Load Testing Results

[Maine Sail]

Originally Posted by estarzinger

I plan to look at the bunt line hitch today, but because I had some splices left over from yesterday I did some quick bends:

Can't wait to see how it does. I stopped breaking wire with it, compared to a bowline, but we are talking two different materials....

[estarzinger]

I am assuming all these results will be very different in dyneema, higher modulus more like your wire.

[estarzinger]

Buntline series (5 pulls in each rope, buntlines on each end)

LS : 86% of rated strength (3% stdev) vs test 1 83% for bowline & 84% for figure 8. Statistically no difference
XLS: 70% of rated strength (3% stdev) vs test 2 71% for bowline & 69% for figure 8. Again, statistically no difference.

tentative conclusion . . . these Dacron double braids are relatively insensitive to knot geometry, and the knots weakens them less than normally assumed. I have some NER dacron double braid coming and can confirm with another mfg's braiding.

[thinwater]

It's always impressive to watch just how far nylon stretches, including drawing up knots, before it breaks. With short samples, at least double.

While you are at it, it might be fun to create a stress/strain graph for the lines in question. Just mark 2 spots and measure at several loads. Bear in mind that this varies from brand to brand and with age, not just material. You knew that. I think I shared these with you. All were moderately used materials (rock and ice climbing equipment still in service) I had; new material results are good for catalogs but moderately used is real.

% strain on left. Load in # at bottom.



Cumulative energy absorbed, ft-pounds left. Load in #, bottom.




It might be interesting to consider how different amounts of energy storage effect sea and ground anchors, looking at different rode materials. Smart folks have suggested that polyester might sometimes strike a smart balance.

[estarzinger]

gripper hitches

1/2" dyneema single braid main line, with 3/16" dacron double braid gripper line (samson LS - rated at 1200lbs)

rolling hitch slipped at 200lbs
rolling hitch with 3 turns slipped at 370 lbs
prusik slipped at 860 lbs
Icicle hitch did not slip, broke at 900lbs

Not sure if there is a statistical significant difference between prusik and Icicle (would have to do more pulls) but definite difference to the rolling hitch.

^^Yes, I am amazed at how much stretch there is even in the dacron double braid. Some of it is the knot pulling up, but the line elongates quite a bit. I will do some measurements of that, but right now I am still in the 'repeatibility and calibration and testing stage' to make sure everything is right. I did notice I needed (and put in) a bigger pin at one end, because the bend radius was just slightly too small and the bunt lines were all breaking at that end (I tossed those test results out). That was not happening with the bowline and figure 8, so the buntline is more sensitive to a small pin, probably because it tightens up around the pin

[thinwater]

Originally Posted by estarzinger

... so the buntline is more sensitive to a small pin, probably because it tightens up around the pin

On the other hand, if every practical application is around a carabiner-sized object, then that weakness counts. Yes, I too have noticed that some knots are more sensitive to rod diameter; those that function as a noose. I have read that this is even more true in dynamic (fall) loading, though I can't recall where. It has to do with the way the line moves through the knot and over the pin as it draws up. Possibly irrelevant to sailing.

[HappyMdRSailor]

Evans,

I have years of real world tensile testing many moons ago... Your first couple of groups and their data sets came out amazingly well... Honestly, 30 samples is only going to marginally decrease your SD...

Great job!

[estarzinger]

^^

Thanks.

Yes, the Dacron groupings are tight enough that somewhere in the 5 to 10 pull range seems to practically speaking capture the results.

I will be curious to see how the dyneema testing goes. I am expecting it will be more sensitive to small changes and that I will have to be more careful to get repeatable results.

[HappyMdRSailor]

Originally Posted by estarzinger

^^

Thanks.

Yes, the Dacron groupings are tight enough that somewhere in the 5 to 10 pull range seems to practically speaking capture the results.

I will be curious to see how the dyneema testing goes. I am expecting it will be more sensitive to small changes and that I will have to be more careful to get repeatable results.

You're welcome!

Your intuition is right on the money with regards to my experiences.... Like materials i.e. line, tend to form tighter groups in the midrange/average strengths.... With the higher loadings of dyneema it will probably be tougher to get such a pretty grouping....

Same goes for high elongation/low load stuff... I hope not, but I think the nylon is going to give you fits....

As you have discovered, large mandrels are one of the secrets... Bet you could easily make a couple of different sets with schedule 80 filled with epoxy/cabo with a reamed pipe centered to freely spin on a pin...

PS: This is no longer my day job! Just hundreds of hours sitting behind a tensile machine control panel in my youth...

[JayH]

Originally Posted by estarzinger

gripper hitches

1/2" dyneema single braid main line, with 3/16" dacron double braid gripper line (samson LS - rated at 1200lbs)

rolling hitch slipped at 200lbs
rolling hitch with 3 turns slipped at 370 lbs
prusik slipped at 860 lbs
Icicle hitch did not slip, broke at 900lbs

Evans, what about some hitches on chain? Probably less of a slippage issue, but I seem to recall some reports of snubbers attached with rolling hitches breaking at the hitch.

[estarzinger]

so, soft shackle . . .

I just pulled one - 1/8" amsteel - spec sheet says 2500lbs average break strength (with splices).

I expected the soft shackle to break at about 2500lbs.

But I got 4220lbs!

One leg broke at the diamond. The other leg was still intact but probably would have broken quite quickly.

Pull - I need to get an larger diam oval shackle for this:



Broken - the diamond knot came apart -

[estarzinger]

Second one.

4380lbs

This one broke first at the diamond, and then immediately at the lock point.

[estarzinger]

Shackle 3:

4130lbs broke like the first one (one strand at the diamond . . .if we could get both strands truly evenly tensioned we could probably greatly increase the strength)

I will check the line strength tomorrow . . .but Samson's dacron line strengths were spot on so I trust the rated numbers.

[estarzinger]

Amsteel line strength - Samsons rated load (2500lbs) look spot on again.

For 5 pieces with bury splices on each end: I got average 2428lbs or 97% (133 stdev), and I did not do really careful tapers at the end of the bury's.

Simple hand sewn sewn splices do not works as well with dyneema as with dacron. I got full strength with the dacron sewn splices while I got 81% (5 pull average) in dyneema.

[Jim Cate]

Originally Posted by estarzinger

Amsteel line strength - Samsons rated load (2500lbs) look spot on again.

For 5 pieces with bury splices on each end: I got average 2428lbs or 97% (133 stdev), and I did not do really careful tapers at the end of the bury's.

Simple hand sewn sewn splices do not works as well with dyneema as with dacron. I got full strength with the dacron sewn splices while I got 81% (5 pull average) in dyneema.

Evans, could you describe the technique that you use in your stitched splices, please? Lots of different approaches for that style!

Cheers,

Jim