UKC

I thought this issue was worth starting a separate thread for, but thanks to Pete Callaghan who started this thread https://www.ukclimbing.com/forums/rocktalk/incident_report_101-705772 about the BMC near-miss and accident reporting service, where I saw the following.

Someone has reported after taking a repeated fall onto a Friend, that they noticed the extendable sling was nearly cut through. See the pictures at the bottom of the report https://www.incidents.thebmc.co.uk/responses/703649a5-f1c6-496a-8ec7-d31311...

That does look really bad, but can it possibly be the repeated fall on to a well placed cam that could do that? I've seen a less bad but similar looking cut that I think came from the sling being rubbed against a crystal in the crack wall, but if you can get that level of damage from a sling rubbing against itself, it is a bit worrying.

Anyone seen or done similar?

Slightly at a tangent but Dan did a local Peak BMC area test demo where a new sling rubbed hard on grit for a few seconds was weaker than a new sling cut half through. You can assume anything old and furry is well below half its rated breaking strength. As another aside I took a lead fall on The Link and sliced an extender half through as it rubbed along the lip of the roof as I swung.

If it was actually caused by the sling just rubbing against itself, as one of their pictures seems to suggest is their theory, then I reckon Wild Country's testing would have picked it up as a fatal flaw for the design. I think it must have been loaded over a sharp edge or scraped along the rock, either in this incident or maybe unnoticed on a previous occasion.

To me that looks like wear from where the cam was in a horizontal break and  loaded over an edge so that the force pinched the sling between the wire and the rock.  I can't see how this wear could happen otherwise. It does make me wonder what would happen with a dragon cam in a similar situation with the "pigs nose" forming an anvil. It also highlights the good design of the old tech friends with a steel attachment and the sling positioned so that it wouldn't be cut is such a way.

It's the classic failure of slings doubled so one strand lies over the other with a tight radius. Slackliners are familiar with the problem.

For some reason I can't see any pictures. 

Why don't we see this more often then Jim? I had DMM 4CUs for a decade and used them a lot. They had similar style slings, but the slings remained in good condition. 

If it's a classic form of damage, are cans with extendable slings not a flawed design?

I second this question. But it seems worth pointing out that not all extendable sling cams feature the tight radius and sling running over itself that are the features Jim mentioned as possibly causing the damage. I believe that was DMM's stated reasoning behind avoiding thumb loops on Dragons and Demon.

So the damage is caused by tape against tape or tape against wire, but not tape against rock? Or is the tight radius to blame? Or indeed a combination?

Interesting that DMM's Demons and Dragons both seemed to be designed very specifically to have an extendable sling, but not to let it lie on-top of itself. I assumed that this sort of thing was the reason why. But with the Dragonfly they they've gone back the the wire loop approach. So I guess they think it's not too bad, or that they have a different way to control it.

Dmm dragon flys are also rated to a lower strength 10kn for sizes 5 and 6 lower for smaller sizes so perhaps the sling fold isnt the weak point of the cam.

Friends have a dual strength rating of 12kn normal and 10kn extended.

Really? I have never come across this before. What kind of forces would cause that damage, presumably something fairly dramatic?

Chris

If you rub a full section of surface nearly all the fibres are damaged but a half way cut only damages half.

Maybe you don´t fall off often enough?

It´s an interesting phenomenon (well if you are geeky I guess). I first noticed it when I was testing stakes and one bent so far I had to double the sling to keep pulling whereupon the sling immediately failed. This was kind of interesting because I´d always learnt or assumed doubled up they were twice as strong, possibly true when I learnt to climb as slings were rope or cord but clearly with tape things are different. Back in the workshop I tested a few more and noted that particularly with modern karabiners wider slings break at lower figures than they should as the fibres are unevenly loaded so they effectively rip across rather than break as they would with the standard test pins. Double the sling and the position is worse as the loaded outside strand puts pressure on the underneath layer which cuts through (you can research the plastic bearing industry to find the tables of pressure melting, it´s a common occurance for them).

Sometime later I was doing work on line-lockers  for a slackline company and dicovered this is a well known phenomenon, they use sacrificial layers between the wraps of tape to prevent it cutting when they are using shackles as line-lockers but it´s still really hard to get right up to the tape rating (in fact impossible but you can get close).

With a doubled sling on a cam I´d expect the manufacturer to de-rate the sling strength (and they do) but repeated falls don´t come into the standard, you are expected to inspect your gear between falls, the instructions tell you the equipment may only be good for one fall.

Cams often have to be reset after holding falls---I know of a few cases in which the cam failed on the second or third catch.  So while we're at it, seems best to rotate an extendible sling so that the impact isn't applied to exactly the same spot over and over again.

Very interesting... thanks Jim.

Interesting... and maybe worth testing. But... to my mind it seems far more likely that this part of the sling was being repeatedly loaded over a sharp gritstone edge (a flake or the lip of a crack), rather than simply rubbing against itself. It's a deep but narrow slice and I just don't feel that it would look like that if it were the sling cutting through itself in some way. I also don't think there would be the kind of movement in the sling that would potentially create that kind of friction. But, I'm not claiming any special knowledge here, so...!

It's tape that has the 'furriness problem' because in standard tape every fibre is on the surface at some point due to the plain woven construction, therefore an old piece that's furry all over a large area invariably has a high percentage of damaged fibres.  Kernmantel rope has a load bearing core which is protected from abrasion by the sheath thus doesn't suffer such a drastic loss of strength. I believe you can now buy 'cored tape / slings' with a corresponding increase in longevity.

Im not sure I agree with jim titt on this one as the forces generated in fall are gonna be given by F=MA (mass x aceleration) so let's assume a climber weighs 70 kg so F= 70x9.81×9.81. The force involved is roughly 6.7kn assuming that the rope is a static rope (I know in reality we dont use static ropes for lead). The next stage is to work out the pressure or stress this is equal to force divided area so our area is 0.011x (0.0025x 2 pi x 1.5) so I've done the width of the sling times the contact patch with the thumb loop the 0.0025 being the diameter x 2 x 3.1415 x 1.5 the 1.5 comes from the spiral effect as the sling is wrapped around the thumb loop 1 and a half times and this should give us the stress which is 14.4 KPa this is all roughly done and not taking into account the dynamics of the rope or other external factors such as the climber lessening the fall by scraping down the rock face. Since I've got time on my hands tommorow I might look into this further beyond back of the hand stuff as this is a really interesting failure if it is the thumb loop causing this I'll try to make my assumptions as close to realistic as possible such as the radius of the thumb loop etc and use the conservation of energy theorem and see what numbers get thrown out of the equations but maybe the sling is just being sheared by the thin gauge of the thumb loop after a fall as the outer edge would would be taking all the strain as a sling cant take a compressive load only a tensile load think of the outer edge as being like a stick when you bend it away the edge furthest  from you will splinter but the inner edge does not. Hope this clears up some of the forces involved and perhaps why the sling was cut/sheared 

You're joking, right?! Please be joking because you've already crammed in a truly astonishing number of fundamental physics errors. If you're going to disagree with Jim Titt, this is not a strong start!

No idea if your physics of maths make sense but have a like for effort

FWIW - Jim Titt - has long history of breaking stuff and finding out why - worth listening to.

I'm interested in what today brings!

Can you point them out then itd be a great help, this is all off the cuff I was thinking of using energy methods and impact loading principles but I was tired, as far as I can the first mistake is multiplying by gravity twice the second is not taking into the deceleration time of the climber so I think it would be more like a quarter or  a third of a second to decelerate from the peak speed which in a three meter fall would be V=sqrt(2 x distance x gravity) so let's say a three meter fall, so v is 7.7 m/s so to get deceleration this 7.7/0.33= 23 m/s^2 so f = 70 x 23 => f = 1.610 kn at the point of impact so drastically lower than I calculated last night but I hope this figure looks more correct to you 

Do you have a link or a general idea of where I can find these tables

Not the sort of thing I keep links to. The subject you should be looking at is "glass transition point".

Well, yeah, in your first attempt you didn't consider the length of the fall or the acceleration of the climber, suggesting that any fall of any length stopped by any rope would produce the same force, which should have been a clue that you were on the wrong track. And yes, as you say, the numbers you put into F=ma weren't a mass or an acceleration.

Yeah, the calculation all seems like correct and relevant physics now so your impact force might be a reasonable order of magnitude estimate. I don't think this kind of approach can really clarify anything about what happened to the sling in this case, it involves far too many simplifications and assumptions. The fall distance is a guess, of course, and you've assumed a completely constant deceleration over a time that's also a complete guess. I'm not piling on with criticism, those are both reasonable approaches to take if you want to use this kind of basic mechanics to make an estimate, but it can never be more than a very rough indication.

Good on you for coming back and dramatically improving on your first attempt!

4 to 6kN are always good numbers to work on for the force on the top piece!

4-6kn are quite high i think in general the 1.6 kn number has a much better mathmatical underpinning, the only thing i need to find now is the melting table for nylon sling and rework the numbers for stress in the sling which should still be in the order of kilapascal (pressure) the only source i could find in a quick google search suggested the melting pressures where in excess of 100 mega pascal which is a huge difference 

Oh, for goodness sake, you're deluded! Your back of the envelope estimate is based on vast oversimplifications and numbers plucked out of thin air! It was a lot better than your entirely meaningless first attempt but it's nowhere close to being a good substitute for Jim Titt's extensive knowledge and years of actual physical testing. Give it up, please.

Hmm. You're using (well, mis-using) lower sixth physics and then trying to argue your figures with someone who literally works in gear design & manufacturing...

Your first attempt simply equated f=ma, but the value of a you picked was free fall. So what you were inadvertently calculating was the tension in a rope being used to accelerate someone at a rate equal to free fall. That's not the situation.

You've then had a go at calculating the force by guessing what the time of impact might be. That's a more valid approach, but what justification do you have for your 0.33sec?

Next problem with a kinematics type approach: you're implicitly assuming that the force is constant over time. That's not true; impact forces for stretchy things like ropes are bell shaped. You'd need some way to find the peak force. You also need to take into account a host of real world factors like slippage through the belay plate, movement of the belayer, stretch in other items like the harness...

You could go down the route of calculating energies instead - equating gpe to wd on the rope. But then we need to know things like wd heating the top krab, the belay plate, etc etc.

There's a reason we do this empirically and not theoretically! 

On a tangent, I'm really surprised (and really pleased) at how well the incident reporting is catching on. That's going to become a seriously useful resource.

Whether you think 4-6kN is too high has no relevance, since they are typically the range of values that are obtained by testing AND produced by more accurate mathematical models they are the ones to use.

The melting point of nylon (or more relevant in this discussion Dyneema) is of no interest, as I said before the glass transition point is where to look.

The pressure of a flexible object around a circular shaft is not constant, it is a sinusoidal repartition which will tell you the peak value.

Exactly what I was thinking...

<walks away stroking his chin, trying to look like he has a clue what they are going on about>

Been waiting ten years to slip that one into a UKC thread

as i mentioned in the posts all of this is only very rough and im not trying to argue with anyone

i know i shouldnt just assume but if the maths is wrong then i guess i have to rework the formulas until im confident its correct, trying to find this info online is far more difficult than i first assumed i guess ive bitten off more than i can chew haha. 

i done some more working out today using statics and not using sinosoidal repartition and got some fairly alright numbers nothing crazy but the fall from 3m generated a stress on the contact patch of the 67 Mpa with an axial stress of 8.75 Mpa so a max stress of 75 Mpa this might be enough to heat the material you know more about this than me. however i do know that the youngs modulus changes with temp so i guess even before the glass transistion stage it could fail due to changes in the material properties. 

It is way off and we know from lots of testing that it is way off.  Your first analysis had no connection to the fall distance.  Your second analysis has no connection to the mass of the falling climber, makes up, without justification, the time it takes to stop the faller, assumes deceleration is constant (it isn't), and mistakes that non-existent constant value for the peak load.

With so many mistakes, it is hard to know how to help you in the right direction, but to start with, no analysis can have any validity if it fails to account for the mechanism that actually creates the anchor loads, which is the stretching of the rope.

If you want a simplistic elementary account (written for US undergraduates), have a look at http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;pos... .

For a more sophisticated analysis, see http://www.sigmadewe.com/fileadmin/user_upload/pdf-Dateien/Physics_of_climb... .

Thanks for those links I appreciate the help at moment I study engineering but they dont teach us anything like this in solids and structures or dynamics so it's a bit alien and probably a very specific area to be in 

Just a bit of background regarding this incident - it happened to people in my climbing club. 

The friend (cam) was basically new and the leader took a few falls from the same move. The cam wasnt in a horizontal break as such but more downward facing so the sling wasnt rubbing over the rock.

We speculated that it was the extendable sling being pulled taught over itself which caused the issue. If you look at DMM cams, the sling is extended over the thumb press which means it is not in contact with itself. Its a lot easier to pull out a cam and have a look at what i mean than to try and explain it!

It could just be a design flaw with the extendable sling on the BD/Wild Country type cams. But its still a bit of a scary thought!

Thanks for sharing. What was the fall like? How far? How much rope out?

What have you done with the unit? Is it possible to sent it to Dan Middleton at the BMC to have a look at it?

https://eu.blackdiamondequipment.com/en_GB/qc-labreslinging-camalots-and-c3...

I think this article has the answer 

Look back at your notes on (damped) simple harmonic motion---everything you need for an elementary analysis is there.

Thanks I'll have a look over my first and second order harmonic systems and try to understand it in more detail it's never been my strongest subject 

Bottom line is that materials science is far more relevant than 'back of a fag packet physics' in this case

Given the above why did you start you initial post with an cursory dismissal of Jim's statement and follow with an insistence that 1.6kn was more likely than the real figures of 4 to 6kn?

I just wasnt sure I agreed with his statement about the sling being heated up for all I knew it might well have, but given the hottest recommended working temps are 80-130 Celsius it seems unlikely that the sling would be heated by as much as 110 degrees Celsius from room temp in one fall, if you've looked at the black diamond article it says it's been sheared by the deformation of the thumb loop causing a pinch point on the 11mm dyneema hence why BD use thicker nylon doubled over

You must be aware that knotted dynema can melt during tightening. Also I gather that loaded threads can cut through other loaded threads that are under them but compressed against an unyielding surface (metal)

I said nothing about the sling being heated up, I indicated the direction you should be looking at to start your research into the behavior of these types of materials under pressure and heat. The BD failure shown is not what we are talking about either, it is the outer layer severing the strands of the inner layer, starting with the outermost strands and moving inwards.

Being neither an engineer nor a physicists I'm completely lost on the technical discussion about this BUT...

Jim, do you reckon the damage shown in the photos on the BMC site is caused just by a fall onto an extended-out extendable sling on a cam?

And if so, wouldn't we hear about that sort of damage more often?

Well the OP said it was a repeated fall, not a fall. Why we don't hear about it happening could have many reasons, I expect a lot of things happen you don't know about

I might think about doing repeat drop test to see what happens but it's not really burning issue somehow.

Are any cam manufacturers using these? Knackered slings are the most common reason I retire old cams, and switching to the cored design would seem to offer a major improvement.

Do you not just re-sling them (or have them re-slung)?

The manufacturers won't touch them beyond a certain age.  I have a couple of 4cus that are pretty ancient but have never fallen on and are still in perfect working order except for furry slings.  Given that I really don't like falling off I expect most of my other cams to go the same way eventually.

I could just get some cord I suppose, but it tends to be bulkier.

Re-slinging yourself is by the far the least hassle but you're right that a knotted nylon sling is more bulky than a sewn dyneema on.