Synthetic rope strength as strands break

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Synthetic rope strength as strands break

Post by RealNick » 06 Dec 2016, 9:43 am

Hi guys,

Doesn't need to be a full blown physics lesson but can anyone give me a rough breakdown on the strength of synthetic rope if a strand breaks?

e.g. If you have a 12 strange 12,000lb plasma rope do you lose 1,000lb of pull capacity per broken strand? Or more because they rely on each other for the total?

If you lose one strand to abrasion but aren't pulling anywhere near 12,000lbs is their any real need to replace it?
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Re: Synthetic rope strength as strands break

Post by Gwion » 07 Dec 2016, 9:25 am

I think we'd need to go all Mythbusters on this one to get a true and definitive answer.

A load rating is not a breaking strain rating. At a guess i'd say that one strand isn't going to make a huge difference but lose a few and your rope will be severely compromised beyond any basic ratio of broken strands: strands in tact. Ie: it wont be a linear equation.

Still, I like the idea of going all Mythbusters on it! Just need a coil of 12000lb dynema rope and a hydraulic jig with a force indicator to test the breaking strain of set lengths with X number of strains cut.......
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Re: Synthetic rope strength as strands break

Post by cooker » 07 Dec 2016, 12:34 pm

Gwion wrote:A load rating is not a breaking strain rating.


Was YouTubing some stuff on this earlier actually, can't remember what the brand was but it was a workshop testing the breaking strain of steel vs synthetic.

The 12,000 lb rated strap broke at over 18,000 lb so plenty of slack. Just one example but yeah.
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Re: Synthetic rope strength as strands break

Post by BBJ » 07 Dec 2016, 1:03 pm

Gwion wrote:Ie: it wont be a linear equation.


As soon as you're using it in a real world situation I'd say this is sure to be true.

If you did a lab test e.g perfect strands, perfect anchor point, dead straight pull, 1 strand would have 1/10th the strength of 10 braided in a rope you'd think.

Real world though when the rope is getting dirty, rubbing at different points, different pull angles, stretching, having other strands to share dynamic changes in load through the road, and 100 other little subtle differences, a 10 strand braid would more than 10 times what a single strand could cope with.
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Re: Synthetic rope strength as strands break

Post by Oldbloke » 07 Dec 2016, 2:13 pm

The twist adds strength so I think it will be more than 10 percent. Perhaps about 15 percent. I would toss it out. If 1 is busted there will be other unseen damage. But how much, that's the question.
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Re: Synthetic rope strength as strands break

Post by Blackened » 07 Dec 2016, 3:10 pm

If a strand has snapped I wouldn't continue using it as is while just trying to keep the load down, that's not going to work for long and you're just asking for it to snap completely when you most need it.

Even if they haven't broken now, the neighbouring strands around where the break occurred have probably been degraded to some extent and will follow soon enough.

Depending on where the break is you could still salvage the rope though.

If the break is close to the hook you could cut off the damaged portion of rope splice a new eye and thimble onto the end. There are a thousand videos of how to do this on YouTube so check their for the full procedure, but in short you'd need to cut the line about a foot after the break, and it will cost you about 2 feet of line to re-splice it. At the end of the day hopefully you've only lost a metre or two from your rope so you still have plenty to work with.

If the break is anywhere near the middle it will be a bin job.
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Re: Synthetic rope strength as strands break

Post by <<Genesis93>> » 07 Dec 2016, 3:41 pm

Luckily its not a physics lesson you're after... considering its mechanics and material properties....:lol:

Abrasion you say, oxidation/wear??
else one strand broken from 'strain' would indicate the rope was near its ductile/strain limit when it stretches before snapping.... if so, then the strands will commence giving way, then one goes first.... = whole rope has failed. = bin it.

Alternatively, one strand gone means you have 11/12ths of the capacity left... rejoint at the missing link to return the other 12th.... :thumbsup:
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Re: Synthetic rope strength as strands break

Post by Gwion » 07 Dec 2016, 5:18 pm

From my understanding, the braid system itself will add strength to the line. Each strand on its own will not be equal to (in the OP's example) 1/12th of the strength of the braided line.

That said, I totally agree that if a stand has broken through shear strain then the breaking strain of the line has been too closely tested and the line will be at risk of imminent complete failure. Time to replace. If, however, the loads have been well with in the load rating and some damage has occurred over, for instance, sharp rocks; then the line will retain more strength than the sum of its remaining parts until the braid system itself is compromised. I would be very surprised if each strand in a braid line with a breaking strain of, say, 12000lb would have an individual breaking strain equal to that weight divided by the number of strands; I believe it would be somewhat less. All theoretical guess work unless you go Mythbusters on it.

Still, a damaged line that I have to trust my life to is one that needs replacing.
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Re: Synthetic rope strength as strands break

Post by Oldbloke » 07 Dec 2016, 9:25 pm

I'm not sure in the case, but in most cases lifting equipment such as cable must be capable of withstanding loads 2 1/2 times its nominal rating before it begins to fail.
So, unless this damage is mechanical, eg caused by being struck, it has been over loaded. There will be additional damage that will not be seen by the untrained eye.

I would dispose of it if it could have been damaged by over loading. If it fails in use someone could be seriously injured.
As mentioned earlier if it's not been overloaded and it's mechanical damage at one end, then remove the fault and splice.
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Re: Synthetic rope strength as strands break

Post by <<Genesis93>> » 08 Dec 2016, 8:48 am

The old 'a chain is as strong as its weakest link' applies.... the braids may or may not impart an extra strength (they don't)...

Take the example of a 100 element rope,
The elements of the rope may be twisted, or braided, or straight
Take any point in the line, slice straight through it (metaphorically)
You will see 100 sections of nylon, or Carbon fibre or stainless steel or other....
Each of the sections will have exactly the same tensile strength no matter what the rope looks like....1+1=2

Even if the braid DID impart some extra strength....then the rope would fail at the jointed end, where there is no 'braided action' to rely on... so the mythical superpower is redundant...

However, how the rope is twisted or braided or not, will give it other properties of flexibility, durability, ease of use etc...

:thumbsup:
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