Can slackline carabiners (tri-axial loaded) be put back into a climbing rack (safely)?

I have been using a few carabiners exclusively for my slackline over the last year or so, but been afraid to re-introduce them to my climbing rack. I am afraid because my line is usually anchored to a slung tree. This being the case, the carabiner is being pulled in 3 directions; 1 by the line and 2 by the sling around the tree. Therefore, it is being tri-axial loaded... And i have heard that this weakens carabiners significantly.

So my question is this... does this weakening mean that the structure of the carabiner is weakened, and therefore not able to withstand the forces it was originally created for?
Or, does the weakening mean that the failure point is lowered while being tri-axial loaded? And there being no long term affect on the carabiner?

Thanks for the input!!

Biners are darn cheap for just a few, at the risk of being cliche, why put them back on your rock rack?

It would depend entirely on how it was used, if it was always used in a "block and tackle" to tension it, most likely it would be fine, but that also depends on the weight of the people using it, as well as the amount and extent of the tri axial dynamic loading.
If you were using a winch, or come-along to tighten the line, there is a possibility you could stress the biner to the point of introducing micro fissures at the grain boundaries, most likely in the center of the spine on the outer radius, or the insides of the corners on the inner radius. Without performing an ultra sensitive florescent liquid penetrant examination micro fractures are impossible to see. Use your best judgment, I use the same biners for slacklining and climbing, but I have the means to test them if I have suspicions, I would venture to say it is probably just fine to use (maybe not for anchors, or master points), 25Kn is a whole lot of force, and breaking a biner is ultra rare.

CalmAdrenaline wrote:It would depend entirely on how it was used, if it was always used in a "block and tackle" to tension it, most likely it would be fine, but that also depends on the weight of the people using it, as well as the amount and extent of tri axial dynamic loading. If you were using a winch, or come-along to tighten the line, there is a possibility you could stress the biner to the point of introducing micro fissures at the grain boundaries, most likely in the center of the spine on the outer radius, or the insides of the corners on the inner radius. Without performing an ultra sensitive florescent liquid penetrant examination micro fissures are impossible to see. Use your best judgment, I use the same biners for slacklining and climbing, but I have the means to test them if I have suspicions, I would venture to say it is probably just fine to use (maybe not for anchors, or master points), 25Kn is a whole lot of force, and breaking a biner is ultra rare.

A micro fracture under any other name...

I would probably have to say to the biener has been tweeted/bent/micro fractured. I wouldn't use it on my rack... Just keep it for slacklining.

In my experience only two biners have ever broken. One my buddy was pulling a car out of the mud and 'said' it broke the locker "cuz that dude was stuck." The other time was when a lengendary (RIP) climber came off 'AirGuitar' and his (wiregate) biner sheered in half, maybe due to lack of sling and it bent on an edge.

Great, thanks for all the ideas and opinions! It's nice to get an idea of how carabiners act under tension. I'll probably just leave them as slackline biners and dish out the extra money for a few new biners. Thanks!

I have been using the same set up for the last few years, i finally replaced the biners when i realized the gates would no longer close on their own, haha. Your life is worth way more than a few biners, just replace them.

-Nate

There's actually zero data one way or the other about the impact long-term, high-load situations have on short-term, high-load resilience. It could very well be the case that carabiners used in a slackline are perfectly safe to climb on. Personally, I wouldn't want to find out the hard way, especially because I find bail biners I can throw into my slackline all the time.

Question for the metallurgists out there.

As aluminum (maybe any material???) is cycled through load/no load situations it loses elasticity while breaking strength initially decreases before returning to near spec (despite being more "brittle")??? I walked thru some test results of some 'biners put thru this process a while back and this is my recollection - metallurgists out there, correct me if I'm mis-stating anything. Brian S - the test results I saw seemed to directly contradict your comments if I am reading you correctly...

Anyhoo...while initial risk to a new 'biner of the dreaded "micro-fractures" is essentially nil...after repeated cycling (when the aluminum has lost its initial elastic properties) this actually does become a more significant possibility.

The short version was that there is the potential for risk in this situation according to the engineer I spoke to (who was testing for the exact scenario asked in the OP). Although the risk was not quantified, personally I'd keep the 'biners away from my rack.

bevans wrote:Question for the metallurgists out there. As aluminum (maybe any material???) is cycled through load/no load situations it loses elasticity while breaking strength initially decreases before returning to near spec (despite being more "brittle")??? I walked thru some test results of some 'biners put thru this process a while back and this is my recollection - metallurgists out there, correct me if I'm mis-stating anything. Brian S - the test results I saw seemed to directly contradict your comments if I am reading you correctly... Anyhoo...while initial risk to a new 'biner of the dreaded "micro-fractures" is essentially nil...after repeated cycling (when the aluminum has lost its initial elastic properties) this actually does become a more significant possibility. The short version was that there is the potential for risk in this situation according to the engineer I spoke to (who was testing for the exact scenario asked in the OP). Although the risk was not quantified, personally I'd keep the 'biners away from my rack.

I will try to weigh in on this. My background is in mechanical engineering, though; not metallurgy. I apologize that this is going to be long-winded; I will try to keep it as brief as possible while still covering the basics.

First an introduction to stress-strain and elasticity. Then I will address the questions above.

Here is a stress-strain curve:

This curve relates how much stress is in a material while under tensile strain.

Strain is the amount of deformation, divided by the total length of the material. So the strain measures the fractional amount the material is stretched.

Stress is a measure of the forces inside the material divided by the area over which those forces are spread. Think of stress as the internal pressure in the material induced by the strain.

The first thing to note about the stress-strain curve is that there are two regions: elastic and plastic.

In the elastic region, the material returns to its previous shape when the load is removed. In the plastic region it does not. Think of a paper-clip. If you bend it a little bit, it returns to looking like a paper-clip. If you bend it a lot, it remains deformed.

Now for specific points on the graph.

Point A is the true elastic limit. This is the point where dislocations begin to move within the material. Dislocations are microscopic imperfections in the crystalline structure of the material. These imperfections are, to some extent, sources of weakness in the material (they can also be used to create strength in the material, which I will discuss in a little bit). As the dislocations move around in the material they will encounter other dislocations. When this happens, much more often then not, they will combine to form a larger dislocation, thus creating a larger area of microscopic weakness. This is the primary source of fatigue.

Point B is the limit of proportionality. At loads to the left of this point, the amount of stress is directly proportional to the strain. The constant of proportionality, E, is called the modulus of elasticity (also called Young's Modulus). It is a measure of how elastic the material is. (It is actually the inverse of that---stretchy materials have a low modulus of elasticity).

Point C is the elastic limit; it separates the elastic region from the plastic region. At loads below this point, the material returns to its previous shape. At loads above this point the material is permanently deformed.

Okay, now regarding the specific questions of bevans:

bevans wrote:As aluminum (maybe any material???) is cycled through load/no load situations it loses elasticity while breaking strength initially decreases before returning to near spec (despite being more "brittle")???

You are talking about two different phenomena here. The first is fatigue, and the second is work hardening.

Fatigue (load/no-load cycling) is due to the crack growth from the microscopic movement of dislocations. This happens above the true elastic limit (point A in the graph). The true elastic limit of a material is incredibly small compared to the elastic limit. I don't know what the actual value is for aluminum carabiners, but I would not be surprised if it was body weight. In practice, it is extremely difficult to even measure the true elastic limit.

Work hardening also is due to the movement of dislocations. The difference is that it is on a much larger scale, and occurs under plastic deformation of the material. Work hardening in metals tends to increase the strength of the material, while at the same time making them more "brittle." By "brittle," what we really mean is that the modulus of elasticity is increased. In any case, this is not happening unless you are subjecting the carabiner to such a load that it enters the plastic region, i.e., it isn't returning to its original shape after unloading.

Now to the OP

Neil R wrote:I am afraid because my line is usually anchored to a slung tree. This being the case, the carabiner is being pulled in 3 directions; 1 by the line and 2 by the sling around the tree. Therefore, it is being tri-axial loaded... And i have heard that this weakens carabiners significantly. So my question is this... does this weakening mean that the structure of the carabiner is weakened, and therefore not able to withstand the forces it was originally created for? Or, does the weakening mean that the failure point is lowered while being tri-axial loaded? And there being no long term affect on the carabiner?

I seriously doubt there is any long-term effect on the carabiners. When people say that tri-axial loading weakens the carabiner, what they really mean is that the carabiner is not as strong in that configuration as it is in its design configuration. The reason for this is two-fold: (1) tri-axial loading can actually increase the internal stresses, much in the same fashion as an American Triangle; and (2) the carabiner is designed to carry virtually all of the load along the axis of the spine. Tri-axial loading places loads across the spine resulting in bending forces. Aluminum is not as strong in bending as it is in tension (try stretching a paper-clip, then try bending it...which is easier?).

Aside from this: You don't need to load your 'biners tri-axially with your setup.
1. Put your sling around the tree like you do now;
2. Put a carabiner on each end of the sling (instead of just one on both ends);
3. Attach your slackline to each of these.

OR

3. Attach a short sling to each of these with another carabiner on the the other end;
4. Attach the slackline to that 'biner.

Interesting question here to consider--I wouldn't be overly paranoid to mix them but using other biners (like--booty biners is what I use for this) might give a warmer fuzzy feeling.
In regards to some of the discussion on fatigue and cycling of the biner, here's a paper online that has some actual test data of cycling a carabiner at varying loads:
web.mit.edu/sp255/www/refer…

These tests used all light D biners pulled in their major axis, so it'd be prudent to realize that the results couldn't be extrapolated to the exact tri-axial situation posed (though they include open gate cyclings, which shows effects of a different non-ideal loading.)

One other thing that's nice to know is that these tests are run at pretty high loads (8-20 kN for the closed gate tests). For the average backyard slackline (say one around 30' long), I've seen measured loads that never really exceeded 4 kN (2-3 kN max would be more of an average) while the line was in use.

For a less technical answer than others have provided, if you're asking the question then probably good enough to just dedicate those biners to the slackline setup for life and use others for climbing.

Kinda like if you say, well my harness is cut along the waist belt. It's only cut 1/3 of the way through. Do you think it's good enough? Maybe, maybe not but it would really suck to find out...

Clayton Laramie wrote:For a less technical answer than others have provided, if you're asking the question then probably good enough to just dedicate those biners to the slackline setup for life and use others for climbing. Kinda like if you say, well my harness is cut along the waist belt. It's only cut 1/3 of the way through. Do you think it's good enough? Maybe, maybe not but it would really suck to find out...

+1! Say you're using 4 biners (I don't really know slackline setups, but assume that would be more than enough), and you bought nice expensive ones at $15 each, is your life worth $60 to you? Even if they're not compromised, is the peace of mind in not having to worry about whether your biners will break worth $60 to you? Seems like a gimme answer, considering how much money is spent on climbing in general.

Thanks for the in depth response Bobby. Cool info.

Bobby Hanson wrote: This happens above the true elastic limit (point C in the graph).

You mean point A here don't you?

Bobby Hanson wrote:In any case, this is not happening unless you are subjecting the carabiner to such a load that it enters the plastic region, i.e., it isn't returning to its original shape after unloading.

If I remember correctly, the 'biners in the test I saw were being loaded way beyond a slackline setup's typical load. Sounds like a slackline would never approach load levels necessary to cause any change in the elastic properties of an aluminum carabiner.

Interesting stuff nonetheless.

Brian Scoggins wrote:There's actually zero data one way or the other about the impact long-term, high-load situations have on short-term, high-load resilience. It could very well be the case that carabiners used in a slackline are perfectly safe to climb on. Personally, I wouldn't want to find out the hard way, especially because I find bail biners I can throw into my slackline all the time.

I don't have the materials science background that others do, but it seems to me that in certain slack-line situations (for instance, the very long or very tight lines), you can encounter loads that may enter into that plastic-deformation region.

More importantly, the forces involved in slacklining are not as brief as in a lead fall. Like I said, I'm not a materials scientist, but I know enough physics to understand that the closer you get to an instantaneous increase in force, the more efficient your energy dissipation system has to be to deal with it. That's the whole point of dynamic ropes in climbing. I'm honestly not sure how close to the limit of proportionality (past which you do deal with work hardening) my carabiners get on my lines, and I'd rather not find out the hard way when my life is depending on them.

NO.

there is a new study right now in europe about the vibrations of slacklines actually damaging the molecular integrity of carabiners. pretty strange stuff. I'd keep them off your rack.

dude, biners are cheap. While this discussion is interesting (especially the post above mine), this is exactly the type of question that means you should buy a few new biners. The chances of those biners failing? unknown to moderate I would say(?) - consequences of it failing....? most severe?

bevans wrote:You mean point A here don't you?

Yes. Typo. Thanks. :)

SAL wrote:NO. there is a new study right now in europe about the vibrations of slacklines actually damaging the molecular integrity of carabiners. pretty strange stuff. I'd keep them off your rack.

SAL, can you provide a reference? I'd be interested in reading this.

Wish I could at the moment. I just heard this yesterday. I have to start to do my research on it asap. I will post more once i find out. basically a slackine carabiner broke while the line was being walked. that is related to other stories/theories i heard about military and lowering troops with a cable. that would obviously have much more vibration but they are thinking that the same effect can happen with slacklines.