Introduction

Today we are going to look at an issue that has popped up with our recent Spider Silk MK4 hi-tech slackline webbing. There was an incident where someone was using a LineGrip to tension a longline rigged on this webbing and the LineGrip tore through the sheath and broke the webbing. This occurred at a tension around 6.5 kN, which is far below the MBS of the webbing and below the WLL of the LineGrip.

First, I want to discuss the mechanism behind this failure. What happened was the tilt in the LineGrip when it is under tension, puts a large amount of stress on the webbing right where it exits the device (Line-side, not anchor side). At this point, when tensions are high, the Linegrip will cut through the polyester sheathe on the webbing, exposing the Dyneema threads.

In the lab, this does not result in a full line failure as there is very little potential energy on the sheath in the short distance of the test. However, on a longline, there is a LOT of potential energy in the line while you are tensioning. If the sheath breaks, that potential energy will turn into kinetic energy, producing a TON of heat. Enough heat to melt through the Dyneema core, causing a premature failure of the webbing.

When we heard about this incident, we rushed to the lab to see if we could reproduce it. Sure enough, every single sample we tried with every webbing grip device we have, the webbing broke at that exit point well below the MBS of the webbing (no full line failures in the lab). Beyond that, we tried another Polyester Sheathed Dyneema webbing that's also quite popular in our community and the same thing occurs.

This leads me to believe that polyester-sheathed Dyneema webbings should not be gripped at all with any type of webbing grip!

If you have any sort of length in your line and a sheath failure occurs, you will have a full line failure! Instead of using your high tech webbing at the tensioning end, add a low tech polyester or nylon segment as your last "Rigging Segment". This will alleviate pretty much all concerns with rigging with high tech and hardware compatibility.

We tried several techniques to try and mitigate the occurrence of the sheath destruction with grip usage, but nothing had a significant effect on the outcome. Here are some results of our testing:

Webbing: Spider Silk MK4
Device: LineGrip G4
Sample 1: 7.9 kN
Sample 2: 8.0 kN
Sample 3: 7.4 kN

Webbing: Spider Silk MK4
Device: Wafer
Sample 1: 6.6 kN
Sample 2: 6.8 kN

Webbing: Spider Silk MK4
Device: Wafer XL
Sample 1: 9.8 kN

Webbing: Spider Silk MK4
Device: Double-stacked LineGrips
Sample 1: 10.1 kN
Failure Spot: Front of Linegrip furthest from the anchor

Webbing: Spider Silk MK4
Device: Double LineGrips with Sliding-X
Sample 1: 11.0 kN
Failure Spot: Front of Linegrip furthest from the anchor

Webbing: Spider Silk MK4
Device: Double Wafers with Sliding-X
Sample 1: 7.8 kN
Sample 2: 8.3 kN
Failure Spot: Front of Wafer nearest to the anchor

Webbing: Spider Silk MK4
Device: Double Wafer XL with Sliding-X
Sample 1: 11.8 kN
Failure Spot: Front of Wafer XL furthest from the anchor

Webbing: Other brand Polyester-sheathed Dyneema
Device: LineGrip G4
Sample 1: 13.5 kN

Webbing: Other brand Polyester-sheathed Dyneema
Device: Wafer
Sample 1: 7.3 kN

In conclusion, this demonstrates a major flaw in a Polyester-sheathed Dyneema webbing. The interaction between the highly grippable Polyester and the not-at-all grippable Dyneema inside creates some major conflicts. As soon as higher tensions are reached, there is a TON of strain on the fibers within the webbing that are holding the two different materials together. This is mostly seen when gripping these types of webbing using a standard slackline webbing grip. Even adding more length to your grip surface by stacking multiple devices together, we don't gain enough strength to be safe gripping these types of webbing.

Moving forward, we do not recommend using sheathed Dyneema as your final segment on the tensioning side of any line! On shorter lines where you plan to use sheathed Dyneema, use Nylon as your final segment to gain some dynamics and soften the falls as well as enable standard rigging practices. For longer lines, Polyester works just fine for this.

Most importantly, for longline applications where tensions will be high, absolutely do not grip sheathed Dyneema webbings. This can and will result in a very premature break in your webbing!