Splice throat distortion--Break Test

[moray]

When I bought my first climbing rope about 4 years ago, I got one with a commercially spliced eye at one end. I don't think I had ever seen a splice up close, and I was shocked at the great distortion to the cover strands at the throat of the eye where the bury began. I went about using the rope, trusting that the splicers knew what they were doing, but continued to wonder just how much weaker the rope must be due to all that distortion.

To test this, you need to remove all other factors so any measured weakening of the rope can only be due to the distortion.

I performed this experiment twice using my favorite test rope, 5/16 inch Tenex Tec. It should be mentioned at the outset that this is a very different rope from any climbing rope, and it only has 8 strands in the cover, not the 16 or 24 of most modern climbing ropes. Also it is hollow, whereas climbing ropes are solid or have a core. A splice in this rope, to my uneducated eye, seems to cause much less cover distortion than a splice in a climbing rope. All of which is to say the results of this experiment probably don't tell us much about climbing ropes, but they should be relevant Whoopie slings and Loopies, both of which are weakened where the adjusting tail penetrates the cover.

 

[moray]

Description and Results

The first picture shows an ordinary eye-2-eye sling. The breaking strength of the sling, tested twice in previous experiments, is about 4550 lbs. The tapered slug of rope above the sling will be used to distort the cover for the test.

In the next picture, the blunt end of the slug has been inserted. It is easy to move it inchworm-fashion through the rope with no tools.

The last picture shows the slug in position for the test. The buried part of the slug is tapered as a splice would be to avoid weakening the rope along the bury. We now have what we were after: a full-strength rope with a rope-diameter hole in the side.

Both test ropes broke at the spot where slug entered the rope. The breaking forces were 3860 and 4234 lbs, giving an average break strength of about 4050 lbs. This is 89% of full rope strength, confirming my impression that a splice in this sort of rope does not distort or weaken the cover very much.

After the first test, I could not find the rope slug! It was no longer in the rope, and after a 5-minute thorough search I gave up. The next day, while walking in the woods, I found it 40 feet away. I had a better recoil snubber installed for the second test, and the slug, even though it was expelled from the rope, only traveled a foot or so.

Even though this experiment has been described in terms of cover weakening where a splice bury begins, I was after something else. This represents one of two extreme tests of a spliced eye: The first (this one) is where the entire tensile load is on the cover leg of the eye (the other leg has been cut off! all that remains is just the little slug). The other, which I will report next, is where the entire tensile load is on the buried core. It is this second scenario that gives people nightmares and cold sweats.

 

[moray]

Moray.......to be honest........your speaking to only a select few...

treenoob, thanks for your tactful observation... I do know few people have much interest in splices, and fewer still give a rat's ass about the more-or-less mathematical details. Still, this is a big forum and there are SOME people who care about this stuff, including some who only lurk or find this place as a result of a Net search. (I know, since I sometimes run into my own stuff when I do such a search!). So I try to do experiments that interest me and publish the stuff that I, at least, have not been able to find anywhere else. There is surprisingly little hard information out there about a lot of this stuff, and what there is is often wrong or misleading, so I still have plenty of work to do (and a day job, too).

 

[moray]

The other extreme test of a splice is to put the entire tensile load on the buried leg of the eye. This is the condition that really worries people, since it is not obvious why the buried core would not pull right out. In fact it will pull right out unless the throat is stitched or otherwise secured. Since stitching is the standard method for securing the throat and is the method recommended by manufacturers, that is the method tested here.

A little bit of stitching goes a long way. If your stitching, by itself, provides 100 lbs. of holding force, that does not mean that 101 lbs. of force is enough to pull the splice apart. There is a very large multiplier effect such that the 100 lbs. of stitching might give the splice 1000 lbs. of holding force. The longer the splice, the greater the multiplier effect.

I normally follow Samson's instructions for stitching, but I use only 70-lb. test polyester yarn for stitching, much weaker than what is called for. Samson's formula calls for using stitching yarn the same size as the strands in your rope; for the 8-strand hollow braid in these tests my stitching yarn should be about 600-lb. test! This may sound risky, but remember stitching has no effect on strength except when almost the entire load is on the buried leg of the eye. Since this condition is essentially impossible to achieve in normal use, I don't worry at all about substandard stitching.

Now that I have the means to test the rope to failure, and measure the actual force required, I decided to test one of my substandard splices with the entire load on the core.

 

[moray]

Description and Results

How to apply the entire load to the buried leg of the eye is a bit of a problem. One way would be to make a very large eye (a couple of feet), then sever the cover leg a few inches from the throat. The buried leg, now a long straight piece of rope, could be wrapped around a bollard or terminated with a spliced eye for pulling purposes.

I chose the spliced eye termination.

The picture shows the business end of the setup. The rope ending at "T" goes out of the picture at the top and terminates in an eye. The arrow shows where the lower rope enters the upper one to form the bury. The spot marked "S" locates the stitching that secures the throat. When tension is applied between the two eyes, the entire force will be trying to pull the buried core out of the upper rope.

 

[moray]

In the first experiment the stitching yarn was 70-lb. test polyester, a little more than 10% of the strength Samson calls for. I was actually quite confident the rope would break at the end of the bury like a normal splice, and the force required would be about 4550 lbs., the full rope strength.

Nope. The core pulled out at 3226 lbs. The picture shows the results. the point B indicates the actual buried core that pulled out, S is all the broken stitching near the throat, and T is the inert tail of the main rope.

In the next experiment I decided to triple the yarn size by using 3 strands for the stitching. This is still only about 1/3 the strength Samson calls for, but now I was dead sure the core would not come out and that the rope would break at 4550 lbs.

As the gauge passed 3000 lbs. and I settled in for the final few strokes of the pump handle, suddenly the rope broke. What??! The gauge said 3186 lbs. This was a big surprise, and I was beginning to think I had done something wrong, or there was something defective about that piece of rope. But no, the rope had broken at the stitching, and the stitching was still intact. Of course.

Even though the stitching saw nothing close to 3186 lbs, it saw enough force to be seriously distorted by the cover and the core, which were pulling in opposite directions. In turn, it significantly distorted both the cover and the core. This of course weakened the core, and it lost about 1/3 of its strength. The picture shows the results. T is the tail of the main rope, and S locates the still-intact stitching. B points to the broken core leg at the bottom, and to the still-buried core in the main rope to the right.

 

[blewgrass]

although I've never considered myself a rope geek I find myself reading all of these tests to the end. very interesting stuff moray. one thing that would be awesome for your tests would be the super-slow-motion camera. I know it'd cost an arm and a leg and isn't doable, but it'd be great to watch it fail slowly.

 

[blewgrass]

where are you at anyway moray?

 

[moray]

blewgrass, I wasn't born a rope geek--I didn't know the first thing about ropes or splices 3 or 4 years ago. But I found it really interesting so I dug into it a bit.

The fast camera would be really cool, but as you say, economically totally out of reach. I've read that the tip of a bullwhip exceeds the speed of sound when it cracks. It seems highly likely that these ropes when they break also go supersonic.

I broke a double-braid eye today, the first one I have tested. As in the hollow braids, the break occurred at the end of the bury. When I was trying to pry apart the strands in the break area to expose the core, I noticed that the strands seemed roughened and somewhat glued together. I have an old dissecting microscope I picked up at a yard sale, so I took a closer look at the broken strands. Signs of melting everywhere! This is a whole new dimension I will have to keep track of...

 

[blewgrass]

blewgrass, I wasn't born a rope geek--I didn't know the first thing about ropes or splices 3 or 4 years ago. But I found it really interesting so I dug into it a bit.

I wasn't trying to call you a rope geek (unless you like that) but was just using the "terminology" treenoob used in his initial post. i'll keep reading as long as you keep posting.

 

[moray]

I wasn't trying to call you a rope geek (unless you like that)...

Geek is not a bad word in my book. It's an honor to be called that by someone who thinks they are delivering a knockout insult...

OK, now it would be nice to distill all the numbers down to a few simple ideas. I think the data strongly support the analysis that follows, but I fully realize the data are very sparse. Call it educated opinion. Everything below refers specifically to hollow braid rope such as Samson Tenex, and splices made to manufacturer's specs, which means Samson's splicing intructions. Other manufacturers' ropes and instructions tend to be very similar. As I see it, there are 3 separate loading scenarios that give rise to 3 distinct outcomes.

1. Normal Use. This is almost certainly the only scenario any of us will ever see. Both legs of the eye are more-or-less equally loaded. This splice will absolutely never under any circumstances pull apart, period. The stitching has no role when the splice is loaded. The rope will break at the end of the splice bury. Break strength is near 100% of true rope strength.

2. Cover leg of splice carries all or almost all the load. It is almost impossible to imagine how this could happen with the tight eyes and small eyes arborists use. Splice will not pull apart (there is no load on the core). Stitching plays no role. Cover leg will break right at the throat because rope is disturbed at that point. Break strength is 90% or less of true rope strength, probably less for some other rope types.

3. Core leg of splice carries all or almost all the load. Probably impossible (see #2). Splice will not pull apart. Stitching plays crucial role. Core leg will break just inside the throat at the stitching because both core and stitching are under high tension at that point and core strands are therefore disturbed by the stitching. Break strength is significantly below rope strength, perhaps only 70% or so.

Having seen some of the experimental evidence regarding splice performance, it is interesting to compare the evidence with the manufacturer's specs. Take stitching for example. Since case #1 is the only realistic one, and the stitching has no functional role, its only remaining role is to hold the splice together when it is rattling around in the back of your truck or your dog is playing with it. For that purpose a few stitches of ordinary sewing thread would be enough, so why use 12 stitches of yarn that has 10% of the tensile strength of the rope itself?

I think it is because of case #3. I imagine the nerdy rope engineer sitting down with the corporate lawyer to design the splicing instructions to publish on the Web page. The lawyer tells the engineer, "Look, all ropes break, so we won't worry about breakage. But we absolutely cannot under any circumstances have one of these splices ever come apart. This can only happen in case #3, so specify the yarn size to handle case #3. Make the yarn 3 times stronger than necessary to force the core to break before the core can pull out."

I personally ignore the impossible case #3 and use the same 70-lb. polyester yarn for all my stitching. There is nothing magical about the number 70--I just happen to have a bunch of it on hand from an old rope I took apart, and it conveniently fits in several of my needles. It is also white and big enough to be readily visible on my ropes so I can easily check that it is still there.