Cumulative Stress on Gear

[Toast]

So you take a whipper on a new cam. There's no visible damage. Do you treat it as suspect and toss $50 down the drain or keep it for another fall some other day?

 

I think the CE tests are static pulls until failure. Most leader falls don't ever approach the rated breaking strength, but doesn't that stress add up? Does anybody know whether cumulative stress from multiple impacts is tested? Isn't it relevant?

 

Thanks

[fern]

 

cyclical loading of carabiners:

 

http://www.mit.edu/afs/athena/course/16/16.62x/www/Graham_Jon_622.pdf

 

page 18 has a summary chart

 

[Dru]

i think if you are gonna toss a new cam with one fall you should toss it my way

[EWolfe]

Maybe a schedule of depreciation?

[lummox]

but doesn't that stress add up?

unless one of the impacts starts opening up a defect in the metal no fukin way. at least that is what i understand of fracture failure in materials. then again aluminum is wierd shit maynard. how bout 'dont fall'?

[high_on_rock]

Good question Toast, when you come up with a final answer why don't you post it as a final to this thread, from which the rest of us may learn.

[catbirdseat]

I suggest that you write the folks at Climbing Accidents in North American Mountaineering. They may have a database that would allow a statistical treatment. Here's your contact info:

 

John E. (Jed) Williamson

7 River Ridge Road

Hanover, NH 03755

e-mail: jedwmsn@sover.net

[texplorer]

Retire your cams!!! Send them to :

Texplorer's Home for Fallen Cams

420 Red Cougar Drive

Portland, OR 97203

[catbirdseat]

cyclical loading of carabiners:

 

http://www.mit.edu/afs/athena/course/16/16.62x/www/Graham_Jon_622.pdf

 

page 18 has a summary chart

 

To summarize:

 

Cyclic Load Range [kN], Average Cycles to Failure

8, 10939

10, 5533

12, 2959

14, 1556

16, 1182

18, 751

20, 263

 

While I would suppose that it is possible for someone to take 263 falls of 20 kN on on the same carabiner, I think it is unlikely anyone will approach that number. They claim a person weighing 200 lbs and falling 3 m will generate this force (20 kN), without specifying a fall factor.

[colt45]

Plus the loading was done in 0.5 second intervals. I know that in rope drop testing they let the rope "rest" between drops, which apparently makes a significant difference in the number of falls held. So biners might actually be even more durable in real life.

 

There are a few interesting tidbits in the report:

 

*In order to show evidence for long-term deformation of carabiners, they cite a study where a load <=2kN was applied 500,000 times in succession. This did bend the tested carabiner...by 1/10,000th of a centimeter!!

 

*To quote the article: "Climbers normally attach a carabiner to the rock face every 1.5 meters during their ascent. This means that the furthest distance the climber will ever fall is 3 meters." hmm...was this study actually done by a climber?!

 

*Look at figure 5 on page 13. In their demonstration of a "worst-case scenario fall" it is clearly not worst-case scenario, as the fall factor in the image looks like less than 0.5.

 

*And the best part is the first paragraph of the conclusion:

 

"Only so much could be accomplished within the amount of time allowed for this project. Furthermore, any good experimentation project asks more questions than it answers. Many new questions arose during the course of this project that could potentially form the basis of further work."

 

This is an interesting paragraph in that it could be appended to ANY study of ANY topic! It was a nice way to lengthen the manuscript though without needing to generate additional data to fill the space.

 

But weak conclusion statements notwithstanding, it's always nice to see some data to know how gear might be expected to perform over time.

[catbirdseat]

I'm not a metallurgist, but I believe that an aluminum carabiner would recover very quickly, unlike a rope. Ropes get their springiness by both the elasticicity of the individual stands as well as from the weave. By forcing the strands to bend around one another, more elongation is possible when stress causes a straightening of the individual strands. There is a certain amount of slippage that must occur between strands during relaxation and this is what takes time, I would surmise.

[Gordonb]

I used to do metallurgy as a mechanical engineer and Aluminum will fracture when stress cycled, there is no stress level that is safe, unlike steel. Now very low stresses will put the number of cycles in the billions or higher so it is not that dire.

 

All metals will stress relieve when allowed to rest. A couple of hard falls over the weekend, if not too near the breaking point, will mostly be relieved by the next weekend. I would not worry about cyclic loading, but shock loading. Did the fall overload the cam somehow and cause some sort of damage that has weakened the cam.

[catbirdseat]

What is it about aluminum that makes it behave differently from steel with regard to stress cycling? This effect accounts for why airplanes have to be retired after so many takeoffs and landings. Sometimes the wings fall off.

[Gordonb]

It is just one of the properties of aluminum, cracks also propogate faster.