Comparative Testing of High Strength Cord
by Tom Moyer, Paul Tusting, and Chris Harmston
Complete test data for the results presented in this paper can be found at:
The use of Kevlar, Spectra and Vectran fibers to make high-strength rope was pioneered by the sailing industry, to take advantage of their high tensile strength, low elongation, and low moisture absorption. These materials gradually moved into climbing applications, first as chock cord and more recently as cordelette material, prusik cord, and emergency rappel line. In the climbing world they have been surrounded by a lot of mythology and little hard data. It has been said that tying and untying chock cord weakens it severely, that double fisherman's knots are not secure, triple fisherman's are needed, that Kevlar-based cords will self-abrade and eventually weaken, and that knots will not hold in Spectra/Nylon webbing. In recent years, manufacturers have been combining and improving materials, and climbers have been expanding their applications. Have the materials improved? Are the myths true? And most importantly, are these materials strong enough to use in these applications? This testing will provide some answers to these questions.
Kevlar was one of the first high-strength fibers to be used in rope. It still offers high tensile strength and very low elongation, but has poor fatigue properties. The fibers inside the rope abrade each other, offering little indication of the reduced strength until the rope breaks. Kevlar has a very high melting point, 500 °C or 932 °F. Kevlar-core ropes are sold as escape lines for firefighters - to be used once and discarded. Manufacturers have had some success at solving the self-abrasion problem by combining Kevlar with Spectra.
Seven products were tested for this project. Sterling Vectran, Blue Water Titan, Black Diamond Gemini2, Maxim Spectra A, Mountain Tools Ultratape, Sterling 7mm Nylon accessory cord, and Liberty Mountain 1 inch Nylon Tubular Webbing.
Sterling Vectran has a Vectran core and a Nylon sheath - it is sold in precut lengths labeled "cordelette" in addition to spools. Blue Water Titan has a braided Spectra/Nylon core and a Nylon sheath. Black Diamond Gemini2 has a Technora core and a polyester sheath. It is identical to the product sold as "Tech Cord" by Maxim (New England Rope). The original Black Diamond Gemini was a different product, similar to Maxim's Spectra-A, and has not been sold in several years. Spectra-A has a braided Spectra/Kevlar core and a polyester sheath. It has largely been replaced by Tech Cord, but is still sold, usually at cheaper prices. Ultratape is a Spectra/Nylon webbing, constructed to minimize the amount of Spectra on the outside surface. This helps protect the Spectra from UV damage, and lets knots hold better, since more Nylon is in contact.
Cordelette Anchor Webolette Anchor
(Karl Lew photo)
Test MethodsSlow Pull Tests:
Slow pull tests were done on the 11,000-lb SATEC Apex 11 EMF universal test machine at Black Diamond. These were done on unknotted material over 4" diameter drums, on figure-eight knots, on loops tied with double fisherman's, triple fisherman's and water knots, and on a cordelette loaded on a single arm. Pull rates and fixtures were consistent with CEN standards. The material was not temperature and humidity conditioned, but all tests were done at 29% humidity +/-4% and at 71°F +/-6°. Five samples were tested in each material for each configuration and the results were averaged. One sample of various friction knots was also tested in each material on Black Diamond 10.5mm Cirrus dry-coated rope.
Keep in mind that an average breaking strength (the arithmetic mean) is not a good quantity to use to determine whether a component is strong enough. A minimum breaking strength - three standard deviations below the mean - is much more appropriate. However, five samples are not sufficient to determine a meaningful statistical minimum, so average strength is presented here.
Flex Cycle Test:
Slow Pull Tests
Friction knots were tested to determine whether there were any obvious reasons why any of these materials could not be used for ascending or as a self-rescue rope-grab. For all the cord materials, any of several friction knots work fine and the choice would be based simply on ease of tying and loosening in use. For the two webbing materials, it is tougher to get sufficient holding power. A climber can easily generate forces of 500 lb when ascending. If a hitch will not reliably hold that load, slipping will happen. For the webbing, adding wraps is the only way to get the holding power. The most convenient hitch to do this with is the Kleimheist. The fact that the Spectra/Nylon Ultratape can be used at all for friction knots also contradicts the conventional wisdom.
For the cordelette strength, both the strength of the weak arm (knotted or single-strand sewn) and the strength of the stronger arms are plotted. For most of the materials there is no difference. The material breaks at the pin or in the overhand cordelette knot. For the webolette, the weak arm is a single strand, so the double-strand leg is considerably stronger. Since these are used as anchors, the UIAA spec for maximum dynamic-rope impact force is shown for comparison. This represents a typical worst-case force on the rope. However, if the belay is run through the anchor, force on the anchor is multiplied. A level 170% of the UIAA spec - an assumed maximum - is also shown for comparison. It is apparent from this chart that at least some of these cordelettes would be expected to fail a UIAA drop test.
Flex Cycle Test
How strong should your anchor be? One arm of a Vectran cordelette, for example, fails at only 2600 lb (11.5 kN). This is little stronger than a good carabiner in the open-gate mode - and the material gets weaker with use. Is this strong enough? One can easily argue that the drop test we performed is unduly harsh. First, it uses a completely static belay. A sticht-plate or tube belay-device can reduce the peak impact forces significantly. Second, the lead rope is run through the central anchor point. This practice increases the load at the anchor. Third, the test loaded only one arm of a cordelette. While the cordelette anchor does not equalize when the belayer shifts position, there is typically enough stretch in each arm that all three will be loaded to varying degrees in a major impact. Fourth, Chris Harmston, Black Diamond's Quality Assurance Manager, has reviewed field failures of climbing gear for eight years. He has never seen a stopper rated at over 10 kN fail, and has seen only a few carabiners fail in closedgate mode. He believes that forces exceeding 10kN rarely happen in climbing falls.
All that said, we do not think it is unreasonable to expect one arm of the anchor to hold at least one UIAA fall on a soft rope when both the rope and the cordelette material are new! The decrease in strength with use is a worry for any of the Technora, Kevlar or Vectran materials. The Gemini and the Spectra-A are also extremely stiff and difficult to tie and untie. An 18 -foot piece makes a bulky object hanging from the harness. They make excellent chock cord (where a stiff cord is desirable), but would make a poor cordelette. Among the high-strength cords, Titan seems to be the most suitable material for cordelettes. The Ultratape is even better, and the webolette is an elegant solution to multi-point anchors, although we'd prefer to see slightly higher strength on the single-strand arms. Last, Nylon cord and webbing may be the best of all. Although heavier, they are cheap, strong, universally available, and seem to have a virtually unlimited flex life.
Black Diamond provided test material and made their test lab available for this project. Sterling Rope, Blue Water Rope, New England Rope, and Mountain Tools all provided material for testing. Karl Lew asked these same questions a year ago and provided samples for earlier testing in the BD test lab on this subject. Bill May, Lewis Dahm, John Snyder, Jim Gallo, Scott Whitehead, Dave Hibl and others from the Rocky Mountain Rescue Group in Boulder worked hard to get their new drop tower up and running in time for it to be used for this project. Sensotec provided the load cell used to measure forces at the drop-tower.
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