better idea of the ratio of what is compressing.

 

[palguay]

very interesting, thanks for posting the results and video. It will be interesting to see how does this compare to a say dhs rubber, will it compress as much or lesser