Knot strength
Each knot reduces the tensile strength of a rope , i.e. a rope under tension is most likely to break at a knot. The knot strength is in the knots that size which specifies how many percent of the original tensile strength yet remain a rope with knots. The value depends on the type of knot and the rope used. The higher the percentage, the better the tear resistance. Typical knots reduce the strength by 30 to 50 percent, so the knot strengths are only 50 to 70 percent.
causes
During loading tests, a knotted rope will tear where the curvature is greatest. This point is usually at the beginning of the knot - provided that the rope does not run over sharp edges ( edge strength ), has no manufacturing defects and is not previously damaged (see abrasion resistance , UV resistance, chemical resistance , drop resistance ). Because the rope fibers are unevenly stressed in the knot. Under load, both stretching and squeezing reduce the effective diameter ( transverse contraction ) and thus the tear strength.
If stretching causes excessive stretching, individual fibers can tear. Depending on the force applied and the coefficient of elasticity, the material reaches the yield point and begins to flow . A high crushing pressure can cause the material to exceed the crushing limit and also begin to flow. If individual fibers are damaged, the remaining fibers must bear the entire load, which can then exceed the tensile strength of the remaining fiber bundle.
In a straight, untied rope, the forces are distributed evenly over all fibers with increasing stretch. In the node, however, this distribution does not apply; if a crack occurs, it originates from the most heavily loaded fibers.
Comparison of different nodes
The knot strength is given as a percentage of the strength of the untied single strand. During the tensile tests, a load is increased quasi-statically.
loop
The following table was created by the German Alpine Club in 1999 in tests with a dynamic 10.5 mm climbing rope and Edelrid's 11 mm static rope "Everdry".
| node | image | Dynamic | Static | Name for climbers |
|---|---|---|---|---|
| Nine knots |  |
77% | ||
| Bulin 1.5 |  |
67% | 64% | Bulin 1.5 |
| Figure of eight knot (loop) |  |
63% | 65% | |
| Bowline |  |
64% | Bulin | |
| Leader knot |  |
58% | 59% | Sack stitch |
| Double bowline |  |
56% | Double bulin | |
| Spar stitch |  |
56% | 58% | Spar stitch |
| Loom line |  |
52% | Mast throw |
Rope connection
The German Alpine Association determined the following values for rope connections. The testers created an endless loop by connecting the ends of a piece of rope to the knot to be tested. The resulting noose was tightened with two carabiners and the load increased quasi-statically. The load values determined therefore apply to the rope, which is run twice as a loop, and have therefore been halved for the specification of the knot strength related to the tear strength of the individual strand.
| Knot in an endless loop | 10.5 mm | 7 mm | |
|---|---|---|---|
| Eight knot tied |  |
58% | > 72% |
| Eight knot in teardrop shape |  |
59% | > 73% |
| Sackstitch pinned |  |
63% | > 66% |
| Sack stitch in teardrop shape |  |
44% | 52% |
A 10.5 mm climbing rope and a 7 mm accessory cord served as test ropes. The inaccurate information on the 7 mm cord comes from the fact that the accessory cord broke under this load on the carabiner. The strength in the knot is therefore higher.
Web links
- Comparison of sack stitch, double spar stitch and triple T spar stitch
- Chris Semmel: So check who is tying ... - Knot strength in loops and cords, part 2 . In: German Alpine Association (Ed.): German Alpine Association Panorama . No. 4 , 2007, p. 76–79 ( alpenverein.de [PDF; 573 kB ]).
- Thomas Bock: weak point node . No. 10 . Delius Klasing, 2004, ISSN 0006-7636 , p. 66 ff . ( archive.org [PDF; 403 kB ] Knot breaking load on lines for sailors).
- Dave Richards: Knot Break Strength vs Rope Break Strength. Retrieved April 25, 2018 .
Individual evidence
- ↑ Piotr Pieranski, Sandor Kasas, Giovanni Dietler, Jacques Dubochet, Andrzej Stasiak: Localization of breakage points in knotted strings . In: New Journal of Physics . No. 3 , 2001, doi : 10.1088 / 1367-2630 / 3/1/310 .