Stress-Strain Plots for Braided and Twisted Nylon Twine
When I built my outdoor tensegrity dome, I wanted to be extra careful about measurements, so I took into account the non-linear stress-strain behavior of the nylon twine I was using. This involved creating a look-up table for the stress-strain relationship and using that instead of a linear relationship between stress and percent strain. There are two types of nylon twine I use, braided and twisted. The above plots show the data the look-up table uses for both types. As the chart shows, the braided stretches a little more in response to the same stress value. For smaller structures I generally use braided fishing line and have good results by just assuming the average tendon stress will stretch the line by 2%.
I measured these values by tying one-gallon milk jugs filled with water to a length of twine and measuring the response. A gallon of water weighs 8-1/3 pounds, and I worked in one-gallon increments up to eight gallons. I increased and decremented the load and allowed hours or days for a load to settle. I took all my results and drew a curve through them to get the table data. The table doesn't take into account significant memory effects: when the load is removed, the old value for the unstretched twine can never reached since the twine is permanently stretched. The significant settling time is something to remember when assembling a structure since a tendon which is really tight when first put on will loosen up a little after time passes. Overall nylon is kind of a mess to work with if you want very accurate results. It is somewhat forgiving of mistakes though.
The values for the look-up tables are as below. I linearly interpolate to get intermediate values.
| Stress (pounds) |
Twisted Twine Strain (percent) |
Braided Twine Strain (percent) |
|---|---|---|
| 0.00 | 0.0 | 0.0 |
| 4.17 | 2.9 | 3.3 |
| 8.33 | 4.8 | 6.3 |
| 12.5 | 6.8 | 8.7 |
| 16.7 | 8.2 | 10.8 |
| 20.8 | 9.6 | 12.2 |
| 25.0 | 10.7 | 13.8 |
| 29.2 | 11.8 | 15.3 |
| 33.3 | 13.0 | 16.5 |
| 37.5 | 13.8 | 17.8 |
| 41.7 | 14.8 | 18.8 |
| 45.8 | 15.7 | 19.7 |
| 50.0 | 16.3 | 20.8 |
| 54.2 | 16.9 | 21.3 |
| 58.3 | 17.5 | 22.2 |
| 62.5 | 17.9 | 22.9 |
| 66.7 | 18.3 | 23.8 |
| 70.8 | 18.7 | 24.6 |
The braided twine I've used is T.W. Evans #1 Braided Nylon Mason Line (Item No. 12-503). I've seen many brands of twisted twine. Two brands I've used are Wellington #18 Nylon Twine (Prod. No. 46302) and T.W. Evans #18 Twisted Nylon Mason Line (Item No. 10-184). I find the braided twine is easier to manipulate and unravels less readily. I tie a simple knot in the end of the twisted twine to keep it from unraveling. The end can also be burned with a flame or soldering iron, but take care with the toxic fumes.