20 May 2012

Are knotted Dyneema® slings good for climbing?

DMM have carried out some very interesting static load and drop-tower tests to find out the strength of knotted Dyneema® slings.  Dyneema® is normally found in stitched slings - DMM wanted to find out how slings would fare if made by tying your own knot in a piece of  Dyneema® from a reel.

As the DMM article mentions, Dyneema® is actually just a brand name for Ultra-high-molecular-weight polyethylene (wikipedia) which is a type of polythene (wikipedia) which has very long molecules, which makes it stronger and tougher than polythene with shorter chained molecules. The size of the molecules is key because polythene with shorter molecules will have massively different mechanical properties.

Dyneema® also has a relatively low melting point (between 144 and 152C) and also has a 'low' coefficient of friction. DMM suggests that heat in the knot during dynamic testing could explain why the knotted sling failed at significantly lower loads than the sewn slings. DMM believes this is supported by the result that frozen and wet knotted slings were stronger than dry ones - perhaps suggesting that the water/ice in the knot helps to reduce heat build up.

Based on the results of the tests, DMM strongly recommend against using knotted  Dyneema® slings as they consistently failed at lower loads compared to sewn counterparts.

Take a look at the full article here. The video gives a good overview.

Image from DMM article

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