UKC

In the past we've discussed the potential failure of dyneema slings when larksfooted (heat/sawing action if the sling edge on the flat tape as the larksfoot tightens) and the tests Chris Harmaston (formerly of Black Diamond) did some years ago, larksfooting (girth-hitching) dyneema and nylon slings together.

However, last weekend when climbing in Wales I heard some disturbing news about tests recently carried out, I think at Plas y Brenin.

As I understand it, a dyneema sling was clipped into solid gear, an overhand knot tied in it, and an 80 kg weight applied to the other end. the sling was then set up horizontally from the gear, and the weight released.

The test was repeated a number of times and every time the dyneema melted through at the overhand knot, depositing the weight on the floor.

Now I appreciate that we've discussed not climbing above the belay point on a sling before (I believe someone died last year or the year before, climbing above their first anchor when clipped in on only a sling to set another anchor for the stance). It's probably quite rare for people to do that, but I reckon it'd be quite common for someone anchored with a dyneema sling to be placed with the sling coming at a horizontal angle off the anchor. I've also seen knots tied in slings quite frequently to either shorten them or to create 2 independent sets of loops.

Apparently the dyneema in the above tests failed due to its significantly lower melting point than nylon. Nylon slings don't fail in this manner.

Now admittedly I heard of these tests second hand, but from someone very close to the people who'd done them. I'd be interested to hear from anyone who has any more information on this. When I left Wales on Monday, I left a very experienced climber and former long-standing instructor at PyB so worried about what we'd heard the night before that he was systematically going through his gear removing all dyneema slings and putting them on one side pending more information.

Was he adopting "bird flu-itis" or being sensible?

In reply to Rob Naylor:

Several sites comment on synthetic ropes and cord all highlight teprature and avoidance of shock loads
http://alabamaslingcenter.com/cordage.htm#cordageoverheating
Dynema is the generic name for Gel Spun Polyethylene (GSP.) It is an approximate process name and chemical description.

Spectra by Allied Signal corporation is a trademark name for dynema.

This is raw polyethylene fiber, spun while in gel form, into a crystalline substance is one of the strongest fibers imaginable. It is 10 times stronger than steel. It is indestructible. One would not imagine those polyethylene milk bottles and the six-pack holder rings could be made into ropes strong enough for acrobats, and hang gliding, and towing glider planes. These applications require ropes that do not stretch at all, and Dynema does not stretch like nylon ropes do.
Dyneema is a high performance polyethylene (HPPE) fiber, developed by the Dutch Chemical Group- DSM, and manufactured by DSM High Performance Fibers. It is produced by a unique gel spinning process where molecules are disentangled to enable parallel orientation, giving the fibers it's unique properties. Dyneema's strength and low density makes it one of the world’s strongest commercially available fibers.

The strength and relative high elongation to break gives the fiber the ability to absorb a great amount of energy. The sonic velocity, over 10 km/sec, allows extremely high distribution of energy. Dyneema is used by the US department of defense as absorption of ballistic energy (bullet proof jackets), and by Starboard to increase the lamination strength of our sandwich construction boards.

by far the most detailed answers with links is

http://dyneema.iqnaut.net/

In reply to Rob Naylor:
Not to clear about the test you describe, it sounds like a straight drop test over the length of the sling 80kg load with an overhand knot in the sling.
Could you confirm this please?

In reply to Ian McNeill:

Interesting links, but do not in themselves help answer this particular case.

We've always known that dyneema has a relatively low melting point.

We've also always known that static slings and ropes aren't designed for shock-loading....hence my comment about the guy climbing above his anchor in the OP.

My point here, though, was that the tests I was told about last Sunday replicate a situation that I imagine a lot of climbers wouldn't think twice about putting themselves in.

In fact, on the Saturday I was climbing Christmas Curry at Tremadog and had *exactly* that situation. We were waiting for another party to clear a stance, and I looped a dyneema sling over a spike, put an overhand knot in it and clipped in horizontally, just as it was described the tests were conducted. I wasn't that happy with the spike, so quickly clipped into an additional nut in a crack as a backup, but that was a function of the spike geometry, and not of any possible melt-through if I fell!

So if the reports of the tests were accurate, it's surely something that should be publicised widely, and asap?

In reply to karl walton:

Pretty much what I understand was the case, but with the weighted end of the sling being moved out tight horizontally from the anchor, rather than being dropped from above. Imagine clipping into a nut in a crack then shuffling along the ledge until your sling is tight horizontally between yourself and the anchor.

That's the geometry I'm trying to describe, if I under stood properly the explanation given to me.

In reply to Rob Naylor:
That sounds very scary and a little implausible if you don't mind me being frank. The loads in the sling are very low in that configuration almost laughably so. Also there should be no shock loading at all.

In reply to Rob Naylor:

Yes, worrying - I'd assumed something similar might be the case, given the non-dynamic properties of Dyneema slings, and resultant energy transfer. I wonder how many people are relying on this kind of stuff for cow-tails on Via Ferrate etc, and risking very high factor falls.

In reply to karl walton:

Yes I know...hence my careful choice of the way I put it in the OP.

It's something I've done several times without hesitation, and if my understanding of what was tested and how is correct (and the others with me at the time, including 2 former PyB instructors of long-standing understood it exactly the same way as I did) then yes, it's scary, and I'd like to know asap if the description I heard was correct, or if the person giving it had got something critical wrong.

In reply to Steve Parker:

That's a bit different, I think. I certainly wouldn't (normally) climb above my anchor if I was only clipped in with a sling, and I think most climbers probably know abot high fall-factors on static gear, but this scenario was within the bounds of something I've done myself, and seen many others do, without worrying.

In reply to Rob Naylor:
Fair point mate.
My sceptical nature would lead me to think that the test has been described wrongly. The pendulum action in the test you described builds load in the sling very slowly to quite a low level.
However I am not unaccustomed to being wrong.
I never the less shall make my opinion clear now, and wait for a more clear outcome.
Thanks for your good humour on this.

Why would you want to tie a knot in a perfectly good sling?
It is something I see all the time(I dont do it) I can never understand what this achieves or enables. I ask people why they do ? but no plausable safe reasoning as to why.
I personally would chose a different size sling if I though it would help.

If the problem is tying an over hand to shorten a sling around a spike - reassess you belaying stance.

I wonder if the same applies to the old overhand knot for multiple anchor linking method ?? Have tests been done on this which is more common pratice.

In reply to Steve Parker:
I don't fully understand the situation, but it seems to me that the fall factor calculation does not apply in this situation.
The load in the rope is developed much more slowly and to a lower level than in a straight drop test, as can be seen from the fact that the load does not come to a sudden stop or a stop at all for that matter, as it will continue to swing like a pendulum.

In reply to Ian McNeill:
When using a cows tail it is common to tie Knots in the sling for abseiling etc.

In reply to karl walton:
It this case it might prudent to suggest that one uses slings of an appropriate length for the job rather than improvising, IF there is the possibility of a large fall factor possibly being realised.
Or
carry a good old nylon sling for this purpose...

In reply to Rob Naylor:


Just to be absolutely clear, last weekend - Saturday - was April Fools day. You weren't being wound up, were you?

If not, then it does sound very worrying.

In reply to Ian McNeill:

In this case I did it to "snug" the sling up against a slight depression in the back of the spike, as I wasn't sure that it wouldn't slip off otherwise.

As I said above, I wasn't happy with it anyway, and so immediately clipped into an additional nut as soon as I could arrange one.


I was taught that if you have, say, a doubled long sling and knot the centre, you then get 2 pairs of independent loops. I've you're over a spike or similar, the sling is weighted and the rock cuts through one sling, then the fact that you've knotten it means that the doubled loop doesnt't just slide off the spike under your weight.

admittedly it's not a situation that's likely to occur, but if knotting a doubled overhand gives you that extra (small) margin, then "why not do it?". If we now find out that knotting a sling actually decreases the safty margin, then that's a very good reason *not* to do it.


So would I...if I had one.


I did, on this occasion. Quickly knotted and clipped to the spike, then reassessed and got onto a nut as well.


Dunno...I brought this thread up in the hope that someone might know what tests were done and how they came out, as something heard over a curry in North Wales, while from a source trusted by my eating companions, might have been prone to a misunderstanding somewhere along the line!

In reply to Dominion:

No, Saturday I was climbing Christmas Curry. Sunday I was goggling at "Quarryman" in the pissing rain. Sunday night, 2nd April, was when the conversation occurred.

It did occur to us at the time that the reporter might have been "April fooled" the day before, but he said he'd been told midweek the previous week.

In reply to karl walton:

That'd be my feeling too.


Me too!

No good humour needed...I'm concerned at the report I heard, but also open to the idea that we may have misunderstood, or the original reporter had it wrong from his source.

In reply to Rob Naylor:

could well be a mis understanding of another conversation while chewing the vindalo ? and drinking a cobra ?

In reply to Ian McNeill:
I think you missed my point putting in the knots allows the sling to be used at several different lengths for several different purposes and is standard practice. The failure mode in question here is as yet unconfirmed.
If it comes to high fall factors onto slings then knots are not going to make a huge difference as an un-knotted sling may well fail anyway, hence specialist via ferrata kit.

In reply to Rob Naylor:
Aye lets hope it's wrong.

In reply to karl walton:
'Standard pratice' I would be careful to us this phrase here because I'm sure that it is not standard pratice across the board... a weight drop machine does put excessive loads on to gear over the years I played with them, to test all sorts of things.

I cant agree more on the specalist gear for via ferrata - why put yourself and friends in potential danger for the sake of a few quid on the proper gear.

In reply to Ian McNeill:
Sounds to me like a difference of view point.
Knotting anything makes it weaker. Knot or no knot a climber should not be putting them selves in a position to take any fall onto a sling.
When you say proper gear, what in the context of a multi use cows tail would you suggest.

In reply to karl walton:
Multi use cows tails ?

I used to use dynamic 9mm or 10.5mm climbing ropes for my cows tails caving and in Rope access.
And where I require the same today - i.e. going out with the intention to use cows tails I would do the same, use lengths of dynamic rope.

In reply to Ian McNeill:
I can respect that, and your obvious high level of experience.

I would appreciate your thought on this.
Assuming each is used within it's design limitations, in practice why is knotting a sling any different to knotting a rope (overhand in sling)?

In reply to Rob Naylor:
Sorry for the hijack

In reply to karl walton:
main factors are..


Then profile of each also differentiates strength and performace.

Beyonmd these two basic differences the type and presentation of knots varies some idea of how can be seen on this page link.
http://climb8.com/EDK-Table.htm

where different knots and diameter ropes have been used in testing.
hope this helps..

In reply to Rob Naylor: I am sorry but this has to be bullshit, or I must be very lucky. I've taken several hard falls on tied off dynema slings. For sure, there's no way to untie the knot afterwards at least when I took repeated falls on a tied off dynema sling, but if you've dressed the knot it wont fail.

In reply to Ian McNeill:
Thanks for that.
One more question if I may.
'Excluding the knotting issue' how plausible does this knot failure in this pendulum type test seem to you?

In reply to karl walton:
with out speaking directly to the people who carried out the test and replicating the test Very difficult to pass judgement.

With any test reliability and validity should play their roles, and with out conclusive evidence that knotting dynema weakens it I Iwold take care about disposing of my slings in a hurry !

Oh also - the dynema slings I own and sell are not just made of dynema there are strands of nylon included espically at the edge of the sling, where nicks are more likely to be experienced.

But as I have said beforehand I would rather not knot a sling to shorten one I would choose a shorter one any time

In reply to Ian McNeill:
Cheers.

In reply to Rob Naylor:

So we have a pendulum let go from horizontal. The loss of potential energy is mgl
for mass m and length of sling l. Equate this to kinetic energy = 1/2 mv^2 at
the bottom of the arc. This gives v^2 = 2gl. Equate this to the centrifugal force
for circular motion F = mv^2/l.

The upshot is that the maximum force on the sling is F = 2mg. In other words,
the maximum force is only twice the _static_ force of mg that the weight would
produce, or (for an 80 kg mass) about 1/14th of the static breaking strain.

I find it very implausible that a knot would melt under a load so far below the
static breaking strain, and very implausible that such a severe limitation would
not have been discovered long ago.

In reply to karl walton:

Everything's a bit vague on this thread, seemingly, so I don't quite get what situation you're referring to. I was on about those Via Ferrate which have a hawser to clip into, interrupted at intervals by anchors. If you were clipped in via a short cows tail and fell vertically, say, 3 metres down to the last anchor, surely that would create a high fall factor, probably enough to break a static sling?

In reply to Steve Parker:


Yes. And that is a well-known danger of Via Ferrata.

The situation in this thread is different. Anchor one end of sling. Attach weight
to other end. Hold weight at same level as the anchor, but at sling-length horizontally
from the anchor so that the sling is tight. Let go, and the weight pendulums.

In reply to Coel Hellier:

Yes, I was making a somewhat divergent point earlier. I find it truly astonishing that the pendulum you and the OP have described would generate sufficient energy to melt through the sling, so I think I favour Dominion's April Fool's theory, even if it was the wrong day! In fact, I'm pretty sure I've actually taken similar swings on to Dyneema slings, which I'm still using, and which show no signs of melting. It sounds like a very easy expt to set up anyway.

In reply to Rob Naylor:

However, last weekend when climbing in Wales I heard some disturbing news about tests recently carried out, I think at Plas y Brenin



Not sure how much relevance this has but I was at PYB on Thurs/Fri 6th/7th doing the SPA training

If the origin of this came from there I feel certain that the instructor would have mentioned it. Due to the fact that they were so thorough about everything else they impated to us.

W

In reply to Rob Naylor: I have mailed PYB to see if they can confirm this test. Will further post their reply.
Nige

In reply to Rob Naylor: It's possible that this stems from some demonstrations that I was involved with last year. Attending were various people, including some of the instructional staff from Glenmore Lodge and PyB.

One of the demonstrations looked at a fall factor 1 fall onto a knotted dyneema sling. The sling was 120cm long, and the knot was an overhand at the mid point. We used a steel test mass of 80kg, and basically we raised the mass till it was level with the anchor, and then let it go. The sling failed at the knot.

I thought that this test was worth doing, because it was exactly this set-up that I was using for abseiling off long rock routes in the Alps that summer, with the belay plate clipped at the mid point, a prusik into the abseil loop of my harness, and your krab into the anchor at the end. This is fairly standard practice. The aim of the test was to show the limitations of equipment - it doesn't mean that dyneema slings are dangerous, but that we need to be aware of what we can and can't do with them. If we use this method, then we need to be careful moving around on stances and not get too much slack in the sling.

It's worth pointing out that we didn't have time to replicate the same test with a nylon sling, it may not have broken but the impact force would likely have been high enough to cause serious injury. On the bright side, a fall in the real world would probably involve more of a swing and some sliding against the rock, and of course an 80kg person absorbs lots more fall energy than an 80kg steel mass, but its always worth looking at worst case scenarios. I hope this explains things a little.

In reply to danm:

Ah! This sounds sufficiently like what I was told that it may indeed be the origin of the story. So the mass was "level with the anchor" rather than the sling being at full horizontal stretch? That would be a somewhat different kettle of fish to the implied penduluming of how it was originally explained to me.

And yes, as with all kit, it's knowing the limitations that's important. I was just concerned that in this case, as told to me, it seemed to be that the limitations seemed to fall within my own assessed level of acceptability.

In reply to Coel Hellier:

I agree that it sounds implausible...hence my original post. Looks as if, from the post above, that the origin of the story might have been found at last (good old Rocktalk!) and that the situation described to me was a little different to the actual test done. Chinese whispers through the instructor community?

Though it wouldn't surprise me that a limitation in kit took a long time to "discover": how long did it take for someone to demonstrate the propensity of screwgate krab gates to punch through the screwed up sleeve in certain loading configurations? It may well have been that the designers knew of this limitation, but it didn't become widely known until , IIRC, an abseil accident followed by some formal tests.

In reply to danm:

Thinking about it again, although the test you did was not set up as was described to me last week, the set up you *did* test is something I've seen people do several times. Not just abseiling, but clipped in with a very slack knotted sling that could potentially give a factor 1 fall if they slipped.

So would it not be a good idea to publicise the test you did a bit more widely? Not in a "panicky" was but in the measured and sensible way you described it above? So that people can factor it into their personal assessments of how they use kit.

In reply to Nige G: This story appears to originate from a workshop session held during an MLTE conference at Plas y Brenin on March 25th. Pete Robertson from Lyon Equipment was reporting on a test carried out last year which featured a load dropped onto a knotted dyneema sling. A test wasn't carried out at PyB. I have spoken to Pete and he will add to this thread to clear up the confusion - better if it comes from the horses mouth.

In reply to martin doyle:

Martin, thanks very much for contributing to the thread. It will be good to get the true details of the actual test and result directly from the people involved.

Looking forward to seeing Pete's clarification.

In reply to everyone:

I was at this MLTE workshop, where video footage of a dyneema sling was shown.

The experment was set up with an 8 foot sling that had an overhand knot put into it half way. This sling was then connected to a belay point and an 80kg mass connected above the overhand knot. The mass was then tied off level with the belay point, so the sling was slack.

When the mass was released, the sling failed straight away, due to the heat that was produced when the knot tightened up!

In reply to Paul at work: I have the video footage of the original drop test done at Tebay saved as a quicktime file if anyone wants it I can email it to them.

In reply to nealh: I have emailed you.

In reply to Rob Naylor:
Theoretically a load dropped onto a line with NO stretch whatever would generate an infinite shock load. Any real system has "give" and stretch to absorb the energy. I shaln't be losing any sleep!

In reply to jutting: The give you talk of will depend on the properties of the components in the system. The point of the thread is dyneema slings have very little give and if knotted will fail well below the rating given when shock loaded. The test mentioned simulates what could happen if you slip off a stance clipped into a slack sling. The fall itself looks fairly insignificant however the force generated at the point of impact is actually very high (enough to cause serious injury even if the sling hadn't failed).

In reply to nealh:

Exactly!

The situation's not quite as per my original post (ie penduluming off a tight knotted sling) but it *is* a situation (slack knotted sling with potential for a factor 1 fall) which many climbers would probably not be unhappy about setting up, and therefore, I think, *does* merit getting the straight info about.

I think in general people don't think enough about the stresses gear comes under in various configurations, and the limitations of its use. When I "think out loud" about situations I'm often told "I never worry, this stuff's designed to be able to hold a truck...you don't have to think about it failing".

In reply to nealh:

I'd certainly like a copy, if it's freely available.

In reply to Paul at work:

Thanks for that. this ties in exactly with the original report I had last week, over a curry in Wales, except the impression we were given was that the sling was tight horizontally, more of a pendulum than a factor 1 fall.

But still, I know of people who would happily take a factor 1 fall onto a knotted sling ("the sling's rated to 22KN, the knot will only weaken it 30% at most, and it's only a factor 1 fall not a factor 2").

Worth discussing, IMO, if it makes more people aware.

In reply to Rob Naylor:


Me too, thanks!

In reply to Castleman:

Sobering viewing. There's not even a hint of a check to the 'fall', the sling just fails completely... Guess who won't be knotting his slings in the future!

In reply to Rob Naylor:



Not if they wanted kids afterwards !

Its a sobering thought, though, non the less.

In reply to Rob Naylor:

Another interesting thing is that if you buy dyneema slings in the USA, it states in the intructions not to dry your slings near any heat, as that heat will damage the material!!!!

How many times have you seen people drying their slings on or near the radiators in huts/bunkhouses etc?

It seems that its almost time to go back those heavy nylon slings, in fact i already have on my working / client racks!!!!

In reply to GrahamD:
Id never want a belay with a loose sling in it, but I suppose its fair to say I've used knotted slings in belays statically loads of times - in fact at the top of most grit routes I shorten a big sling to the right length round a boulder by putting an overhand in it - never been a problem yet though. Whats perhaps more worrying is that I always treat a good sling placement on a route, e.g. a solid chockstone or other thread, as absolutely bomber - perhaps not so. But then I don't knot those and theyre rated to 22kN without a knot so I guess should be fine...

In reply to Rob Naylor:


I think that the main point is that most people don't even think about it, as they are not going to slip in their minds! Or they don't realise how much force they will create in a slip of 30cm (or a foot for you imperial types ;-P)

In reply to Clauso:

Why not, i will still do it, as long as you remember to have a static load on the sling/ knot system, it will be fine.

Remember this video clip shows a dynamic situation.

In reply to Paul at work:

And good luck to you. Each to their own and all that. I, however, won't be doing so simply because you are!

In reply to Clauso:

So how are you going to shorten your slings then in future?

In reply to Rob Naylor:
Keeping this in context.

I have always been aware, and believed it to be common knowledge, never put yourself in a position to fall any distance onto just a sling, being static they can fail, with or without a knot.

If you weaken a component by knotting it and then use it in a way that it was not designed to be used in the first place, you should hardly be surprised to see it fail.

Outside this specific failure mode a knotted sling is 'very' unlikely to be subjected to loads that will make it fail, belays etc.

In reply to Paul at work:

I'm going to do my upmost not to have to do so in the first place i.e. use a shorter sling or an alternative to a sling.

In reply to karl walton:

If you look at my OP, you'll see that I mention this...ie, the case of the person who died climbing above his anchor tied in only to a sling. Yes, I think this is probably common knowledge, but the operative comment here is "any distance".

Lots of people would not think of a slack sling onto an anchor horizontal to their harness as representing a fall of "any distance".


Agreed...but we come back again to the very casual understanding many people have of what the design criteria of kit comprises. And very few people think twice about knotting slings...probably ost wouldn't consider whether putting a knot in one would affect the usability beyond the standard "well, an overhand weakens it by 30%", as I said above.


Agreed, but I certainly think it's worth knowing about this specific failure mode.

In reply to Rob Naylor:

Absolutely and thanks for bringing it to our attention... Do you know whether the same would/could be true of nylon slings?

In reply to Clauso:

No it would be the same as nylon isn't affected by heat as much as Dyneema is.

In reply to Paul at work:
see one of my earlier posting the rope and cord site link has lots of useful information

somewhere near the top ...

In reply to Rob Naylor:


Given that this is 50% of the maximum fall for a given sling it seems big enough to me.

When I said "any" I did mean absolutely any not "substantial" or anything else.

Also as has been mentioned before in this thread, it would be very much unlikely to see this failure mode in a real situation, with a real 'bendy' (shock absorbing climber) and a rather unlikely totally clean free FF1 drop.

In reply to Rob Naylor:
Most of the clarification has already come from NHeans and Danm. The results are have not been made freely available as the test's are still on going and the intention was not to alarm people when it was not unnecessary. As Dan has already said the intention was to educate people who already have a wide knowledge and get them to ask questions rather than just using things without question.
When the test's are complete I am sure the information will be available and if any alarming results were obtained during the test's that relating to the normal use of equipment these would be available imediatly.

In reply to Pete Potter:
well i do hope so with so many of these type of sling out there being used on a daily basis.... let hope that it is quick...

In reply to karl walton:

I think people are going to get confused with this statement. Quickdraws are slings, friends and hexes are on slings...

In reply to Dux:

Yep they are they are like all gear in the safety chain, If this Dynema sling issue is a big one It will cost a lot of money in product recalls and clear outs at centres - So I guess thay (the manafacturs) have to get it righ first time no chance for errors.
I'm sure thay will comment widely as possible either way .. when they know the full story ...

In reply to Rob Naylor:

Seems to me the test is being done to prove what some climbers and most in it as professionals, know already. Don't fall directly onto a static system be it Dyneema, nylon or linked up krabs unless there is a dynamic element within said system i.e. climbing rope. If you do, you'll come close to busting the kit or yourself. It's nothing new and shouldn't cause any alarm. The test is simulating a common technique which has a flaw; the kit isn't flawed, it's being ill used. Whilst using lanyards or cowstails to protect from slips where any fall will be encountered, use dynamic rope. The test has nothing to do with the setting up of belay anchors using knots to centralise the tie in point.

In reply to Ian McNeill:

Already posted dynamic ropes where you expect to fall.
never slings or static rope

In reply to Dux:
Yes possibly, I tried to be specific by saying "onto just a sling".
I take your point though.

In reply to brt:

Well said.

In reply to Dux:

It's only confusing if you fail to read it properly: Karl quite clearly warned against falling any distance on to "just a sling". All the slings you mention, you clip a nice energy-absorbing dynamic rope in to if you are going to take a fall on them (ie lead climbing).

One question about the test which was performed (the results of which, now that the test procedure has been clarified, don't sound all that surprising or shocking): why would anyone tie a knot in a sling, and then leave it slack? One of main reasons for tying a knot in a sling - certainly one on a belay anchor - is to get slack out of the setup. Slack-free knotted slings in the connections to my belay anchors are surely prefereable to un-knotted but slack ones.

The point that people should be taking away from this discussion is not that dyneema slings are dangerous, or even that they may be more dangerous than nylon, but that it is easier than some of us may perhaps have realised to deploy them in a way which makes them, effectively, useless. If that makes some folks think a bit more carefully about their belay setups then that's probably a good thing. If it leads to some smartarse at the crag telling me that my knotted dyneema sling is "dangerous" because he's misunderstood something he's read on UKC then that's a bit sad.

In reply to Rob Naylor and others: Very interesting thread, especially as I use a knotted 4' dyneema sling when conducting multi-pitch abseils!

1) Does a un-knotted dyneema sling fail under a Fall Factor 1 or any other FF?

2) Does a knotted/un-knotted nylon sling fail at FF1 or any other FF?

3) What is the peak force required to break dyneema/nylon (knotted/unknotted) slings and what test FF does that result from?

4) The human body in a harness does not behave like a 80kg steel test mass. Can a 'body' still provide a sufficient shock load to break the sling and if so at what fall factors?

5) Would the person survive the fall without breaking their back even if the sling didn't fail?

Until we know the answer to all these questions we are still in the sitaution of "if you are clipped in with a static sling don't let any slack develope". We all already try to do this albeit with varying degrees of success in the real world.

If it happens that a knotted dyneema sling needs a force of 15kN+ to break it then I'm not really worried as that's a 19G shock load and I'd have a broken back anyway - replacing it with a nylon sling or and un-knotted sling would not make it any safer. However, if it happens that you would survice much better with a nylon sling that would be worth knowing. Additionally if it is highly likely that you could severely injure yourself even if the sling holds then that might make me consider using a dynamic rope cow's tail.

Dropping steel weights is a good way make a point but it is not a good representation of a falling climber.

In reply to brt:

But that *is* a point. The number of people I know who say things like "why faff with the rope like that? I just larksfoot a sling onto my harness and clip into that" and then happily stand clipped into a slack cowstail...

In reply to Martin W:

Exactly. And if it makes people think about that, I'm not unhappy that I brought it up.

In reply to Pete Potter:

I'm grateful that the clarifications were posted. I made the initial post precisely because I felt that there may have been something a bit wrong with my initial understanding or the other person's description of the test mode.

This turned out to be the case, as the test was for a fall factor 1 shockload, not a more gentle penduluming fall. The test as clarified isn't so surprising to me...it seems plausible that a short fall onto a knotted static sling with no dynamic gear in the system might result in injury or failure.

However, almost every time I'm out climbing I see people ambling about on ledges, with loose cowstails connecting them to an anchor ('cos I'm a crap climber, and tend to be on climbs with ledges big enough both for ambling on and for multiple occupancy ).


Great. I'll look forward to seeing the results. I still think some people are quite casual abiut this sort of thing, though. They don't look beyond the brute strength of the gear and possibly the most basic sort of weakening such as considering the turn radius of a knot only, without linking in things like material melting point, or abrasiveness.

A few years back, I pointed out that the published "weakening percentage" of a clove hitch was when it was being used as it was designed...ie, as a hitch around a pole, bar or rope several times the diameter of the rope used for the knot. I thought it was quite reasonable to ask whether knotting a clove hitch around a bar (krab) that was actually *thinner* than the rope used for the knot might have a bigger effect on the weakening. I was ridiculed mercilessly for even asking the question, though no-one could give me an answer.

But it's my life, and I want to know enough about the mechanics of my kit to make an informed judgement on how to use it.

In reply to Rob Naylor:


I'm not sure where you were ridiculed for your above suggestion, you were done a great injustice.
To my knowledge it is a generally understood principle that the tighter the turn in a knot/hitch the weaker knot/hitch is. Hence some knots being stronger than others.
The tighter the turn the fewer strands of rope on the outside of the turn actually bear the initial load in any fall, hence they can break more easily.

In reply to karl walton:

By some people at my club.

In reply to Si dH:

The main thing there isn't that you're using them unknotted, but that you're using them as runners, and you have all the dynamic rope out from your second in the system, so yes, they should be fine.

In reply to karl walton:

>The tighter the turn the fewer strands of rope on the outside of the turn actually bear the initial load in any fall, hence they can break more easily.

I had that explained many years ago, tying a fig 8 knot should be done with the loaded side of the rope on the outside of the knot to reduce the risk of it failing.

Reading of the failure of the dyneema sling, there is still the possibility of one failing at much lower loads than expected, IF the knot is not tied tightly and it can slip a bit. Maybe something to be checked in the tests being made.
Marks use of a knotted sling for abseiling could put him in danger IF the rope slipped over an edge dropping him a few inches suddenly, something that happens on certain abseils that are not straight down. Maybe testing of this scenario should be done as well.

In reply to sutty:



I feel the general consensus was that you would have to try very to cause this mode of failure with other dynamic elements in the system, people, dynamic rope, etc..

There does seem to be some uncertainty as to whether the low melting point of dyneema makes it more likely to fail in a slack knot situation than say Nylon.

Am I right in assuming we have two distinct failure modes here one convention tight turn failure at the knot and other when the knot is slack in the Dyneema and energy released in friction as the not tightens in the fall melts the sling.
Or are they two expressions of the same failure in a way I do understand?

In reply to karl walton:

>Am I right in assuming we have two distinct failure modes here one convention tight turn failure at the knot and other when the knot is slack in the Dyneema and energy released in friction as the not tightens in the fall melts the sling.

That is what I am thinking about. We know all ropes and slings are more likely to fail at knots where they are bent at tight angles, it is knowing if these slings snapped or melted, the film may show the details.

In reply to sutty:
An intersting puzzle Sherlock.

In reply to martin doyle: thanks for getting back to me Martin
look forward to Pete's clarification
Nige

In reply to Castleman: Me too if its not too much trouble.

In reply to Rob Naylor: Here's a little bit of physics...

Assumming a 8 ft sling, energy is fall is 1.2 mg = 800 J (taking g as 10). Max force on sling falling into perfect arc is mg + centripetal force (mv^2/r). v squared = 2gr (from conservation of energy) therefore max force = 3mg = 3 x 80 x 10 = 2.4 kN. This is just over a tenth of the breaking strain of your average dyneema sling.

But, 800 J is a fair bit of energy. Specific heat capacity of water is 4.2 J/gram degreeC - therefore the energy of the fall could raise the temperature of 2 grams of water by approx 95 degrees.

Now a skinny dyneema sling weighs about 0.3 g/cm length. The overhand knot will only occupy ~ 6 cm of sling length at most - ie less than 2 g of sling. The heat capacity of dyneema is probably less than water (water has a high s.h.c.) - so there's clearly enough energy is the fall for the friction in the knot tightening to get part of the sling to the dyneema melting point (even if only half the energy is lost into the knot tightening.

An 8mm sling has approx half as much material in the knot as a 12mm sling - so failure is more likely with the skinny sling.

In other words, failure by said means is unsurprising - assuming there's enough movement possible in the knot. So if you knot a sling, knot it tightly!

Kevin

For all that this is interesting and potentially a little worrying. Surely there is not enough emphasis being put on the 'real world' situation here.

In the test as described the 8 ft knotted sling was the most dynamic element. The test rig, steel weight (and probably) steel krabs will deform very little. In the real world the bulk of the energy in the fall would be absorbed by the climber's body.

As to how much peak force would be generated in that type of fall (and energy released to melt the sling for that matter) that is almost entirely down to the distance fallen. A real world 'factor 1' onto a 30cm sling is NOT the same as a 'factor 1' onto a 240cm static sling as, in both cases, almost all the energy released is absorbed in (very roughly) the same amount of deformation of the body. in the latter case there is 8 times the energy to be absorbed and consequently a much higher loading on the body and the sling. Note that I'm not actually claiming either is safe just that I would be much less concerned about falling 30cm onto my arse than 8ft!

I would be very shocked if the results could be replicated by dropping a human 'soft' mass onto a knotted 30cm sling. I would be much less shocked to hear that an identical test with a knotted 240 and 'soft' mass produced a failure in both the knot and a damaging/lethal shock load.

Who actually uses a slack 8ft cows-tail?

Perhaps I'm being complacent?

In reply to kevinknights:

Not to be too pedantic and don't get me wrong, you make a good point. However, if you fall on a truely static sling in a 'prefect arc' you will not be dissipating your 800J of potential energy as heat into the knot as you will have converted the majority of your potential energy into (horizontal) kinetic at the bottom of the arc.

If you are describing somebody falling straight down I can see your point and it's well made.

In reply to jkarran: You'll lose some energy into the knot as stretch against friction - how much I don't know. You'd get some kind of figure for this from a non breaking case by how far up it swings once its past the low point. It would be interesting to see a video of the test to see at what point in the arc it breaks and how fast (and direction) the mass is moving at that point.

Kevin

In reply to jkarran:


Um just to clarify, a factor one fall on an 8ft sling would see you fall 4 feet. (8 feet is the length of the material used to make the loop not the length of the finished sling)

In reply to kevinknights:


It has already been established that the test was fot a factor one fall, not a pendulum swing.


Not if it is a pendulum swing as your initial analysis assumed, because the sling doesn't have to absorb the energy in that case - the swinging mass will just continue to swing until the kinetic energy is converted back to potential energy, then the cycle will reverse. But in the case of a factor 1 fall then you are right, the sling will end up absorbing the energy of the fall and in this case your calculation may have some value in explaining why the sling failed at the knot.


But can you pull the knot tight enough by hand to stop it slipping under the load of a factor 1 fall? Here's some more physics: The young's modulus of Dyneema is around 100GPa. I'd estimate that a 12mm dyneema sling is about 1.5mm thick, giving a cross-sectional area of about 0.00002 square meters. That gives you an elastic modulus of 2000kN. Say it's half that because of the weave of the sling etc. An 80kg mass in a factor 1 fall on to such a sling will experience an impact force of roughly 40kN - enough to break the sling, and to break you if you were that 80kg mass. Even if you halve the elastic modulus of the sling, the impact force is still approximately 29kN. Even if you take the elastic modulus of the sling as being one tenth the modulus of solid Dyneema, the impact force is 18kN which is still enough to break a sling made 30% weaker by knotting.

So I think the answer is: no!

By the way, the Youngs modulus of Nylon is around 2-4 GPa. Similar calculations to the above (assuming a 25mm sling) give an impact force of about 12kN, which would probably still hurt you quite a bit but the sling itself might survive...

All finger-in-the-air, back-of-a-(banned-in-public-places)-fag-packet stuff, of course, but it gives some idea of the possible differences in behaviour of the two materials.

In reply to Rob Naylor:

Come on Pete R of Lyon where is you contribution to this, people including myself are aching for an answer - I have said pretty much all I have to say at the top of this posting.

But where is the manafacturing / distribution industry answer reply will it be with the board sooon ?


I know weight drop towers can be very destructive in the way that they load gear - in an unnatural manner and all the science fact is WOW but at the end of the day.

From a guiding working perspective are the super thin cordlet slings worth their weight? and will we all have to send back these dynema slings ?

Look forward to a reply soon,

Thank you for all the fish and all that ....

In reply to jkarran:


We could use cragfast sheep as crash test dummies.

"Bleat if this hurts, Maisie".

In reply to Rob Naylor:
I think it is time this thread was left to die peacefully in it's sleep.

It can be seen by anyone that reads it in it's entirety that it has been going round in circles for 2 days now.

Either that or read the thread before posting.

Thanks.

In reply to karl walton:

Agreed. Unless/ until a Lyon rep posts something more definitive.

In reply to Rob Naylor:
Good point.

This was a good thread BTW mate, I think it's always better to keep all issues of safety as current as possible.

In reply to Rob Naylor:
i won't be loosing any sleep over tieing overhand knots in dyneema slings that's for sure!...

Stevo

In reply to Martin W:
"An 80kg mass in a factor 1 fall on to such a sling will experience an impact force of roughly 40kN"

Can you explain why people (including yourself) keep metioning factor 1 falls? Surely the theory of fall factor does not really apply to a static sling and only applies really to lead ropes. In the case of a static sling surely the distance fallen is far more important than the fall factor.

On normal nylon ropes I can see that a 1 metre fall on to 1 metre of rope is the same force to the climber/gear as a 100 metre fall on to 100 metres of rope, however this just does not apply to dyndeema slings does it!

Stevo