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?