/ 8.5mm rope breaking load?

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FatRob 09 Aug 2019

Just tested some 8,5 mm dynamic half ropes to destruction on the tensile tester (because I can).

Double overhand knots both ends (uses less rope).

Breaking load was 8,0kN extension was measured at 230mm for a sample of around 250mm eye to eye.

Strain rate was 50mm / min.

Given that the double overhand de-rates the rope strength to 45-55% of the original strength then this would possibly suggest that the rope has a static strength of 14,4 - 17,8 kN.

A 10,5mm semi static EN1891 rope is required to make 22,0kN static.

Given a reduction in cross sectional area of 1,5 times, and similar core/sheath ratios I'd expect a reduction in strength proportional to the loss of cross sectional area.    22,0/1,5=14,7 kN.

I was surprised that the rope failed at 8kN I was expecting higher even with the evil overhand knots.  I was impressed by the elasticity, and yes I know they are designed to stretch, Max Impact Force is quoted at 5,5kN.   

My question is:  Does anyone have any similar data from other ropes they can share?

Post edited at 18:32
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In reply to FatRob:

On a vaguely related note... I used a retired mammut serenity 8.7 to tow a people carrier behind a land rover. Did the job surprisingly well. I wonder what peak force was through the rope... 3kn at a guess?

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FatRob 12 Aug 2019
In reply to A Longleat Boulderer:

Yep probably very little peak force when towing to be fair.

I'm primarily interested in how we can measure deterioration of rope over time.  I've read a lot of stuff about this on these and other forums.

Does anyone know where 8,5 dynamic rope is sold off the reel by the meter?

Cheers

R

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jkarran 12 Aug 2019
In reply to FatRob:

When you say double overhand both ends, did you form loops for 10mm pins to go through? Did it break at a knot?

Only rope I ever broke (10.5 Mammut) stopped a car (~60kJ into ~15m of rope), twice before it let go over an edge, it was also quite tired having spent a couple of years lying in a field after a full life on the crag. As anecdotes go I don't suppose it'll add a very useful data point to your collection but I've always found it reassuring

jk

Post edited at 13:12
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iainJ 12 Aug 2019
In reply to FatRob:

Pretty sure Dick's Climbing sell it off the reel.

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FatRob 12 Aug 2019
In reply to jkarran:

Double overhand knots both ends to form loops, yes, I used 8mm pins though i appreciate that the standards say 12mm, and yes it broke at the knot all 4 samples.  Ropes will, in my experience, always break at the knot unless damaged or cut, e.g. by an edge, and I've broken many in my time, mainly semi static. 

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FatRob 12 Aug 2019
In reply to iainJ:

Cheers, I'll check them out.

Yes they have a 10.2, but I'm interested in 8,5mm half ropes.

Cheers.

Post edited at 21:23
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pass and peak 15 Aug 2019
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Toerag 15 Aug 2019
In reply to FatRob:

>  Given that the double overhand de-rates the rope strength to 45-55% of the original strength

Are you sure? A single overhand is widely quoted in angling literature as reducing the BS of fishing line by ~50%. It does this by its lack of internal friction allowing the knot to pull ridiculously tight into a tiny bend radius. A double overhand has loads more friction and wider bend radius.

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FatRob 18 Aug 2019
In reply to Toerag:

Yes, read the HSE report done by Lyon.

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FatRob 18 Aug 2019
In reply to Toerag:

Yes, read the HSE report done by Lyon.

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FatRob 18 Aug 2019
In reply to pass and peak:

Cheers,

have ordered some, will be very interesting to pull it.

Post edited at 21:39
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krikoman 19 Aug 2019
In reply to FatRob:

> , will be very interesting to pull it.

phanarr phanarr

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FatRob 22:18 Fri

Update:

Got the beal verdon 9mm this week.

Tested same method as above:

160mm test sample with double OH knots both ends:

4 samples tested:

Average BS 10,300N, extension was ~350mm from 10N to break.

Conclusions:

This here dynamic rope is seriously stretchy.

20+ year old nylon fibres retain at least 90% of the strength of new nylon fibres based on a simple stress relation - Break load / Cross sectional Area.  This relation also holds based on BS / mass per metre. 

Differences in the rope types tested and core to sheath percentages 42/49% may skew these results.  

The data is interesting, but ropes are cheap enough not to risk climbing on ropes that are suspect.

Cheers,

Rob

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