Testing Rope Sheath Failure

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Testing Rope Sheath Failure

Postby KeyserSoze » Oct 10, 2014 8:24 pm

After watching a video from PMI demonstrating rope sheath failure from a simulated shock load - video link is below - I became very concerned about the integrity of the PMI ropes we've been using. At 1:04, the video shows the demonstrator making a single vertical cut down the rope, not more than 8 inches in length, and the sheath immediately slides off the core like a stick of butter after approaching the croll about 5 or 6 inches away. After a discussion about this on cavechat, I decided I wanted to test the rope sheath failure for myself so see how it would react. I apologize that I may have raised more questions I've answered, but I think people may find the results interesting anyway.

PMI demonstration video
https://www.facebook.com/video.php?v=711504208888564


Below is a video of our test. I cut the rope vertically several times and was surprised to find that the sheath did not slide off like butter at all. It held firm as a matter of fact. I notice now that I did make the cuts higher above my croll than they did in the video, but you will see that I did eventually move the croll up higher and even bounced on it. We were only able to slide the sheath down after the ascender was placed on the core of the rope directly above the cut sheath.

https://www.youtube.com/watch?v=ym8DHFkNbOI&list=UU1lZUN_Vwae8OBYx56iZoTg



While we were at it, here are a couple of other things we tested.

Rope sheath strength test. I was surprised to find that the rope sheath holds together quite well on it's own and will carry the weight of a person.

https://www.youtube.com/watch?v=PiWZEmF3dMA&list=UU1lZUN_Vwae8OBYx56iZoTg


Ascender on kermantle test. The ascender held tight as long as the cord stayed in the center. If the strands move too much to the side, it will slip.

https://www.youtube.com/watch?v=rQEFFsZ0D3Y&list=UU1lZUN_Vwae8OBYx56iZoTg


We did some other tests, which we neglected to film, in which we cut the rope sheath much further above the tail end. The sheath did not bunch up into a ball or fall apart as predicted. In fact, we were able to climb and even rappel past the break in the sheath with very little issue at all.


In conclusion of our testing, I will have to say that I have gained back some confidence in the PMI pit rope. This test was not performed in a scientific manner, but it raises some scientific questions that I find interesting.

Question: why did our test seem to perform differently than the one from the PMI video?

Certainly I'm not suggesting that PMI played any trickery. I can see several other important factors at stake: The length of rope under the croll, the type of croll, the depth and length of cut, how close the cut is made to the croll, the type of rope, the age of the rope, and whether the rope has been used / washed.

I hypothesize that the age of the rope and the fact that it is used could be the most important factor. The rope we used in this video is 6 years old and has had heavy use. I've been told that PMI rope gets stiffer and stronger after getting some use and washing. I would like to know the extent to which that is true. What do you think?
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Re: Testing Rope Sheath Failure

Postby GroundquestMSA » Oct 11, 2014 9:42 pm

KeyserSoze wrote:In conclusion of our testing, I will have to say that I have gained back some confidence in the PMI pit rope. This test was not performed in a scientific manner, but it raises some scientific questions that I find interesting. Question: why did our test seem to perform differently than the one from the PMI video? Certainly I'm not suggesting that PMI played any trickery. I can see several other important factors at stake: The length of rope under the croll, the type of croll, the depth and length of cut, how close the cut is made to the croll, the type of rope, the age of the rope, and whether the rope has been used / washed. I hypothesize that the age of the rope and the fact that it is used could be the most important factor. The rope we used in this video is 6 years old and has had heavy use. I've been told that PMI rope gets stiffer and stronger after getting some use and washing. I would like to know the extent to which that is true. What do you think?


PMI's track record suggests that there's no reason to lack confidence in their rope. They still need to push new products. Call it advertising instead of trickery if you want... is there really any difference?

The type of rope in the PMI video seems to me to be an important factor. They were careful not to claim that it was regular PMI, only "typical kernmantle rope". PMI does, by the way, stiffen with use, and washing/drying. I can't imagine how it would strengthen, but the sheath may shrink and tighten around the core.

How a vertical slice in the sheath is an adequate simulation of an ascender shock load I'm not sure. It wouldn't be hard to actually shock load the rope for a test.

While sheath slippage has been an occasional issue for cavers, this looks like another example of a salesman inventing a problem that can be solved by his new product. The demonstration reminds me of the goofy ads for fancy water hose that show a poor lady being wrestled to the ground and strangled by her possessed conventional hose. Even if the demo was completely legitimate, how likely is any caver to shock load an ascender?
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Re: Testing Rope Sheath Failure

Postby Tim White » Oct 16, 2014 11:38 am

While I'm no expert on all PMI rope, I can say that I do not know of them making a white rope with blue tracer like the one here. I'd bet that the "typical kernmantle rope" is just that...some off brand that failed due to poor construction.


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Re: Testing Rope Sheath Failure

Postby Anonymous_Coward » Oct 16, 2014 2:45 pm

It sure looks a lot like New England KMIII. If so, I would not say it is typical. In my experience, the KMIII is susceptible to all manner of sheath problems, including glazing and poor abrasion resistance.
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