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brenta
Apr 29, 2009, 4:21 AM
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Concerning p. 158, the confusion arises from the different test procedures for UIAA/CE certification of slings and accessory cord. With slings, the force already refers to a (sewn) sling. With accessory cord, the force is measured on one strand. Hence, when you tie a loop of accessory cord, you get more strength. It's not a matter of high-tensile strength fibers versus nylon.
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theguy
Apr 29, 2009, 12:25 PM
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Is this relevant to the results cited?
While the study does say that for the Mtn. Tools webolette "the weak arm is a single strand with a sewn eye", Technora, Kevlar and Vectran are tested in cordelette.
The single-strand strength test on p.4 shows a strength of about 3k lbs for 7mm nylon cord, and about 3.5 k lbs for Blue Water Titan (Spectra) cordelette.
The loop knot test (i.e. no sewing) on p. 5 shows a strength of about 5k lbs (almost double single-strand) for 7mm nylon cord, and almost 4k lbs (<25% increase) for Blue Water Titan (Spectra) cordellette.
It's not clear from this test whether the limitation in the tech cord is due to the knot or the cord, but the result is the same...
(This post was edited by theguy on Apr 29, 2009, 1:20 PM)
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brenta
Apr 29, 2009, 3:29 PM
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As I read that inset on p. 158, first it discusses tests done at Sterling, then goes on to explain the effect of looping, and finally turns to Moyer's paper. Note the use of past tense in reporting specific results versus present tense in discussing the general effects of looping.
Let's focus on the looping. The usual sewn Spectra slings are rated 22 kN. The doubling of the strength is already included. Seven mm accessory cord should not break below 9.8 kN. You tie that in a loop and you get about the same strength as the sewn sling.
Bottom line: sewn Spectra/Dyneema slings are not stronger than tied 7 mm nylon cord. Rather, they are lighter. This is what I take that comment to mean: To address a common misunderstanding. Perhaps John Long had in mind the effect of knots, which is more pronounced in high tensile strength materials. Maybe he'll clarify himself.
As for the results of Moyer et al. for the cordelettes, I don't understand how they were obtained; hence, I have trouble drawing conclusions from them. Specifically, they talk about the cordelettes breaking at the pin of the test fixture. That seems to suggest that the high-tech materials have problems with sharp turns. I wish the authors provided more details.
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theguy
Apr 29, 2009, 4:16 PM
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I really don't understand why you're bringing sewn slings into this.
To re-iterate, the book says "a looped leg of high-tensile cord still fails at 5,000 pounds, not 10,000 as you would expect".
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brenta
Apr 30, 2009, 4:01 PM
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You are right. On second reading, there are not one, but two clear indications that Long is not thinking about sewn slings. (The other is at the end of page 157.) I stand corrected.
Back to the original question. First p. 190. The reference there is to the difference in load between the two legs of a two-point anchor. The text says that the quad equalizes just about as well as the equalette. It does not say that it is about as strong.
Concerning p. 172, the load will be split among four strands when clipping 3 and 4. Each strand is worth about 10 kN. Each contributes because the system is self-equalizing.
I see no obvious contradiction.
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