Dyneema sling failure under low load when girth hitched
Submitted By: Andy Laakmann on Oct 22, 2006
From John Sherman:
Dyneema sling failure under very low load when girth hitched
On Oct 19th I experienced a partial rappel anchor failure when a Dyneema sling used as part of the anchor broke at the point where it was girth hitched to a wider Spectra sling. The anchor was set on Oct 15th, 30 feet back from the top of the cliff and used only for rappels and was never subject to any sudden impacts. The anchor was equalized for the direction of pull with the girth hitched point of failure on one side. In theory this part of the anchor should have only been subject to half the loads, however because the equalization point was a clove hitch it is possible that one side could have taken the full load (body weight only) if the angle of the rope changed. The rappel line was a static rope and I was using a Grigri as my rappel device. Where the rope ran over the lip of the cliff it was padded with a piece of carpet. Every morning the anchor was inspected for soundness (especially any chewing by rodents) and seemed sound. When loaded, the Dyneema sling was suspended in mid-air with absolutely no contact with the rock to cause abrasion. The anchor was rappelled on a total of seven times (sling failed on 7th rap). As we were cleaning routes, the time of rappels varied from about 10 minutes to one hour. There was a small amount of rain on the 17th and possibly some showers on the 18th, however the rappel line felt dry on the 19th. The failed sling was in use less than one year and showed no signs of damage prior to this incident. The girth hitch feels quite tight after the break. When the sling finally failed I was partway down a slab with multiple points of contact between the rope and the rock to reduce the force on the anchor. I was stationary at the time and suddenly dropped a foot when the sling broke. The point where the Dyneema failed was Dyneema pinching Dyneema and not in contact with the Spectra (see photos). The Dyneema sling that broke was an 8mm Mammut runner. The sling it was girth hitched to was a 5/8" Misty Mountain Spectra runner. The broken ends of the Dyneema feel soft, not fused.
What factors might have increased the load beyond body weight?
- Pendulums across the face - 10 foot lateral swing at most, 30 feet below the point the rope ran over the top of face.
- Extra force applied when prying loose blocks of with a prybar. In theory this force could not exceed the amount I could deadlift. In this situation I don't believe I ever exerted more than 100 pounds of additional downward force on the rope.
- Sudden drops onto rappel line after it was unweighted when standing on ledges. At times the rap line was partially unweighted, but because I am nervous/careful when it comes to ropework I always sucked up the rope through the Grigri before reweighting it.
- Extra weight of bolting gear, prybar, etc - 40 pounds at most.
I don't believe any of the above factors is significant, especially since all of these take place below the lip where the friction of the rope on the lip would reduce the load on the anchor.
At times the sling may have been stored in the same pack as a Bosch battery. Is there any evidence that NiCad cells emit anything that can damage Dyneema?
At present my best guess as to why the sling failed is that when girth hitched tightly such a small diameter sling can cut through itself (the sling suffered a very clean break). In this incident I can imagine that the girth hitch received numerous small tugs under low load (body weight and less) and this might have caused a repeated microscopic nipping or sawing action that eventually cut through the whole sling (the wider Spectra sling it was girth hitched to also had some fibers cut at the point of contact with the Dyneema). I am trying to contact the equipment manager from Mammut to have this sling failure expertly analyzed. Until then I suggest that Dyneema slings should never be girth hitched or otherwise knotted.
| || Failed at the girth hitch |
|Comments on Dyneema sling failure under low load when girth hitched
Oct 23, 2006
I believe the sling failure is due to the use of the static rope. There have been some very interesting test that show the dynema slings are very easy to make fail when a dynamic element such as a rope are not involved(ie .5 factor falls).
|By adam tschida|
Oct 23, 2006
The cut looks very clean...not like it was abraded.
|By Tony B|
From: Around Boulder, CO
Oct 23, 2006
The failure point of this webbing is OUTSIDE of the knot and is not at the point of the maximum compressive stress or even within the area of impinging force. The assertion is further supported by the observation that was made by J.S. that the knot was hard and secure even after the failure. Had the sling been cut by the force of compression, one would expect that the sling would "squirt" off to either side of the point where the force was so high as to "cut" the sling. So I don't think that's it.
I also believe that the sling would have a "rough" break if failed under tension alone. One little "tuft" of this sort of break is visible in the photo, but that is not how the whole thing looks?!?!?!?
Are you sure this sling had never accidentally been "precut" and then used without noticing it?
From: Boulder, CO
Oct 23, 2006
At first glance, it appears clear that the failure (1) did not occur inside the weakest part of the sling (the girth hitched portion) and the (2) separation point of the fibers look very clean. These factors would seem to point toward some kind of cut or pre-existing condition rather than a result of simple break-failure under tension or shock load. Let us wait to hear what the fiber analysis can tell us before making any generalizations about dyneema materials or products. Personally, I tend to avoid girth hitching where possible in my rappell and climbing anchors due to the fact that it needlessly reduces the strength of the material by such a large percentage. Plus, there are better ways of accomplishing the same mission with continuous tubular webbing tied off by a water knot or just using a longer sling if possible. Webbing is cheap, custom adjustable for equalization purposes and not very difficult to carry extra. For rappel anchors, static rope itself tied in a tensionless hitch around bombproof trees / boulders or other anchor is much preferable to stringing together multiple slings with girth hitches. Just my 2 cents.
|By E Johnson|
From: Boulder, CO
Oct 24, 2006
Sounds and looks suspiciously like the recent break of the rope in the climbing gym, also Mammut if I remember correctly. They did find the same acid on that rope as used to clean holds.
|By Chris Owen|
From: La Crescenta and Big Bear Lake
Oct 24, 2006
Don't assume because the knot was hard and tight that it didn't play a role in this. It may have failed just below the girth hitch, this kind of knot will half the rating of the sling by about 50%, from 22kN to 11kN, obviously still quite a reasonable safety factor (about 10 assuming a force of about 110N on the anchor). The static rope will obviously not absorb as much of the load as a dynamic rope. Besides bouncing on the rope, I can't think of any other load multipliers.
Even so at first glance it seems like the sling had some kind of flaw which caused it to fail below its rated load of about 11kN in this situation.
|By Seth Finkelstein|
From: Boulder, CO
Oct 26, 2006
The photo of the failure surface is so clean that one can likely conclude that this is not a failure due to gross overload, even in the presence of a stress concentrator such as a girth hitch. There is no notable elongation present - this situation is akin to brittle metallic failure. Ductile failures (including fatigue) nearly always show evidence of elongation prior to failure in tension - judging by the location of the failure this is clearly in tension. If I were a betting man I would guess the presence of a pre-existing condition such as mechanical cutting or chemical exposure. NiCd batteries possess a valve to release gas created when the cells are in an overcharged condition. Hydrogen gas can evolve on an overcharged anode, and oxygen evolution can occur at the cathode. Either of these gases could result in highly acidic conditions once released and allowed to go into solution with water present on a surface (such as a sling stuffed into a backpack after bailing from the rain).
John - were the batteries you mentioned weak or not holding a charge?
|By Rob "Roberto" Dowse|
From: Albuquerque, NM
Oct 26, 2006
Quote: E Johnson
2 days ago
Sounds and looks suspiciously like the recent break of the rope in the climbing gym, also Mammut if I remember correctly. They did find the same acid on that rope as used to clean holds. Unquote.
In the interest of accuracy and to insure a good company is not hurt by mistaken statements I'd like to mention this from the report of the broken rope:
Rope – the rope is a 10.5mm Black Diamond rope – circa 2001. It appears to be a “Cirque” – at this time Black Diamond ropes were manufactured by PMI, with Nylon supplied by Beal.
Oct 26, 2006
check out www.ukclimbing.com/forums/t.php?t=177030 to read about recent drop tests using the dyneema slings. Under a static load the dyneema slings are reqularly breaking with a .5 factor fall. that means 80kg drop from parallel to the anchor, using a double length runner (no dynamic rope)is regularly breaking. The same thing is happening with dyneema daisy chains. The lack of "elongation evidence" is exactly the problem. The Nature of the dyneema material is that it does not stretch, thus the low breaking point. At least that is they way I understand it.
Oct 28, 2006
the tail of the sling not on the girth seems like it was cut. From the strands on the left side of it, and the rest being cut. I cut 75% of a quickdraw's sling after i retired it, and weighted it till it failed. it looks exactly the same, clean where i cut it but frayed where it actually broke. maybe somebody messed with it and because it was so close to the girth hitch it wasn't seen during the inspection. but i don't know how you can get a break point to look like that without cutting part so the rest fails.
|By Mark Michaels|
From: Draper, UT
Oct 30, 2006
I would suspect sabotage. Looks like a razor cut. A thin slice with a razor blade just below the girth hitch could be difficult to spot...especially if one was looking for frayed, fuzzy "rodent" damage.
Reports of dyneema "snapping" under a .5 factor fall are rather alarming. Use your gear wisely....clip anchors with your rope, not a static sling. Don't leave anchors/fixed rope where it is possible for some idiot to mess with them.
Glad no one was killed!
|By David Champion|
From: Centennial, CO
Oct 30, 2006
Certainly it appears from the photo that the breakage, clean as it is, occurred under load (as previously noted here). One possibility apparently not yet considered: A falling object striking the loaded sling may have cut it, e.g. a falling rock. Under load, the cutting edge of this object would not have to have been particularly sharp, either. This possibility gains more weight, IMO, if a chemical analysis of the sling reveals no damaging chemical exposure, as with the recent gym rope incident.
|By Rick Shull|
From: Arcata, CA & Dyer,NV
Oct 31, 2006
After thinking about this one and talking to a few friends, I have to come to the conclusion that the use of a static line with a GRI-GRI may have much to do with slings and belay loops breaking. If dyneema slings have been proven to break with factor .5 falls this is certainly not out of the question. Any one who has rappeled overhanging terrain on a dynamic single line rap with a GRI-GRI understands how quickly it can lock and give a big bounce. When this happens it is creating a hard "shock load" on the system and the weakest link can fail. As other rap devices have much more "give" when descending the forces are much smaller when stopping mid rap. As most climbers are used to setting anchors that are buffered by dynamic ropes, people may want to consider "beefing" up anchor points when using static ropes.
|By E Johnson|
From: Boulder, CO
Nov 2, 2006
Here is a link you all may find of interest, directly related to using a girth hitch to connect two slings. www.climerware.com/cknot1.htm
It provides data on various combinations (nylon+nylon, nylon+spectra, etc.) based on tests done by BD.
|By rob coppolillo|
Nov 8, 2006
well, first off....phew! glad you didn't crater...
couple thoughts.....i contributed an article to R&I last year comparing nylon to Spectra/Dyneema and was surprised to learn lots about Spectra/Dyneema's weaknesses as a material....for clarity i'll use S/D to indicate Spectra and/or Dyneema--they are essentially the same material; Dyneema being manufactured in Europe; Spectra in the states.
as one poster mentioned, any static loading of S/D can be catastrophic..more on this in a sec....another poster explained that most materials show signs of elongation before failure---but the problem with S/D is that it does not elongate (less than 5% before failure, compared to nylon's nearly 30%) at all, and is therefore more susceptible to failure due to static loading than nylon.
so, using a GriGri and a static line would be more likely to result in failure using S/D rather than nylon. part of the reason this is true is due to another characteristic of S/D, namely its low melting point. S/D melts at nearly 100 degrees below nylon--so, putting S/D into a static situation stresses it in a unique way---all of the force is applied at once. if a system has a dynamic component (rope, nylon daisy, nylon sling, nylon cordalette), then the loading rate is greater, meaning the force applied is spread out over time. imagine when you load a dynamic climbing rope, the force of a fall, for example, is spread over a second or so.....with a static system (S/D runners, daisies; steel cable; etc), all of the force is effectively applied INSTANTLY---and it's thought that S/D fails because the force heats the material and that heating can cause it to melt and then fail..........add a knot to the system that further comprormises the system..........
with that said, I AM NOT AN ENGINEER, nor could i possibly know through your description what actually caused the sling to fail. i'm just trying to throw out some ideas that might shed a little light on the situation, and also prevent another accident.....
there has been a lot of informal research going around regarding chemicals and nylon-S/D.....who knows if that affected your sling, but one poster did mention that NiCd batteries have some type of vent on them. even with a fully static system, it seems to me a sling that isn't in contact with rock would not be able to fail under normal forces generated by rappelling. perhaps that particular sling was weakened by exposure to the battery acid? who knows? again--i'm not an authority, just a dirtbag journalist who did his research for an article. i will say, however, S/D slings failed quite easily under static drop tests, at weights less than our less-than-svelte bodies..........
bottom line, from my POV: S/D slings are great, but are compromised in any static system. i use 'em on runners/QDs these days, but i ditched my S/D daisy, and i'm always wary using them any time they're not part of a system that includes the rope. these limitations, furthermore, apply to S/D cordage---i gotta be honest, using S/D material for a cordalette is asking for it, in my opinion. without something dynamic in the system, the stuff can fail at BODY WEIGHT. also---S/D cordage (and webbing for that matter) does not knot well--any time you knot the stuff, it should be a TRIPLE fishermans. one manufacturer told me of watching S/D cordage simply slide through itself when gradually loaded, and tied together with a dbl fishermans. point being: if you do use the stuff, and need to knot it for some reason, TIE IT WITH A TRPL FISHERMANS.....
hope this casts a bit of info on the situation. my take-away from the article in R&I was that S/D is a great material, but not without its drawbacks. so know the limitations of the material and use it accordingly. always have nylon with you in case you need to leave rap anchors, etc. i also think making the sling or draw on your first piece leaving a belay NYLON, rather than S/D, is a good idea. any situation in which the loading might be static or extremely high (like a fall factor approaching 2), it might be smarter to use nylon.
glad you're all right and keep flying the flag, homes........
From: San Diego, CA
Nov 14, 2006
I got intrigued by the photo of the broken sling. Since I had a similar dyneema sling of the same age which I wanted to retire due to a slight friction burn, I decided to put it through the static load test machine at work. The sling broke slightly below its 22 KN rating at 20.8 KN probably due to the friction burn. It broke at the location of the burn.
Notice that the break is very frayed, not at all clean like the other one that failed under use. I heard what sounded like individual fibers breaking or re-orienting themselves from 2,000 lb to the failure at 4,667 lb. Then SNAP, it went very quickly.
The example which broke under body weight in actual use looks like sabotage to me. The break is way too clean and the load way too low. Even chemicals would probably attack the fibers more randomly than this. I expect that when you leave your rappel setup in place for weeks at a time you never know if a rat is chewing on it or somebody you upset because you took a crow bar to the cliff wants to teach you a lesson.
Shock loading has been mentioned as a culprit. In a very stiff system it can cause high loads from a relatively low weight. Rappelling does not create this, more like a fall factor 2 on a sling girth hitched to your harness and clipped to a bolt. The only way to know would be to simulate this with a force indicator in line with the system. I doubt you could generate 5,000 lb on such a short sling even with a heavy guy. Note that this sling stretched 2.5 inches before it broke.
The girth hitch as a saw theory has some merit. It seems unlikely to me, but who knows. I only have so may slings to break for entertainment so I will have to forgo this test. Do not do this if you are worried, it will probably cause premature wear if it does not break your sling.
To my credibility I actually am an engineer working with mostly composite materials.
| || Load Displacement Curve (MAX = 4,667 Lb or 20.8 KN) |
| || Test setup |
| || Post test #3. Note freyed ends. A sling that fails cleanly was most likely cut. |
|By Rui Ferreira|
From: St Ismier (Grenoble), France
Nov 16, 2006
This incident has also intrigued me enough to do some background research of my own. Both Dyneema and Spectra slings are made of ultra high molecular weight polyethylene (UHMWPE) fibers; Dyneema is the registered trademark for DSM products and Spectra the registered trademark for Honeywell products. It is very likely that the fibers in the Spectra webbing are different from the fibers in the Dyneema webbing, given that they originate from two different manufacturers, each of whom list different grades within their own product portfolios, but in general they share many of the same general characteristics and general fiber performance.
UHMWPE is a super strong lightweight polymer (less dense than water), used in armor (ballistics), fishing lines, parachute lines, webbing, etc. It is about ten time stronger than steel (same weight basis) and has a specific strength 40% greater than aramid (Kevlar fibers). It has outstanding cut resistance performance and is used in industrial gloves to prevent cutting injuries...which in my opinion eliminates the theory that abrasion of the anchor slings resulted in the failure of this particular sling - this stuff is hard to cut.
Another aspect of the UHMWPE polymer is that it has outstanding solvent resistance to different organics, acids and bases. In fact polyolefins in general are superior to nylon (standard webbing and rope fibers) when it comes to chemical resistance and are very hydrophobic, eliminating the theory that exposure to moisture/rain and/or battery acid gases could have caused this failure (microscopic analysis of the point of failure should easily confirm this point).
[ as aside: another interesting aspect regarding battery fluids and gases: keep all your nylon webbing and ropes away from your bolt drills and battery packs because, nylon does break down in battery electrolyte solution (all battery types). I do not recommend storing your drill and batteries in a (nylon) stuff sack inside your climbing pack...]
Nylon does have an advantage over UHMWPE when it comes to the melting point however, most UHMWPE melts around 150C, whereas typically nylon 6,6 melts around 210C. This does introduce the possibility that Dyneema and Spectra could be affected by a heat source (from friction, induced dymanic forces, etc.) and potentially lead to a failure, or at a minimum result in a decrease in performance as shown in the previous posting (static load testing of Dyneema sling with abrasion burn broke at 1.2 KN less than listed break strength of 22KN).
This brings us to the break strength and dynamic performance of the Dyneema (UHMWPE)slings, as illustrated by the previous posting (by mschlocker), Dyneema is failing very close to the listed specification under static loading conditions (even when there is an abrasion burn). Other postings have questioned the low elongation associated with UHMWPE slings. From what I have been able to read, UHMWPE under high loads decreases in elongation, but also gains about 10% in strength as the polymer realigns and there is some degree of fusing in the filament tow (from testing of marine ropes). Low elongation in itself is not an indicator of low strength despite some postings attempting to make this link. It appears that after the initial load deformation, UHMWPE will break at its listed tensile strength (maxima in the Stress/Strain curve).
Based on these facts I also suspect that the failure in this particular incident is due to a deliberate cut of the sling, since it does not appear that the loads induced on the anchor system by the climber come anywhere near the breaking point for the slings (even when girth hitched).
The postings about Dyneema slings breaking under certain test conditions are interesting, but unfortunately not much is known of these test protocols to actually calculate the load forces involved and the rate of load. Has anyone actually been able to duplicate these tests and/or calculate the forces involved?
In regards to my background, I am a chemical engineer by training and have spent the last 15 years working with different materials, fibers and polymers in product design of advanced composites, fiber reinforced plastics, battery separators, etc.
|By rob coppolillo|
Nov 20, 2006
great comments by the gang at large, thanks.
my own reference to informal, improvised testing amounted to a magazine editor's own experiments in-house. this editor contacted me regarding his "results"--he had been able to snap thin Dyneema runners by affixing a 150-lb weight to the sling, then holding the weight the length of the sling above a bolt, dropping the weight and observing the results.......which invariably resulted in the sling snapping.......so, that'd be a factor-2 fall without any dynamic component to the system. the failure of the sling, according to sources i used in my piece for R&I, all seemed to agree that the failure of the sling was the loading rate--that is, without a dynamic component to the system the sling is loaded essentially instantly which generates a tremendous amount of heat which then causes failure in the material......
anyway, hope that helps to some degree......my "qualifications" are nothing more than being a journalist....so hopefully my research and sources provide adequate info on the topic!
be careful and have fun,
From: San Diego, CA
Nov 22, 2006
About dynamic failure.
Pretty amazing that a Dyneema sling can be broken by a fall factor 2 with 150 Lb with no dynamic component in the system. I hope to shed a little light on this phenomenon. There are two things to consider.
1) Strain rate. How fast the material is stretching. Typically materials exhibit greater strength and stiffness at high strain rates. Strain rate is is not the problem here.
2) Shock dynamics. This is the culprit. Newton's law of an apple falling is (Force) = (Mass) * (Acceleration). In our case it is a huge deceleration because, as Rob says, our mass is stopping almost instantly. Our huge deceleration with a 150 Lb mass causes a huge force on the sling. Although our mass is only 150 Lb, the sling at some instant in time thinks that there is a 5,000+ Lb guy hanging on the end. Things may heat up a little as they strain, but this I do not believe is causing the failure, it is the huge resultant force. With an actual person, there is still the dynamic component of the harness squeezing their guts and fat, people are more squishy than metal. Seriously, a big gut may add the dynamic component you need to save your life!
The conclusions do not change: do not lead on a static line, do not fall on a sling alone, do not worry about it when rappelling. Just trying to share some knowledge.
Dec 4, 2006
I don't know if this was covered in any of the other post, as I just skimmed over them but dyneema is far more susceptible to degradation due to sunlight than a regular nylon sling, and having sat outside for a year could certainly weaken the sling to a catastrophic point. I'm not sure why everyone seems to be overlooking this fact.
From: San Diego, CA
Dec 5, 2006
The following is pulled from the DSM website (maker of Dyneema).
"Dyneema® floats on water and is extremely durable and resistant to moisture, UV light and chemicals."
The dyneema sling I pull tested was also about one year old with significant sunlight hours and still held extreme load. I am surprised how many Spectra/Dyneema products are actually sold with all of its surrounding fear and mysticism. I would rule out UV as a root cause here.
|By John Easterling|
Dec 6, 2006
For the past 3 weeks i have been breaking climbing gear for a living. I've broken a lot of runners, webbing, spectra (never any Dyneema) and other various things. I would say that this break is suspect because of how clean it is. Webbing (unless it fails at the stitching) usually becomes very frayed when it breaks.
|By doug haller|
From: Boulder, Colorado
Dec 9, 2006
Interesting discussion. Keep it simple. Based on the picture, the sling was cut. Luckily no one was hurt.
Jan 17, 2007
"Seriously, a big gut may add the dynamic component you need to save your life!
Well, I'd think the elasticity in your spine would play a bigger role that your gut. But I think I'll keep my guy just Incas.
You bring up a good point, dropping a 150lb chuck of steel is by no means an accurate simulation.
"Notice that the break is very frayed, not at all clean like the other one that failed under use. I heard what sounded like individual fibers breaking or re-orienting themselves from 2,000 lb to the failure at 4,667 lb. Then SNAP, it went very quickly. "
I'm an electrical engineering student currently building a portable force monitoring system for field testing climbing anchors and my loadcells will have Spectra dogbones as attachment points. I was hoping to set them up to measure up to 12-15kN, but sounds like I may have to drop that spec to 8 or 10kN. Do you know what effect this "realigning" has when the sling is unloaded?
|By Avery N|
From: Boulder, CO
Sep 3, 2007
FYI -- The formal Mammut report/findings can be found in pdf format at:
www.mammut.ch > technology > harnesses > sling report
Jul 24, 2010
I keep hearing about tests where dynema slings fail in a static environment, but I've seen these tests and they are not at all realistic because they drop a solid lead weight. The human body is way more dynamic than a solid lead weight. That's hugely important because the force felt by the sling is given by F=dp/dt where dp is the change in momentum of the falling object and dt is the time interval over which the momentum change is distributed. In these tests, by setting up extremely static situations, they are making dt extremely small so that F=dp/dt becomes extremely large for a brief moment (causing failure). But again, the human body in a harness would NEVER generate the same jerk that a lead weight of the same mass can because the human body is basically floppy and squishy (if it wasn't, then we could kill ourselves by jumping 6 inches off the ground). Anyway, I just wanted to interject some physics in the conversation and point out what I think is false reasoning.