Sonntag, 28. Januar 2018

cyclyc loads on semistatic ropes

I receivend a request from Gethin:
Hi Walter, ...  I’ve still the burning question relating to whether /how a semi static rope may degrade following repeated exposure to relatively high (4kN) loads. This relates to the use of Tyroleans in adventure (not rope access) work. I was wondering/hoping I could persuade you to run some tests in a batch of type a rope. I’d be interested to test a sample of ropes, all of the same age and type, one set initially loaded to 4 kN for say 15 seconds and relaxed 10 times, a second batch exposed to the same proof loading but for 20 times, and a final sample pulled and relaxed 50 times. Then for all samples (including a batch that has not been subjected to any loading) to be pulled to destruction to see if there’s any dip in performance. ...

I already had done such tests during my research, where I found out that if you load a rope with 80-90% of the breaking load you can cycle load it about 150 time until it breaks.

Therefore I thought that 4 kN is not a "relatively high load".

However, I did the tests.

The breaking load of the 10 mm Edelrid semistatic rope was 20 kN in the figure-8-knot.

Then I did cyclic loads:
5 kN load, drops to 4 kN for 15 seconds, relax to 0,5 kN for 10 seconds.
After this I did the breaking load test.
The breaking load was 19 kN.

20 times: 19,5 kN.
50 times: 18,5 kN.

Remember: The breaking load of a knot can be +/- 20%.

What I did afterwards:
Three pieces of rope with the knot that did not break were left. They had been loaded almost to their breaking load.
I took them and loaded one of them with 10 kN and 2 with 15 kN. 
The 10 kN held 500 cycles, then the breaking load was still 19 kN.

The 15 kN loaded knots held 30 and 50 cycles.

My suggestion for tyrolean traverses:

I f you open your knots after you dismantled a tyrolean traverse and check the ropes before the next construction, you can use them as long as the core is not visible or damaged.

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