If you haven't had enough of this...
I did two things: a redneck engineering experiment and some web research.
I took a single strand of Dyneema '02, marked off 50", and used it to hang a 45# weight. (The toughest part: finding a knot that wouldn't slip!)
As soon as the weight was hung, and while the strand was under tension, I measured the distance between the marks as 50-1/4". I took the weight off and the marks were still 50-1/4" apart. I put the weight back on. Several hours later, measured under tension: still 50-1/4" -- maybe a tiny bit (the width of the mark drawn with a ballpoint pen) more. So: I saw 1/2% elongation.
I found some guys thesis on the web. One chapter dealt with elongation of polyethylene fibers. His thesis included this diagram
which depicts three phases of elongation vs. constant stress:
I. increasing at a decreasing rate,
II. increasing at a (virtually) constant rate, and
III. increasing at an increasing rate (quickly resulting in failure.)
Phase I doesn't last long - he measured it in fractions of a second.
His conclusions were interesting:
- Phase I includes reversible (elastic) and irreversible (creep) elongation.
- Phase II consists entirely of irreversible (creep) elongation.
- The reversible elongation is miniscule compared to irreversible elongation.
The differences from one brand to another? There have been at least two varients of Dyneema and Spectra: Dyneema SK60, Dyneema SK66, Spectra, and Spectra 1000. Although made of exactly the same molecule, the differences between the finished products are trade secrets -- but it seems that the differences are the number of filaments per strand (hundreds) and the technique used for drawing the strands during the manufacturing process.
Then, of course, the folks who resell the material as bowstring making stuff add whatever wax they think belongs on the product.
Now, you know what I know...
Topic: ff string question (Read 379 times)
