Sunday, 11 April 2010

Dyneema Slings breaking under direct loads / falls

A couple of months ago there was a long discussion on UKC about how dyneema slings performed when loaded directly. There are a couple of common situations where this could happen:
  • Clipping directly into a belay with a sling, not keeping it under tension, allowing slack to develop and then slipping or falling onto the belay i.e. on a mutipitch route, clipping into the belay with a sling and then moving around to get comfortable/keep the climber in sight. If you then slip or have to hold an unexpected fall the belay will be shock loaded via the sling.
  • Working a move on a route whilst on the lead and using a sling to keep yourself in position, then trying the move with the sling still attached to the anchor. If the rope is slack and you fall you will be falling directly onto the sling.
The guys at DMM had been meaning to perform a series of tests to highlight the huge forces that can be generated in these and similar situations, but the poor weather and the fact that the drop tower is outside put the plans on hold.

The arrival of spring allowed DMM to dust off the drop tower and invite Ray Wood along to film a short video article that should appear on the DMM web site fairly shortly.

Dyneema and nylon slings were both tested using a 80 kg mass in a variety of situations that invoved anchors being loaded directly via dynamic force applied to the sling. Slings of different lengths were tested both with and without knots in the system. The force at the anchor point was measured with a load cell.

The nylon slings generally were a lot more forgiving than the equivilent length of dyneema sling because there is more natural stretch in the nylon fibres that can help absorb energy and reduce impact forces (Dyneema slings are a mix of Dyneema and nylon fibres, but the Dyneema fibres tend to characterise their performance).

The loads generated even with relatively little slack in the system were fairly massive. A fall factor 1 using an unknotted 120cm x 11mm dyneema sling registered 25 kN (surprisingly the sling survived, but whether the climber or belay would have is another matter), a fall factor 1 with an unknotted 11mm x 60cm dyneema slings generated 15.5 kN.

Adding a knot into the system severely increased the risk of the sling breaking i.e. a fall factor 1 onto a 11mm x 60cm dyneema sling with a loose overhand knot in it broke at 11.6kN when loaded.

A test was also performed with a 25cm Dyneema QD sling with carabiners attached - in a fall factor 1 fall this generated 12.5kN.

The tests were simplified and there are always parts of the chain that will absorb energy (the climber/the harness), but the forces are still pretty big considering that nuts and cams max out at 12kN and harnesses are tested to 15kN.

There is risk that the tests will be taken the wrong way and people scream "Dyneema- danger!", but it is all about understanding advantages and disadvantages. Dyneema can be used to make lighter, thinner and less bulky slings compared to nylon plus it is very UV resistant and very resistant to being cut, but it does have disadvantages - it has less inherant stretch and melts at relatively low temperatures (plus it tends to be a bit less durable).

It is easier to mess up with Dyneema, but knowing its weaknesses allows you to act accordingly.

You would have to be a bit of a numpty to get into any of the situations that will be outlined in the forthcoming DMM video, but anyone one can be a numpty at times.