Textile ropes, slings, webbing, and accessory cords are standard pieces of climbing equipment and almost always knotted when used. The material and construction, as well as the knot type and load direction on the knot affect the extent to which the strength of the starter material is reduced by the knot. All gear should naturally hold, but how much safety reserve remains? Which textile with which knot is particularly ideal for which purpose? To answer these questions, we have tested the most important combinations.
Essentially, a knot reduces the strength of textile materials due to the imbalanced distribution of the load on the fibers. The knot's bend radius is a key factor and is influenced by both the knot type and the direction of the load on the knot. The fibers in bent material are compressed, meaning that some are subject to less tension than others (see Figure 1). Tension peaks occur in the material, which can break at these points when subjected to a certain load. The cross-section shape (round, rectangular) and dimensions together with the material's elongation at break affect the impact of the uneven load distribution due to the knot. A highly stretchy material can compensate for these tension peaks more so than a static material as the fibers subject to the most tension stretch, placing the load on additional fibers.
Figure 1: fibers subject to even tension in an unknotted rope; fibers subject to uneven tension at the knot's bend radius; these areas of peak tension are then the predetermined breaking point:
Four materials are used for the textile fibers in ropes, slings, and accessory cords:
Polyamide (PA) is the most widely used material. At 15–30%, it has the highest elongation at break of the four materials but only a medium tensile strength of 800 N/mm2. This is why—compared to high-strength materials such as Dyneema® (UHMWPE) or aramid—more material has to be incorporated into products made from polyamide to achieve the same strength. It is both intertwined into ropes and accessory cords with a kernmantle construction and interwoven into webbing constructions.
Polyester has the same breaking strength as polyamide and a lower elongation at break (10–20%). It is slightly more abrasion resistant and primarily used in sewn and non-sewn formats in webbing constructions.
Dyneema® is the brand name for ultra-high-molecular-weight polyethylene (UHMWPE). It is extremely strong with a tensile strength of 3,400 N/mm2 but has a very low elongation at break of 3.8%. As the surface is very smooth, knots slip easily.
Dyneema® is used in stitched slings and accessory cords. As it is high-strength, the slings offer the necessary breaking load even with a very small diameter.
Aramid is an aromatic polyamide, the properties of which differ from those of its aforementioned relative. It is extremely strong with a breaking strength of 3,300 N/mm2 but has a low elongation at break of 3.5%. The fibers are golden yellow and are mainly used to create the core of accessory cords.
The tests were performed using two ropes and all common types of EDELRID webbing, accessory cords, and slings.
The Apus Eco Dry is a thin, 7.9 mm half and twin rope with a meter weight of 44 g/m. It withstands nine standard falls as a half rope and 30 as a twin rope.
The triple-certified Swift Eco Dry is 8.9 mm thick and has a meter weight of 52 g/m. It withstands seven standard falls as a single rope and 22 as a half or twin rope.
The webbing materials tested were polyamide webbing with a width of 16, 19, and 25 mm, 12-mm-wide Tech Web composite fabric webbing with a polyamide sheath and a Dyneema® core, and Dyneema® webbing with a width of 8 and 11 mm. However, despite the common name, the 11 mm Dyneema® webbing is actually composite fabric webbing that contains 57% polyamide.
The accessory cords used were a 6 mm accessory cord made of polyamide, an accessory cord with a Dyneema® core and polyester sheath (Hardline), an accessory cord with an aramid core and a polyamide sheath (Aramid Cord), and an accessory cord with a polyamide core and an aramid/polyamide sheath (Rap Line Protect Pro Dry).
The belay station setups with sewn material were created using 120-cm-long slings made from the 16 mm polyamide, Tech Web composite fabric, and 8 and 11 mm Dyneema® webbing described above, as well as the Aramid Cord.
To assess the extent to which the knots reduces the strength of the materials, the breaking load and elongation of the unknotted materials were first determined as points of reference in accordance with EN 566 and EN 565 using sheaves.
The materials were subsequently tested with a range of common mountaineering knots: the flat offset overhand bend and double flat overhand bend, the offset overhand bend and figure-eight loop subject to roping-up load, the double fisherman's knot as a connection between two ends and the clove hitch on a carabiner.
Tests were also performed on several knot combinations, as used on belay station setups or as subjected to loads at the belay if an anchor blows.
Prior to testing, all materials were conditioned in accordance with EN 892:2012, point 5.2. To this end, they were stored for 24 hours in conditions of 23 ± 2°C and 50% relative humidity.
The tensile tests to obtain reference measurements with the unknotted materials were carried out at 300 mm/min. The elongation was measured visually. The knotted structures were implemented at 500 mm/min. All tensile tests were performed three times and the results were averaged. The standard deviation between the three values was between 0.2 kN and 1.8 kN.
To depict the reduction more clearly, in the case of knots on a single strand, the percentage reduction in strength was presented in comparison with the unknotted single strand. When subjected to the load, some knots did not tear but instead slipped until the loose end came through the knot. These values are marked with an *. In the case of Dyneema® in particular, knots start to slip quickly. Back-up protection to prevent complete slipping is therefore urgently required.
During the tests, many of the knots slipped until this was prevented. In the case of the offset overhand and double overhand bends, the slippage was stopped by a carabiner hung in the 'eyelet'. In the case of the belay station setups, it was stopped by the blown second anchor (in the case of the test, a knotted eight).
In the case of the Hardline, the sheath mainly tore and the Dyneema® core was pulled out of the knot.
In the case of all materials with polyamide as the outer layer, the knot only slipped partially and, if it did, then one to three times in a jerky manner. The friction of the jerkily slipping knot melted the material to such a great extent that the knot stuck together and the fabric tore.
Major differences were recorded in knot strength depending on the material and the type of knot. In the case of some materials or uses, the absolute strength values significantly exceed requirements, meaning it is ultimately irrelevant which knot is used and the decision can be taken based on practical usage aspects. With some combinations of material and usage, it makes sense to consider the strength reducing effect of a knot when making a selection.
Knots have the least strength reducing effect on ropes: a maximum of -43% with single strands. Knots have a particularly strong strength reducing effect on Aramid Cord (-64% on average across all knots), Hardline (-62% on average across all knots) and Dyneema® 8 mm (-54% on average across all knots). It is clear that with the remaining constructions, the presence of PA with greater elongation positively affects the strength reduction caused by knots.
When comparing knot types, the flat offset overhand and double flat overhand bends have the greatest strength reducing effect of -38% to -75%, with the offset overhand bend causing slightly greater strength reduction. The load type on the knot has a significant influence on the knot's strength reducing effect. If an offset overhand bend is subjected to load as a roping-up knot, the strength reduction is about 20% less than when the load is placed on the ring. The double fisherman's knot causes less strength reduction than the double flat overhand bend.
When creating a series connection, even if an anchor blows, all potential materials (sewn as a sling) still have breaking loads of over 12 kN, or if the load is placed on a Bowline on a bight, usually of over 24 kN. It was not possible to determine a breaking load with the Dyneema® slings as the Bowline on a bight started to slip. However, the strength with an anchor was over 12 kN, which leads to the expectation that the Bowline on a bight would have a breaking strength of over 20 kN. The South Tyrolean belay station with a girth hitch demonstrated slightly greater strengths than that with the clove hitch, although there is no major difference and the values with the clove hitch are also acceptable for a belay station with questionable anchors. With a load applied to one strand only, the fixed triangle of forces has a strength of over 11 kN with all materials.
The article 'Schlingenrisse an Standplätzen' ('Sling Tears at Belay Stations', Berg und Steigen #107) assumes that in the case of a station fall by a leader secured on a double rope using a Munter hitch, depending on the roughness of the rope sheath and the strength of the belayer, the belay station can be subjected to a load of up to 5.5 kN. If an anchor then blows, the pendulum movement of the belayer can add 1 kN and result in a load of 6.5 kN.
All tests were performed with new material. The effects of aging, such as mechanical walking or UV stress, reduce the breaking load. Such effects must be particularly considered in the case of 8 mm Dyneema®.
As described under 'Test Parameters', the tests were performed quasi-statically. That means that the force was applied very slowly. In the case of a fall, however, the force is applied jerkily, therefore reducing the strength slightly more.
Depending on the type of use, it is therefore important to build in sufficient safety margins when selecting a knot.