More of an idle musing than a serious issue I think (hope) but something I came across at work today got me thinking about fretting fatigue and karabiners. In my experience most cavers used steel maillons up until relatively recently when the trend has been towards alloy karabiners. It seems to me that we are using karabiners in a way that is fundamentally different to their ?normal? use in climbing, perhaps with the exception of their use on fixed ropes on big mountain expeditions. In climbing the krab takes one-off loadings on an occasional basis but in caving we subject them to repeated cyclical loading while in contact with a steel anchor. It seems likely therefore that there will be fretting/galling at the contact point and I guess most krabs will be used in the same orientation each time.

Now with fatigue limits of the order of millions of cycles at caving loads it would take some serious prusiking to get close the failure limit of aluminium alloys but I wonder if fretting fatigue or corrosion could reduce the fatigue limit? I?ve had a bit of search and can?t find any work on this.

A I said I think the FoS is such that a dramatic failure is extremely unlikely but just interested to know if this is something that anyone has investigated.

I was all keen on replacing our clubs maillons with krabs because they are quicker and I am lazy, but there was a thread before about cyclic loading that kind of put me off the whole idea. Sure they are fine as you say re. FoS, but an interesting point regarding the additional fretting fatigue.

I am no fan of crabs of any material for pitch rigging, and dont rue carrying the extra weight of an item that inspires me with extra confidence. I have had problems however with stainless maillons.

The OP echoes my own perception of the intended purpose of krabs entirely.
Just watching them as someone comes up a pitch rigged in that way gives me nightmares.

Try http://web.mit.edu/sp255/www/reference_vault/Fatigue_Presentation.pdf

As far as I'm aware, there have been exactly zero carabiner failures in caving (at least, carabiners in sane condition used sensibly).

I would suggest people worry about the things that do cause accidents, which are almost exclusively human error.

Modern equipment has been improved to the point where it's incredibly safe. The human brain has not enjoyed a similar improvement.

So what happens if they get accidentally loaded on the gate?

royfellows said:

So what happens if they get accidentally loaded on the gate?

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Depends on the kind of gate-loading.

Normally you're just looking at cross-loading. For example, this could happen when you sit down onto your descender. This is best avoided as the carabiner is much weaker (say, 8 kN rather than 20+), but that's still plenty strong enough. Potentially there is a risk of detachment, however. Using a locking carabiner greatly reduces this risk but does not eliminate it.

Cross-loading shouldn't normally happen with rigging crabs. It's a good habit to always look behind you as you pass rebelays, pitch heads, etc.

Inwards loading against the gate is very dangerous, since the carabiner can fail under body weight. There is no rating for this; only a few carabiners have a rated gate push-in strength, and even that doesn't necessarily cover a levering action. This video demonstrates a descender breaking its krab. This is unlikely to happen with a bobbin descender, but I wouldn't completely rule it out.

(Note this is not a good reason for using a maillon on your Stop/Simple. Petzl forbid this due to the risk of the maillon opening the descender sideplate. At least one death has been attributed to this.)

This could also happen with bad rigging, if the gate is being pressed into the rock. Even loading the spine against the rock is...concerning. If you must rig in this way, steel maillons are much more suitable. Alternatively extend the anchor with a wire trace or dyneema.

Bob Mehew said:

Try http://web.mit.edu/sp255/www/reference_vault/Fatigue_Presentation.pdf

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Thanks Bob, that's exactly the thing    I thought someone would have done something like that.

If carabiniers couldn't cope with the repeated cyclical loading generated by prussiking, would Petzl sell what are essentially carabiniers (omnis) to replace central maillons?

Sent from my XT1700 using Tapatalk

And of course if you fail to do a a screwgate, it is still full strength (provided the gate doesn't open, and strong enough even if it does), unlike a maillon...

If any one is interested, then BCA has a tensometer capable of exerting 45kN and measuring mm sized deflections.  Or a good topic for a final year undergraduate project.  The point Mike was making underlies Archimedes quote of "Give me the place to stand, and I shall move the earth".

royfellows said:

Just watching them as someone comes up a pitch rigged in that way gives me nightmares.

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Why, what do they look like? I've never noticed anything scary while watching the krabs as somebody prussics.

Speleotron said:

royfellows said:

Just watching them as someone comes up a pitch rigged in that way gives me nightmares.

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Why, what do they look like? I've never noticed anything scary while watching the krabs as somebody prussics.

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You can watch them flex with every prussik!

OK fair one, I never noticed that. So the idea of the OP is that, while the flexing is OK, it might not be OK after loads and loads of cycles?

Personally as a climber I sometimes wonder why all pitch rigging is always done with screwgates when snapgates should be acceptable, since any single bolt failure should be tolerated...

I can see the benefit of the krab as something to clip into and the general reassuring security of a screwgate though...

MarkS said:

Speleotron said:

royfellows said:

Just watching them as someone comes up a pitch rigged in that way gives me nightmares.

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Why, what do they look like? I've never noticed anything scary while watching the krabs as somebody prussics.

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You can watch them flex with every prussik!

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Yes, and however I am assured that this is OK, I personally find it scary.

As I usually explore alone, I am accountable to no one else but myself and use equipment in a way that I feel personally comfortable, which in turn reinforces my personal confidence.

andrewmc said:

...since any single bolt failure should be tolerated...

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Ah.  Some pitches have single anchor rebelays.  And of course links on yourself are 'single' not 'double'.  There has been at least one accident in the past where a guy managed to open his crab gate when crabbed directly into an anchor and fell.  (I can't find a link to show how this can occur.)

TheBitterEnd said:

Bob Mehew said:

Try http://web.mit.edu/sp255/www/reference_vault/Fatigue_Presentation.pdf

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Thanks Bob, that's exactly the thing    I thought someone would have done something like that.

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Just to go back to the study linked to above, it's interesting but I'm not sure it's especially relevant to this discussion. In their lowest-force gate-closed tests, they cycle the repeated loads up to 8 kN. Let's say a caver + kit + tacklesack weighs 120 kg, which puts a force of ca. 1.2 kN on a rope. Assuming poor prussiking technique and rigging from a single anchor, that force could be amplified to about 2.5 kN1 whilst climbing a rope, which is only about a third of the minimum force they tested in that study.

From the 8 kN tests, the carabiners tested survived an average of about 11,000 cycles, and the graph appears to be levelling off as the force is reduced. Testing at lower forces is described as, "prohibitively time consuming", suggesting many more cycles would be required for the sorts of forces SRT would impart.

1 page 58 of http://www.speleo.no/redning/Life%20on%20a%20line%20part1.pdf

Didn't someone drop down a 40m shaft (and survive) cos a snapgate cowstail opened (Jules rings a bell as a name?? maybe - some pics somewhere online) and also didn't someone die when they fell off a traverse line when their safety cord to the jammer disconnected (Switzerland maybe, a lady caver?).

Slightly off topic, but snap gates, whilst I have considered rigging off them, do have some issues.  See low anchor warning in CNCC rigging guides for example.

MJenkinson quote:

See low anchor warning in CNCC rigging guides for example.

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I wonder if this is this the example that Bob's referring to just above?