What weakens a canyon rope?  Spoiler: for PET and Dyneema, worry more about dirt than about water.

11/01/2024 update: more tests.

Briefly, a friend (Celes) dropped a 200’ Canyonprime rope (Sterling 8.5 mm polyester) into a pothole in Illusions Canyon, where it sat for 10 months, until retrieved by a nice canyoneer (Nick). The pool is deep and dark, and the rope was bagged, so the UV damage was nil.

Afterwards, none of her friends would rappel on that rope, even though there was no apparent damage.  She decided to sacrifice that rope for break-testing, along with another used section of Canyonprime (purchased in the same time frame) that had NOT been waterlogged. I bought some brand-new, never-used Canyonprime as a control, and we did figure 8 – to – figure 8 break tests. Three of the new (never used in canyons) samples were subjected to a brief dirtying process, described below. The results(tap image for full-sized graph):

Three observations:

…dry used vs 10 months underwater used -- no obvious effect within 1 SD

… used vs pristine new, 1/3 strength loss

… dirty vs clean new, up to 1/3 strength loss (note sand dirt may have more effect than gym chalk).

OK, how about dirt that you can't readily see easily? I used sand from our backyard for dirtying 5 samples, and Black Diamond climbers chalk (mainly hydromagnesite, other reports suggest particle size for 12-100 microns) for 1 sample.

Here’s the dirtying process for all materials—basically I worked along the rope and compressed each 2” piece along the axis, hoping to draw some suspended dirt into the sheath each time the compression was released. For the fine sand, probably only very small particles lofted into the supernatant water would have a chance of entering between the rope strands.

After that, I rinsed the rope segments to remove obvious surface particles.

After the tests, I took apart the new but “dirtied” Canyonprime and laid the sheath and core out (from the few cm near the break). There were sparse tiny grit particles on the inside of the sheath  – though the core was slightly grayer or browner than core strands from pristine rope. I did the same for my friend’s used Canyonprime, and had the same observations – slightly duller-grayer core strands, but any dirt particles were visible only with a macro lens.

Inside the sheath of the dirtied new Canyonprime, post break test (click or touch image for more detail):

The core strands of the dirtied new Canyonprime:

Inside the sheath of the the old used Canyonprime:

One the comments Celes got on her YT video was “we should get Dyneema ropes.” Well, I also tested pristine 1/8” Amsteel (MBS 2500 lbs) against very slightly dirtied Amsteel (backyard sand), with these result (eye-splice-to-eye-splice):

This what the 3 dirtied pieces of Amsteel looked like after breaking:

The break-test method is shown in the link. I used my Jeep in 4low to pull the rope, against a solid anchor (a thick galvanized pipe set in 16” concrete), through a linescale 3 sampling at 640 Hz. There is buffer of white, stretchy, very strong nylon rope at my jeep. All soft goods are connected with softshackles. The softshackles are dual strands of 6mm Dyneema, sheathed in 1” nylon webbing. NOTE that the test specimens always broke within the figure 8 knots (for PET; Dyneema broke in the middle), never in the loops, so it is irrelevant that we did not use metal shackles.

Ropes used in canyoneering and climbing are expected to lose strength, but the users are unclear on which process causes the most degradation, or how much and how quickly the rope is degraded.

The role of mineral dirt is well-known, though it is hard to say what constitutes a “realistic” test. The work at Teufelberger showed that a fairly simple process could reduce rope strength by up to 50%, and washing with detergents did not restore the strength, even if it made the ropes look nicer – the speculation is that once mineral grit enters the core, it cannot be easily removed. Other comments can be found at this link.  Powerwashing is claimed to clean the ropes without driving particles into the core, but, there are no break tests on the washed ropes. Compared to Teufelberger’s experiments, my dirtying process was not aggressive and was completed quickly. The ropes were softly brushed  and rinsed to remove obvious grains before the PET samples were terminated with figure 8 knots (Dyneema samples were terminated with eyes before dirtying, and dirtied between the splices). These samples were dramatically weaker than the pristine Canyonprime sample in my tests. They differed little in appearance, except the white strands had been turned duller and slightly browner.

Tests on 3-strand pure polyester rope.

I purchased 1/4" (~6mm) twisted 3-strand polyester rope from SGT.KNOTS, quoted as 1490 lb tensile strength. I felt this might be most analogous to polyester core fibers, while still being manageable. I made 2 sets of 3 for tests, 3 samples clean, and 3 samples very slightly dirtied with fine sand from the backyard as described above; the samples were rinsed and brushed. One sample each from the ditried and clean sets ws tested to breaking with a capstan-clamp-type set-up, with my jeep providing the force. The remaining samples has sewn eye terminations (100 lb Spectra thread, at least 70 stitches each termination). Here are the results; once again the slight dirtying seeems to have markedly reduced strength. From my previous experience, 3-strand has a much wider scatter of breaking strengths, compared to braided, consistent with these results.