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The Climbing Club • View topic - Car extraction with climbing gear

The Climbing Club

at the University of Washington
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PostPosted: Mon Sep 03, 2012 2:00 pm 
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UW Climber

Joined: Thu Aug 05, 2010 8:01 pm
Posts: 115
Location: UW, Seattle
I backed my ford ranger over an embankment near tieton saturday night.
I used 1/2inch webbing, a thin dynema sling, two carabiners, a section of 10.2mm dynamic rope, two ratchet straps and a come-along to unstick my car.
It took about an hour.
The set up was roughly like this-
oak tree as an anchor with ratchet strap webbing tied off to it. the climbing rope attached to the ratchet strap with a figure 8 on a bight. then a kleimheist attaching the dynema (and also the sling) and the webbing to the rope (4 turns of the kliemeist didn't slip a bit). the dynema/webbing was attached to the clip on the come-along. the other clip on the come-along was attached to another ratchet strap attached to the frame of the truck.

all of the above climbing gear was loaded with some fraction of the weight of the truck for some period of time. my question is what should i retire?
here are my thoughts:
rope: should probably cut off the section that was stretched out for ~1hr. its an old rope to begin with
slings: they're probably ok? not to dynamic to begin with
carabiners: also probably ok?

ok physics nerds what do you all think?

-Nate J


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PostPosted: Tue Sep 04, 2012 12:10 pm 
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Raging Alpoholic
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Joined: Thu Jun 14, 2007 10:15 pm
Posts: 812
It's kind of fun to engineer fixes like that using some of these knots that we rarely use. It's unfortunate that it killed some of your gear though. I'd retire everything if were you. The rope was old anyway, and one dynema sling doesn't cost that much to replace. I'd mark the biners as leaver-biners and just ditch them next time you need to fix up a rappel station.

I'm curious too about what the physics nerds have to say. :)


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PostPosted: Tue Sep 04, 2012 12:55 pm 
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Longshanks
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Joined: Wed Jan 25, 2006 7:47 pm
Posts: 927
Location: Denver
I'm no mechanical engineer, but I agree with your initial thoughts. The webbing and 'biners should be fine, but I might worry about the rope. To put this in prospective, 10 kN is 2248 lbs of force, so most locking 'biners and those dyneema slings (20 kN +) are rated to support your entire truck. I doubt you would have even approached these numbers pulling it out of a ditch.

On the other hand it's not that much gear to weigh against your life, so Val's more conservative approach would also be reasonable.


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PostPosted: Wed Sep 12, 2012 11:01 pm 
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Rodrigo
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Joined: Wed Oct 13, 2010 4:07 pm
Posts: 109
This is an interesting question. I will preface my longer response but saying I would likely retire all of the gear given that one's life depends on it. Disclaimer: I've studied this stuff a bit and am certainly not an expert. Disclaimer 2: This is long (I got excited), sorry.

Note, I do have a background in mechanical engineering and this materials stuff was my least favorite part. That said, it can still be pretty cool. One question that I hadn't previously thought about was the load rating for the biners. What exactly is the failure mechanism that causes companies to give their biners the load ratings? I did a quick search for this and didn't find an answers. What I would like to know is whether the ratings are based on deformation or catastrophic failure.

The rope: I would retire it for sure. I don't know much about the materials and that scares me.

Biners: To me, this is the most interesting part. I would retire them if it was me but after thinking a long time about it I can't really justify from a mechanical standpoint. Again, given a life depends on the biners and they aren't too expensive, it doesn't seem silly to put aside any logical analysis.

There are a couple different mechanisms the lead to failure of these types of materials. The first is a simple catastrophic failure that results from a large load. If the load exceeds the overall strength of the material, it will fail. This isn't surprising. However, the same type of failure can occur at a lower load than expected if there is some sort of imperfection. More specifically, if a large crack exists in the material any stresses will be concentrated at the imperfection and can lead to failure. In this case, clearly this didn't occur.

The second key mechanism is fatigue. Here minor imperfections in the material are subjected to cyclical stresses over many cycles. These periodic stresses cause a crack or other impection to grow. This growth increases stress concentration factors at the imperfection and in the long term they reduce the overall strength of the material. Since the biners were not subjected to many cycles. Typically a significant reduction in strength would only result from thousands to hundreds of thousands of cycles or more. Again, not really an issue in these circumstances.

I can't see how compressive loads could be important in this situation.

The most important consideration is my mind is the total peak load relative to the overall all strength of the material. For a metal, you have two critical measures of the stress: the yeild strength and the ulimate strength. If the total load places on the biner was less than the yield stress, any strains or change is shape would not affect that biner long term. That is, upon reducing the load the biner would return to it's original shape and in all likelihood only microscopic cracks that could be related to long term fatigue are created. If this is the case, the overall strength over a couple of loading cycles would not be decreased in any significant manner.

The key question relates here to whether the biners were subjected to forces that caused plastic deformation. Here you have stressed the material to a degree that results in a permanent deformation of the material. This obviously isn't good. If that is the case, you want to retire the biners.

In my judgement, and I could be wrong, unless plastic deformation occurred the biners are probably fine. I agree with Obadiah here, you probably didn't even come close to this unless you shock loaded the system (which I bet you tried to avoid at all costs).

On to the slings: I know almost nothing about these materials and would retire them. My biggest concern would be whether creep (slow elongation of the material when stressed continuously) occurs quickly. This could be a sign of the material getting weaker (or stronger). Although I said I'd retire them, my guess is they are fine and it would be awesome to see a test confirming that (having removed any other important environmental variables like UV).

I hope other folks have more comments to add...I'm looking at your Polagye.


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PostPosted: Wed Sep 12, 2012 11:04 pm 
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Rodrigo
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Joined: Wed Oct 13, 2010 4:07 pm
Posts: 109
By the way, any pictures of the setup?

This seems like a very appropriate time to go back to Physics 101 and draw some FBDs and consider the forces.


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PostPosted: Thu Sep 13, 2012 6:11 pm 
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UW Climber

Joined: Thu May 17, 2007 6:11 pm
Posts: 137
To answer some of Rodrigo's questions:

It seems that the values given on a carabiner give the ultimate tensile strength. In the test setup, the force is applied between two ground steel pins of 12mm diameter (see figures in the linked PDFs). The realistic situation, where the load is transferred to the carabiner via a rope or a sling, does not yield reproducible results. In fact, depending on where exactly inside the carabiner a sling is positioned, the geometry can be much worse than in the standardized test (even in the case of a longitudinal load) and a carabiner can break before reaching the specified load. Together with the comparatively low strength of a carabiner with the gate open, this means that they can break at pretty scarily low loads when the gate isn't fully closed. But that's a different topic.

In case anyone can read German, these articles are very interesting:




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