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Author Topic: Question on the Fred Pearce method for testing rubber  (Read 1292 times)
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« Reply #25 on: March 11, 2017, 09:28:00 AM »

Back to the original question. Looking at the test methodology Fred Pearce published in 1979 I have a couple thoughts on selecting the F1 and F2 forces.

I expect the 45 used for F1 and the 430 used in the F2 equations are empirically derived. Pearce states the first pull is to stretch to near the maximum stretch and the second pull is to bring it near the breaking point. With that in mind I would do a few test and derive new factors for both the F1 and F2 equations.

I suggest you test a few samples the way you have been then do one more pull much like the second pull but keep going until you find the breaking point. With this data I would determine a factor that typically gives an F1 force about half way up the final steep slope and a factor that typically gives an F2 a bit short of the breaking point. If you make the third pull to destruction a regular part of the test procedure you add breaking point to the accumulated knowledge and you get feedback on how close your second pull is coming to the breaking point. Update the factors as necessary so the F1 and F2 guide numbers put you where you want to be on the curve.

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« Reply #26 on: March 31, 2017, 01:59:36 PM »

I just stumbled across yet another way to test rubber. "A Quick Rubber Test" by Hank Cole ran in the May 1997 issue of the "Bat Sheet".  It had been filed away years ago; I discovered it going through some old articles on rubber.  The title is right--it is a very quick test; here are the steps; (I've included numbers I got from a test of June 2016 Super Sport.)

* Measure out exactly 12 inches of rubber, weigh on accurate scale and note the weight (wt = .73)

* Mark and accurate 1 inch section in the middle of the 12 inch strip and pull to limit and measure the max stretch (S = 9.5)

* Subtract 1 from the stretch number and multiply by 0.58. (9.5-1 = 8.5 x 0.58 = 5.93) This is the pull-to number.

* Tie loops in both ends of the 12 inch strip. Hook one end to a firmly-anchored peg or hook; fasten the other end to a pull scale. Position a ruler under the rubber strip and start pulling until the distance between the two marks on the rubber are at the pull-to distance. Note the force needed to reach the pull-to length in grams; this is the weight W.  (W = 353.8g)

* Multiply the weight W by the stretch length S-1 and dived that by the rubber weight wt. This gives the energy E in foot-pounds per pound. ( 353.8 x 8.5 divided by 0.73 = 4119)

I did my tests at 66 degrees F and used Paul Roster's temperature correction factor described in his 2016 Sympo paper (1.3% per degree C).  That changed my 4119 figure to 4178 ftlb/lb at 20 degrees C. (Paul did not test June 2016, but a recent numbers I have seen elsewhere for June 2016 is 4106.)

While I had Paul's Sympo paper open I checked his number against some of the batches of Super Sport that we both had tested. Here are the results for the only two overlaps:

June 2013  Cole method by Louis Joyner (3964)  Paul Rossiter (4050)

April 2015  Cole method (4715) Rossiter (4720)
(All energy results corrected to 20 degrees C)

The two problems I had with the Cole method of testing were trying to estimate the exact middle of the marks on the rubber (the two marks made an inch apart). When the rubber is stretched the very narrow mark made with an ultra fine point Sharpie becomes a wide smudge. Also both marks move away from the anchor as the rubber is stretched out to the pull-to distance; I had to constantly move the ruler to keep the zero mark directly under the center of one of the marks on the rubber. All the tests were done with 3/32 strip, but Cole does not specify strip width.

I have no idea how Hank developed this test but I found quick and easy, and, surprisingly accurate. It is, as with any test, dependent on the accuracy of the test equipment (digital weigh scale and digital pull scale) and the test operator.


PS  I also tested November 2012 and came up with 4374 corrected.

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