Dyneema failure

[Tim (Slowhike) Garner]

I learned something this weekend.

I was excited about using a set off Dyneema webbing straps I had been given for my hammock.

The set of two straps with two short toggles made form arrow shaft (if I wanted to experiment with the Marlin Spike Hitch) weighed 1.7 oz!

To my surprise, one of them snapped.
I learned later that Dyneema cannot take very much heat.
I had used a Becket Hitch and probably not pulled it as tight as I should have.
When I put my body weight into it, it pulled tight and snapped.
As you can see from the close up, it fused a small piece of the Dyneema webbing to the continuous loop.

[Tim (Slowhike) Garner]

[Tim (Slowhike) Garner]

[Jerry Adams]

I don’t understand the connection to heat

When you got into it and applied tension it generated heat from friction?

[Matthew / BPL]

It’s such a weird failure. If it was heat/friction I would think these failures would be pretty common. Time for a repeatable test maybe!?

[Alan W]

Thanks Tim!

We’ll see what Tim says about amount of slipping and heating versus possible jerking (see below.)

For another hypothesis besides heat, Google search “impact loads on dyneema tensile strength”.  There is so little yield (give, stretching) in UHMWPE fibers that the molecular structure deteriorates markedly with impact loads.

I’ve experienced 1.8 mm Samson cord snapping when I tried to jerk a Vargo tent stake up after I had unknowingly driven it into a juniper root in very hard & rocky ground. There’s no way I am strong enough to have broken it with a steadily applied pull, which I indeed had tried several times before jerking and snapping on that 1st try.

ASTM tensile testing is done at relatively slow strain (stretching) rates, and UHMWPE is remarkably strong per unit weight tested this way. (Metal impact resistance is a completely different testing method.)

Add another reason to why climbers don’t want the lightness of Dyneema in their ropes, in addition to knot creep under steady loads and lack of stretch cushioning during arrest loading events.

[Jeff McWilliams]

Add another reason to why climbers don’t want the lightness of Dyneema in their ropes, in addition to knot creep under steady loads and lack of stretch cushioning during arrest loading events.

Yes, but climbers use dyneema slings ALL THE TIME.  They’re used for building anchors, they’re used to construct alpine draws.  Some harnesses use a loop of sewn dyneema for the belay loop instead of traditional nylon tubular webbing.

If dyneema failed under impact loads, we’d know about it.  The HowNOT2 guys do drop tests on shit ALL THE TIME, plus dyneema slings and alpine draws are UIAA rated.

 

[Tim (Slowhike) Garner]

heat/friction is what was suggested by a cottage vendor that works with lightweight materials.

So, my best assumption is that even though I sat and laid down easy, the tightening of the Becket Hitch under load created heat and friction.

I was totally surprised by it, so I hope to learn more about what happened.

[Jeff McWilliams]

Some shock load testing of dyneema slings by mammut:  https://youtu.be/Vrgadjo9niY

Dyneema slings can fail as they show in the video.  It can depend on how you build your anchor, what type of knots you tie in the sling, etc.  Also, keep in mind that the angle of your dyneema hammock straps relative to the tree can multiply the forces as shown in this diagram:

In climbing, we’re instructed to build anchors with an angle between the two components of 90 degrees or less.  For hammocks, you’re told that an optimum strap angle is 30 degrees, which results in a 120 degree angle between the straps.  If you poorly set your hammock straps to a greater angle, you can from the diagram how this would multiply the forces on each dyneema strap.

I also have no idea what the rated breaking strength of dyneema hammock straps is.  Has anyone tested?  Is there variability?  Is it possible that the dyneema straps you were given were poorly made or “counterfit” from some shady producer in China?

I also don’t know if anyone has done break testing to see how a becket hitch weakens the sling relative to common climbing knots like the overhand, figure-8, and clove hitch.

It’s possible that your failure was a combination of things:  strap angle resulting in multiplied forces, use of a becket hitch significantly weakening the “anchor”, not dressing the becket hitch knot properly before stressing it, and maybe even you abruptly sitting down in the hammock to shock load the system.

[Dan]

Or maybe just a defective piece of material.

[Slimbobaggins]

I really want to adopt the Beckett hitch setup to shave weight off my straps… but man it seems finicky to me, and this failure is one more reason why I’m dragging my feet on it.  It would definitely shave 4 ounces off my setup, vs poly straps and dutch buckles.

[Jeff McWilliams]

I guess I”m looking the other way, Slimbo.  With dyneema straps and the becket hitch, you have to account for the fact that it’s going to “slip” a little as the knot tightens.  Jeff Myers of “Myers Tech Hammock Lab” is a big fan but as a casual hammock camper I’m just not very proficient at it.  Dutch’s new beetle buckles and straps or Superior Hammock’s buckles and hammocks look awfully tempting for their ease of use and adjustability.   Or Dutch’s new light weight daisy chain straps, possibly combined with the Mantis hardware may be a good options too.

I guess that’s the thing with Hammocks.  There are so many different ways to configure the suspension with different pros and cons.

 

[Kevin Babione]

I hope you weren’t hurt when you fell…Many times in PA I’m forced to put my hammock above rocks (leaving the cleared spaces for my tenting friends).  I had a cuben fiber hammock that I was trying out that was fine the first night but the second night it split neatly in half and deposited me on the soft duff underneath (I had anticipated that failure was an option) – I was just happy that my underquilt didn’t tear.

My setup has remained the same for more than 10 years…I have two poly straps (one 8′ and the other 10′ to accommodate bigger trees) and use a Marlin spike hitch and whoopie slings.  It may not be the lightest, but as I become less young, I’m happy to have something that simply works for me and don’t need to save another ounce or two for something else.

[Tim (Slowhike) Garner]

No pain from the fall. I landed on a thick layer of leaves.

Jeff Myers is sending me 3o’ of the Polyester UHMWP.

I will cut and sew loops and try the Marlin Hitch this time.

The set of two straps with two short toggles made form arrow shaft weighed 1.7 oz!

My friend Two Speed who gifted me the first set (above) uses the Marlin Spike.

[JCH]

For my Chammock hammock chair I’ve had good luck with the 1” spider web 1.5 that Dutch sells. It doesn’t reliably hold a becket hitch and “collapses” into more of a rope than a strap after a couple uses, but it’s strong, light and doesn’t stretch. Works great with a couple of carbon fiber toggles.

[Author]

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