[jwi]

As per title, my google fu has abandoned me. Does anyone have a force-elongation graph for semi-static 11 mm kernmantle ropes?

I could ask a friend to make some measurements but it would be great if someone already did this and put it in public domain...

[danm]

I'd assume it's fairly linear in the normal working range, so you can estimate using the static elongation given for the rope and extrapolating. The given figure is the % change in length from 50 to 150 daN load. You can substitute daN for kg unless you're feeling fussy, so my fag packet says Petzl Axis 11mm is 3% at 150kg and 6% and 250kg.

What is your reason for wanting to know the answer? Your question is the sort which used to worry me when I worked at Lyon, it usually meant someone was attempting something sketchy!

[SA Chris]

Will he get to the ground on stretch on a long abseiling I'm hoping?

[duncan]

Will he get to a belay anchor half way down the Verdon Gorge on stretch on a long abseil I'm hoping?

Some things have to be tested empirically.

[jwi]

What is your reason for wanting to know the answer? Your question is the sort which used to worry me when I worked at Lyon, it usually meant someone was attempting something sketchy!

I want to know about how far I can assume that Hook's law describe the relationship between force and elongation. I am doing a simple macro to give a rough estimate how much a suspended weightless rope loaded with something along it will dip. It will not be used for safety calculations, just to give a quick initial estimate for setups, but I wanted to add a layer checking when the configuration abuses the assumptions so much that it makes no sense to try to set up the system.

Will he get to a belay anchor half way down the Verdon Gorge on stretch on a long abseil I'm hoping?

Some things have to be tested empirically.

Quite

[Johnny Brown]

I think Dan is probably right about the normal working range being broadly linear, but it certainly isn't linear on the way there. I.E. I would expect (much) more stretch going from 0-50kg than 50-100kg. And likewise progressively less as loads increase beyond that.

As an aside, the most interesting piece of research I've seen in recent years was on cutting resistance vs load. That relationship has generally been assumed to be broadly linear, but isn't at all - ropes under high tension lose almost all resistance to cutting. It makes a strong argument for placing an absolute limit on any tension loads for hauling or rigging, and ensuring if falls are possible the rope is well away from any hazards at the moment of peak force. Not always easy in rock climbing....

[danm]

You could set the upper bound as the SWL then you don't need to go to the effort of producing a graph, just assume it's linear up to that point. Used as PPE the SWL is one person but for hoisting I'd apply a safety factor of 5 to the MBL and use that as the upper limit?

Yeah the cutting research by Edelrid is really interesting, I think it means guides can't lower both clients together at the same time if there is rock around, which probably makes using a stacked abseil the quicker/safer option.