Forums: Climbing Information: The Lab: Re: [kennoyce] Rope Elongation: Edit Log




USnavy


Nov 5, 2013, 5:18 PM

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Registered: Nov 5, 2007
Posts: 2661

Re: [kennoyce] Rope Elongation
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kennoyce wrote:
amarius wrote:
If the two ropes are identical, half. Simplifying this quite a bit - assume both ropes get the same tension, hence each one is experiencing half the weight, hence half elongation.

This is only correct for a static mass hanging from the ropes (static elongation), for dynamic elongation (ie elongation in a fall) this doesn't hold true.
I am not sure it is true period. Ropes do not have a linear modulus of elasticity, and adding another rope changes the modulus of the system all together. Consider the specs of the Beal Joker, which is UIAA certified as a single, twin and half:

Number of falls factor 1.77 - Béal Guarantee 5 (with 80 kg) 20(with 55 kg/1 strand) >25 (with 80 kg/2 strands)

Sheath slippage - 0 mm

Extension during the first fall - 37 % 32 % 29 %

Static elongation 80 kg - 8% 8%/1 strand 7%/2 strands

So adding the second strand only reduces the static elongation from 8% to 7%, and the dynamic elongation from 37% to 29%. The impact force rating for the rope is 8.5kN (single) and 9.2kN (twin).

The following is a graph of some testing I did on webbing awhile back. I tested the elongation of 1" webbing, 3/4" webbing, and 1" webbing combined with 3/4" webbing. While webbing is not a dynamic rope, it is still made out of the same material (Nylon 6-6), and it illustrates the general point I am trying to make.




(This post was edited by USnavy on Nov 5, 2013, 5:34 PM)



Edit Log:
Post edited by USnavy () on Nov 5, 2013, 5:26 PM
Post edited by USnavy () on Nov 5, 2013, 5:27 PM
Post edited by USnavy () on Nov 5, 2013, 5:30 PM
Post edited by USnavy () on Nov 5, 2013, 5:34 PM


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