Rope/knot/splice load testing

[estarzinger]

I am starting a series of tests.

Today I ran what I had expected to be some simple calibration tests to see how repeatable the results were.

I made up 10 samples, Samson LS Yacht braid 1/4", with a bowline on one end and a figure 8 loop on the other.

I had expected them all to break at the bowline, at about 50% of line strength, in a relatively tight grouping of breaking loads.

5 of the samples broke at the bowline at 83% of the rated line strength, with a 5% standard deviation

And 5 broke at the figure 8 at 84% of the line strength, with 3% standard deviation

It is really too small a sample for any sort of statistical calculation, but the test results were relatively tightly grouped, so I draw two conclusions (a) the breaking loads were higher than I expected, ( b ) there was no significant difference between the bowline and figure 8 in this line

 

[irwinr]

It looks like the 2nd from the right failed in the middle, not adjacent the knots. Is that right?

Is there a way to do a straight pull to test the actual line strength versus "rated" line strength to establish a baseline for how much a knot decreases the strength?

 

[allene222]

I think if you pull on a line that has an eye splice at each end you will find it breaking in the middle. The 2nd from the right looks like it failed at the figure 8 but there is a long tail that makes it hard to tell.

Knots fail where there is a line under load that has a tight bend. A fishermans knot has lots of bends but they are not under load so it tends to be strong. The figure 8 does as well so it does seem strange that the bowline was just as strong. Perhaps these bowlines were tied in a special way

 

[estarzinger]

The 'rated strength' is 'average with two splices' according to samson. I can and will (a bit later) replicate that test to check the rated strengths because I want them comparable between mfgs.

None of those lines broke in the middle ( in the above or below test). They all broke near the knot exits. On some of the breaks it was 'clean' and on others it was progressive, with a core tail that broke last.

Second test:

Samson XLS yacht braid 1/4"

Again 50% broke with the bowline at 83% rated strength, 3% standard deviation

And 50% broke at the figure 8 with 79% rated strength, 2% standard deviation

Conclusions . . results similar to test 1 but a bit tighter grouped. Bowline = figure 8, Dacron double braid with both knots about 80% of rated average splice strength.

 

[irwinr]

What do you make of the bowlines reliably breaking at ~80% versus "accepted wisdom" that a bowline would fail at 50%?

Are you having fun yet?

 

[allene222]

I notice that your bowlines are pulled tight. If it would not be much trouble, can you test a bowline where the loop is not snugged up. That may be closer to the way they were tested when the 60% to 66% numbers was observed.

I have a couple of useful articles referenced on my "Rope Reference Articles" page.

Another thing to consider is that some of the testing done in the literature is on climbing rope. That rope has a lot of stretch and that may be different.

 

[estarzinger]

Stupid spread sheet error in test 2 (test 1 still correct).

Correct test 2 results are a little lower, but still higher than expected:

50% broke with the bowline at 71% rated strength, 3% standard deviation

50% broke at the figure 8 with 69% rated strength, 2% standard deviation

I just did the double end splice test (5 with each line) to check rated strength. They are very close (statistically equal to) to Samson's ratings.

XLS 98% of rated with 1% Stdev

And

LS 99% with 4% Stdev.

These were sewn splices - I did that because they are more repeatable (at least for me) than class 1 DB splices. All broke in the middle of the line, or near the first stitch (nearest the middle of the line).

I have some nylon line to test but I need to increase (about double) my hydraulic travel. Right now I can just handle the stretch of Dacron, and I will be perfectly fine with dyneema, but nylon will stretch and not break with my current travel. I have a longer ram ordered. I have it in my plan to test less dressed knots, including not pulled tight bowlines . . .but right now I am trying for the highest repeat-ability possible.

Not yet sure what to make of the higher than expected bowline results in dacron double braid. It is still early days in testing.

 

[allene222]

The test I found had bowlines at 66% of line and that is pretty close to what you are getting although it listed the figure 8 at 80%. However, these were tested by different people. The dressing of the figure 8 probably matters as well. I think the important thing would be that both strands grip the line as it comes into the knot, if that makes sense.

I broke a lot of bowlines in spectra when I was developing the double soft shackle. I found that the bowline was weaker than a fisherman's knot but did not have a calibrated setup. I did calibrated testing at NE Ropes with Brion Toss but by then I was not using a traditional bowline and they broke in the diamond knot in the stopper loop. The diamond knot is very weak compared to what you are doing.

Allen

 

[estarzinger]

Someone asked about my test bench set-up. I have twin 10ton hydraulic puller rams (standard auto body shop gear) and a calibrated Dillon load cell.

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:

Samson LS (dacron double braid)
Sheet Bend: Slipped at 51% of rated strength
Double Sheet Bend: broke at 65%
Double Fisherman: Broke at 74%

Samson XLS (dacron double braid)
Sheet Bend: Slipped at 50% of rated strength
Double Sheet Bend: broke at 65%
Double Fisherman: Broke at 65%

Since the sheet bends slipped in both lines I think it will be worth looking at the other 'quick' bends like the ashley and zeppelin.

I have various dyneema on the way, but it has not arrived yet.

 

[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.

Note on the buntline tests . . . I did notice while doing them that I needed (and did 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.

 

[estarzinger]

The diamond knot is very weak compared to what you are doing.

Allen

so, you are the soft shackle expert . . .

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! Are you surprised by that?

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 -

 

[allene222]

The diamond knot is very weak compared to what you are doing.

Allen

so, you are the soft shackle expert . . .

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! Are you surprised by that?

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:

pull.jpg

Broken - the diamond knot came apart -

softshackle.jpg

I am the guy you are thinking of. An expert? That is a different discussion.

I had several soft shackles tested at New England Rope courtesy of Brion Toss. They were 3/16 Dyneema supplied by New England Rope, not Amsteel. They broke between 6000 and 7000 pounds. The testing I have done myself has always been a comparison of different lines. For a soft shackle, I would have a link of the same line with two eye splices and that test link would always break first so I would say that the soft shackle was stronger than the line it is made of. The New England Rope testing showed strengths from 103% to 116% of line strength so I stuck with my statement about it being stronger than the line it is made of. To tell you the truth, I expected them to be stronger than what NE Rope found.

The construction of a soft shackle is theoretically four times the strength of the line so what you did broke at 42% of its theoretical strength instead of the ~30% at NE. There was a lot of variability in the NE testing so you might want to run some more samples. To answer your question I would have to say I am pleasantly surprised but what you found is closer to my expectation than what I actually found at NE Rope, but how can I argue with them?

I have also broken a lot of soft shackles and they always break at the diamond knot right where the line enters the knot.

Allen

 

[estarzinger]

Second one.

4380lbs

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



Allen,

don't be shy . . you have done us all a service by innovating and explaining soft shackles . . .and I love your weather site also.

 

[allene222]

Can you test the strength of the line itself?

 

[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)

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

 

[allene222]

Interesting point. You may be getting higher strength because of the long tails you have leading to the diamond knot. Amsteel does stretch ( maybe 2% in your test) and that can even out the load.

 

[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.


allen, on 31 Dec 2013 - 17:11, said:
Interesting point. You may be getting higher strength because of the long tails you have leading to the diamond knot. Amsteel does stretch ( maybe 2% in your test) and that can even out the load.

That's a possibility. I also only hand tightened the diamond knots. So they may have tightened up more evenly with the slow hydraulic pull.

 

[estarzinger]

Back to dacron double braid for a minute - adding New England Ropes Stayset data.

Bowline still statistically equal to Figure 8

NER ropes breaking at lower % of rated strength, but that may be because they are using a different measure of rated strength. I have asked, and I will check with my own "full strength" breaks.

NER ropes appear to have lower stdev (perhaps higher quality consistency?)

The NER line is 2.33x times higher in (retail) price than the Samson LS

Bowline
Samson LS 1/4": 1582lbs, 83% of rated strength, 5% StDev
Samson XLS 1/4": 1582, 71% of rated strength, 3% StDev
New England Ropes StaySet 1/4": 1575lbs, 67% of rated strength, 1% stdev

Figure 8 loop
Samson LS 1/4": 1590lbs, 84% of rated strength, 3% StDev
Samson XLS 1/4": 1590lbs, 69% of rated strength, 2% StDev
New England Ropes StaySet 1/4": 1558lbs of rated strength, 66%, 1% StDev

 

[glowmaster]

I must be a nerd but I find this interesting. Thanks for sharing. Soft shackles rule.