Testing the break-strength of equipment is commonplace at Treetools. It's quite normal for us to pull rope tools and/or hardware apart until they break - all in the name of tree climber safety.
Last week, with help from Drew Bristow, we tested the new polyester core 8mm Armor-Prus from Donahys. Rather than pull a spliced eye-to-eye friction cord apart we thought it might be more appropriate to test a complete system, as it would be used in the field.
To replicate a common climbing configuration, two DMM steel oval carabiners were connected together with a 22kN webbing quickdraw (to represent the pulley).
The working end of a pre-loved length of Tachyon (donated by Drew) was knotted to the first carabiner with a Fisherman knot and threaded through a shackle at the opposing end of the testbed. This shackle represented the anchor.
The running end of the rope was secured to the second carabiner via a friction hitch (Valdôtain Tress or VT in this case). Representing the climber connection, the second carabiner was attached to the ram-shackle as can be seen in the photograph below.
Please note, in the test setup above, both the anchor and climber connection points CANNOT possibly be broken - that situation is quite different in real life!
The completed system was then loaded with Chain & Rigging's 150kN hydraulic ram. Try as we might we could not break this configuration.
How come? you ask. The answer: slippage, friction hitch slippage.
The VT starts to slip at about 10kN and continued to slip while pressure was applied. Have a look at the graph below - three different tests, all showing remarkably similar paths to destruction.
The only way to break this configuration was to place a stopper knot below the VT (which is not typical). Each major dip in the graph line depicts friction hitch slippage as the stopper knot was drawn into the 'legs' of the Valdôtain Tress.
As expected, all of the test setups broke in the same place; at the base of the Fishermans knot on the working end of the rope: 25kN, 28kN and 29kN respectively.
In real terms, these break-test figures are almost irrelevant. The climbing system would never be loaded in this way (with a stopper knot below the friction hitch).
The relevance lies in the consistency of the 10kN hitch slipping point.
Short of climbing on a damaged rope, or an incorrectly tied (or spliced) friction cord this system simply will not break, it will slip instead at about 10kN. Assuming your anchor and harness connection points are rock solid the rope will finally break at the working end knot (or splice).
The different time frames to break-point can be attributed to varying lengths of Tachyon. In future tests we will endeavor to be more precise with our measurements.
ADDENDUM: As a point of reference, safe maximum arrest force (MAF) in Europe is 6kN. In the USA and Canada it is 8kN.