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    How to Test the Breaking Strength of a Matieral for a Bow String?

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    banuvatt
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    Post by banuvatt on Sat Nov 23, 2019 11:52 am

    I was wondering how to test the breaking strength of a material I was going to use for a bow string?
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    Post by Alan Case on Sat Nov 23, 2019 2:32 pm

    I have built up a pretty extensive database of breaking strengths and tensile stiffness properties of a large number of natural materials (Flax/linen, silk, hemp, ramie) and some synthetic materials for bow strings. What I do is test the sample with a large bucket hanging from a hook. I slowly fill the bucket with water until the string breaks, and then I weigh the bucket.  

    I don’t let the bucket spin and unravel the string material while filling it. The bucket only falls an inch or less when the string breaks so it doesn’t flood the garage!  I find that I get more consistent results this way. I do not rely on a knot to hold the string to the hook on the bucket and hanging hook because knots will always create an inconsistent weak spot. So I wrap the test sample several times around the smooth round hooks between where I tie off the string, and where it is fixed to the hooks. I do not count any tests if the sample breaks at where it touches one of the hooks.

    In order to fairly compare one material to another, I measure the weight per length of string, and divide the test strength by the weight per length. This allows direct comparison of one sample to another regardless of strand thickness. 

    From this testing, I found that B50 should be renamed B25, because I have never had a strand hold anything close to 50 pounds. I also found that some exceptional linen samples had significantly better strength-to-weight properties compared to B50. 

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    Post by banuvatt on Sat Nov 23, 2019 5:13 pm

    Okay thank you very much, your information was very detailed which made it very helpful.
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    Post by c sitas on Wed Mar 04, 2020 9:26 pm

    Just a little late to the game here but, For all intents and purpose I don't bother with dacron any more. Dynema costs a little more but you never break it. It would be the same as super fishing line to a layman. I t will cut you easily and deeply. Don't ask how I know . Been making bow strings many years. When done making one , I'll stretch at least overnight. with 400 lbs on it.
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    Post by banuvatt on Thu Mar 05, 2020 4:06 am

    I might use b55 dacron since I think b50 is getting harder to find.

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