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General => Practical Knots => Topic started by: knot rigger on April 28, 2015, 02:14:12 AM

Title: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on April 28, 2015, 02:14:12 AM
I recently posted a method of tying the Alpine Butterfly Knot, which lead to some interesting discussion of the breaking strength of the ABK.  Specifically, whether or not there is a stronger and weaker side of the knot due to the slight internal asymmetry of the knot.  I did some break testing and have concluded that the strength of the ABK is symmetrical: there is not a weaker and stronger side.

I've attached a report on my results.

I have a larger file size report, containing pictures for clarity, that I am happy to provide to anyone interested.  Or, if someone can explain to me a convenient way of posting a 400kb file to this forum, I'll provide the full pdf file of the report.

[modified 4/28/15]

I have overcome my technological challenges :)

check out this link:

http://tinyurl.com/ohlsd43

you can view and download the report, with the pictures.  there is also a folder of all the pictures I took, and a video.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on April 28, 2015, 06:45:53 AM
"strength is symmetric(al)" :: This sounds quite peculiar, IMO,
vs. "is equal" and so on.

Can't you post photos of your knots, in <100kb-size files,
w/o much to-do?  --or cite ones that are already posted
in the forum somewhere.  Just "butterfly knot" doesn't
denote a specific geometry, and certainly not the particular
one(s) that met your testing.  It can be edifying to see how
knots transform when push comes to shove.

Thanks,
--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 28, 2015, 02:37:33 PM
kr your document claims to disprove the notion that they are unequal.  In technical factual sense not only does it not do this, but this is impossible to do.

An asymmetry can never be measured to be exactly zero.  It can only be measured to be consistent with zero with the precision of the experiment.

Actually your two results agree too well, to the point that a suspicious person would have a right to be suspicious. 

With 14 trials you can expect the range to represent about 1.5 standard deviations (an estimate which itself has a larger error).  But if I go with that, then that gives you a standard error of about 88 pounds which produces an error in the mean  of about 24 lbs.  In other words if you repeat the exact same experiment, you could it expect it to be off by about 24 lbs more or less.  Of course you COULD get the same result within 1/4 of a pound but the odds on that are probably about 1 in 50.

Anyway, the accuracy of your measurement is probably not better than about 23 lbs so a little over 2%.  All you can really say is that they are the same, at the level of a couple of percent (give or take a little).

HOWEVER

The accuracy of this experiment is already far better than the certainly you can have with any one rope you are hanging from at a given moment, and it may even turn out that the asymmetry is smaller than you could ever practically measure with ANY number of breaks, because all that statistical precision can only be trusted so far.  Statistics alone cannot reach .00001 standard deviation measurements in most any practical applications even if you do have enough trials , because except in extreme laboratory conditions studying fundamental results, you can never trust the systematics that well.  So there is a point where I say if the difference can NEVER be measured then it has no observable meaning, ie the difference is meaningless, which I think in standard parlance means there is no difference.  I am perfectly fine with that. 

You are NOT at that level but you are at the level where you can already say the difference can have no bearing on how you decide to use the knot.  That's good enough for me.  So yes, I think as far as anything that matters goes, these are the same.  Use a proper safety factor and don't worry about which way you point the knot.  If you have even the slightest reason to think it matter, you should stop and do something safer.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on April 28, 2015, 11:20:43 PM
DL:
Quote
Can't you post photos of your knots, in <100kb-size files,
w/o much to-do?  --or cite ones that are already posted
in the forum somewhere.  Just "butterfly knot" doesn't
denote a specific geometry, and certainly not the particular
one(s) that met your testing.  It can be edifying to see how
knots transform when push comes to shove.

I seemed to have been suffering from technological tunnel vision yesterday. :)

check out this link:

http://tinyurl.com/ohlsd43

The report as a PDF as well as all the pics I took, and even a video. There is a whole sequence of pictures of a severely loaded, but unbroken ABK if you're curious how the knot behaves under heavy loading.
 I updated my original post, and put the link there as well.

TeX:
Quote
Actually your two results agree too well, to the point that a suspicious person would have a right to be suspicious.

To a person more suspicious than I, that statement could be taken as an accusation of dis-honorable behaviour  ;)  You'll just have to take my data at face value, and trust me that I didn't doctor it.

Overall, I completely agree with what you say Tex.  Perhaps I shouldn't have used the word "disproved", or clarified "disproved within the accuracy of this experiment"

Quote
...the (tiny) difference can have no bearing on how you decide to use the knot.

That's pretty much exactly what I was looking to answer.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 29, 2015, 12:33:26 AM
Quote
To a person more suspicious than I, that statement could be taken as an accusation of dis-honorable behaviour  ;)  You'll just have to take my data at face value, and trust me that I didn't doctor it.

Well I decided not to qualify it and let you guess, but you guessed right.  It certainly was not an accusation, just to raise awareness that a passerby should not be too impressed by that incredible agreement.  You CANNOT conclude that they are within a fraction of a pound of the same from that data.  You can only conclude that they are within 25 lbs OR SO (could be less, could be 75 too though with low probability) of the same.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 29, 2015, 01:32:54 AM
Anyway this is great data and great work.  If you want to take your report to pro level, produce a mostly unassailable conclusion, and avoid much unneeded criticism (xarax is coming) then do the following little extra bit of work that will take 5 minutes.  If you ignore it, obviously that's fine too.

1) put all the numbers in excel and ask it for the standard deviation for each test, s1 and s2
2) divide each by the square root of 14 to get the standard error e1 and e2
3) Find the mutual error  em=square root of ( e1 squared + e2 squared), or just choose the biggest one and multiply by 1.4 (close enough and could have just used 28 in step 2 above to get the same result edit: scratch that, got carried away)

4) multiply em by 2 (2 sigma is two sided 95% confidence interval) and write the result, f, as a percent of the average value.

5) Instead of stating you proved they are not different, state "With 95% confidence level I find that the strengths are within f% of identical"

I did not make the 2 sigma correction nor the 1.4 correction in my quick estimate earlier so it will get worse than what I said.  At 95% maybe it's a 5% measurement, but you have to do the math and see.

Xarax will stipulate that he has always said that they differ by less than 1% and this will be far less your sensitivity. When you state things this way he can be happy to obsess over his 1% and you can be happy that you made a now document conclusion at the level everyone else cares about.    Someone else will argue about the 5% confidence and that's fine too.  People can take the statement for what they want but the statement will be valid and you can shrug it off.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 29, 2015, 01:56:56 AM
Worrying about exactly what distribution you have (I assumed gaussian of course) is by far second fiddle to first making an estimation of the general scale of the spread of the distribution and converting that into some error scale, as I described.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on April 29, 2015, 02:31:17 AM
Quote
(xarax is coming)

 :D LOL  ;D
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on April 29, 2015, 09:38:28 PM
Worrying about exactly what distribution you have (I assumed gaussian of course) is by far second fiddle to first making an estimation of the general scale of the spread of the distribution and converting that into some error scale, as I described.
The first priority is to understand what knot is tested.
Here, it is the "legs-abutting" version of the butterfly.
In this case, I hope that my prior remarks about
the orientations, "the geometries" of the "butterfly knot"
have been understood; alas, though, for there to be
several comments on the report without any mention
of this, I'm afraid not.
(Please refer to that image shown by Alan Lee, e.g. :
that is of a different, "legs-crossed" geometry.)

TO ME, I must see the relatively good results of this
tested orientation to challenge my notions of better
curvature and so on which I see for the "legs-crossed"
version (by either end loaded).

To Xarax, I still don't understand what HE thinks is
the difference, of the 1-dia vs. 2-dia curvatures, for
the shown-here orientation --both seem more of the
1-diameter U-turn, IMO.

To Knot_Rigger, thank you for quite some work!!
(--as I've done similar or not even, and know it to
be a chore).  You've "outdone yourself", and raised
the bar!  That said, the black cord makes discerning
the details of the material difficult if not impossible.
We might need to ask you to report what your eyeballs
up close & personal see of the Real McCoy(s), if you
can abide such query.

I note that the fig.8 eye knots seem to be loaded
in what I have called "the strong form" of what Xarax
in another thread posted a clear image of (well, w/o
indication of loading; but of the "perfect form" geometry)
in response to my remarking on two other images.
IMO, this orientation will benefit from hard loading
of the tails, in setting --something that might be not
practical in many cases, given strength of material
(i.e., that one cannot manually do "hard" for it).
My thought is that, in this orientation, the S.Part
bears against its twin part (i.e., the tail) as it
flows into the knot and goes then to its U-turn
around the eye legs (which is where the break
appears, in your one case --or maybe we should
see that in-knot-broken-end as needing to be
strettttttched farther along, so nearer the entry!?).

There is more that we can wonder about, in the spirit
of pure testing, at least --vs. practical significance-- :
would it matter were the mid-line eye-knot first loaded
end-2-end / "through", and then loaded qua eye knot?!
Essentially, what a hard setting by loading ends would
do!?  --and vice versa : how the knot would perform
end-2-end after taking a hard eye load and whatever
distortions this might impart.  We'd be concerned about
such things mostly (most reasonably) if there were some
practical circumstance in which these are loadings.


--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on April 29, 2015, 10:07:51 PM
   To Xarax, I still don't understand what HE thinks is the difference, of the 1-dia vs. 2-dia curvatures, for the shown-here orientation --both seem more of the 1-diameter U-turn, IMO.

  You do not have to understand it, you have to see it - if you still do not, you need a new pair of glasses, I am afraid.
  I have taken those pictures, and I have marked them drawing the diameters of the two first curves with red ( for the wider ) and yellow ( for the narrower) circles - I can do no more !

http://igkt.net/sm/index.php?topic=3204.msg34668#msg34668
http://igkt.net/sm/index.php?topic=5269.msg34592#msg34592

P.S. Perhaps this difference will remain in place, only if the knot is first loaded end-to-end, so the shown geometrical difference will be "fixed", and only afterwards end-to-eye, as you notice. I can not tell, because I have never loaded a Butterfly loop ( or any other knot ) near its strength limits...
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on April 30, 2015, 02:14:27 AM
Hi again

Tex, thanks for your encouragement, and for the brief lesson in Statistics.  My appologies, but some of that is over my head.  I get it in theory, but the i'm not great with the math.  Perhaps you could recommend a resource to me, so that I can re-figure the data I have and understand the steps I'm taking to do it.  And/or perhaps you could run you're analysis on the data in my report... maybe that would help me understand the details.  I added the raw data in a google spread sheet file to the shared folder in my google drive if anyone is interested in looking at it closer.  The link is in the original post.

DL:  Sorry that the pictures suck!  You're right that the black line is problematic, but it's what I had for free ;)  I'll try and give you some qualitative analysis from the breaking.

So there were two, very similar modes of breaking:  1) the line broke where the standing part entered the knot. breaking right at the collar, leaving the body of the knot fully intact. 2) the breaking point was under the collar.  the point of rupture visible stretched out and after the break you could see a little daylight through the collar where the standing part used to be.  These two modes of breaking are VERY similar, but I did make the distinction in recording the data.  In none of the tests did the line break inside the ABK.

The breaking point in the figure 8 knot was inside the body of the knot.  Where the stading part takes its sharpest bend, at the loop side of the body of the knot.  I did tie each fig 8 in it's "strong" form intentionally.  You can see in the video how the fig 8 reacted to the high load, the standing part slips down and under it's twin (the tail) at the loop end of the body of the knot.  I consistently saw this behaviour during the test but didn't record it as data, as I was concentrating on the ABK.  I've also seen this deformation on heavily loaded fig 8 knots in larger rope, "in the field".  As an aside, I'm not convinced that there is a stronger and weaker way to tie the figure 8 loop knot.  I've seen it said in one source that there is, and I've seen it said in another source that they are equal.  Most sources don't mention a distinction at all.  Perhaps that will be my next break testing experiment (tex i may need your help with the math)

It's hard to see the pictures of the heavily loaded ABK in the shared files (sorry, black line).  I looked closedly at each of the ABK that survived the testing, the ones where the break happened at the fig 8.  Both are heavily deformed, but maintain the same basic shape as a set and dressed ABK (the version I tied, without the loop legs crossing, and dressed with the X on the "back side"... )  Specifically I was looking to see if the X was still there, or if maybe it slips out under heavy loading, it was there on both.  I found the same thing with all the broken ABK in the test as well, I  did not see any capsizing of the form of the knot under heavy loading.

As you can (sorta) see in the video,  as the tension was applied these things happened in approximately  this order: 1) the ABK tightens up, yielding line to the standing part 2) the fig 8 tightens up, yielding line to the standing part 3)  the ABK tightens some more, yielding line to the loop of the ABK 4) rupture.

  With most samples there were two "pops".  The first pop was at peak force, and after it the tension was lower, then a little more pulling yielded a final rupture, but not at the full force.  This is due to first, the load bearing core of the line rupturing, but leaving the jacket, then the jacket rupturing.  I've seen this behavior before in other break testing of similar construction of rope, where the core provides most of the strength (ie kernmantle)

hope this info is interesting and helpful
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 30, 2015, 01:41:53 PM
References, ok, type error analysis in amazon.com.  Choose the first book that comes up.  It's actually a great one (older editions are cheaper though).  The next 10 or so all look good too though.  Wikipedia is a is too dense but it will do too once you know what to look for.

You need basically three concepts.
1) standard deviation, you know that one (excel knows how to calculate this, and wikipedia too)
2) standard error  in the mean.  = SD/sqrt(N), simple. (let's call this e1 and e2)

This is what you would expect the standard deviation of the AVERAGE itself to be if you repeated the whole experiment 10 times. (and for this reason it still gets written with the same symbol, sigma, which can confuse people) It also thus tells you how close you can expect your average to be to the true value.  This number is smaller than SD because if you draw a big gaussian curve, even a randomly bumpy one, you can tell where the middle is (even by eye) with much better acurracy than 1 SD.  An SD is sort of half the width of the whole distribution, loosely speaking.  You know where the middle is much better than that, hence the division by something.  Obviously that doesn't prove the formula, just the idea.

Now you can calculate the ratio of the two values (knot up this way or that way) or the difference.  I went with difference because the error calculation is SLIGHTLY more basic.

3) The error in the sum or difference of two numbers is sqaure_root_of (e1^2+e2^2).  e1 SHOULD be about equal to e2 since the situations were clearly pretty equal.  So this is also just e1*square_root_of two =e1*1.4  I said choose the bigger of e1 or e2, just to be generous.

You can find a nice proof of this in any of those books, but that won't help you do the calculation.  You just need the formula.  If you want it in writing from a less respectable source:

http://en.wikipedia.org/wiki/Propagation_of_uncertainty#Simplification (http://en.wikipedia.org/wiki/Propagation_of_uncertainty#Simplification)

Scroll down to the big example formulas table (as usual wikipedia over-generalizes the formalism until it's useless for learning purposes.  That's too bad.)
You want the one f=aA+bB.  The lower-case letters are meant to be constants, 1 (and -1) in our case. you can ignore sigma_ab, the last term in the answer.  That's for  correlated results. Now you see the formula I gave you. 

Long story short simply square the numbers from step 2, add them, and take the square root.


putting it all together, 14 trials, using root 2 method, what we have so far amounts simply to  SD*0.38 .

That's it.  That is your one sigma error in the difference. 3 easy steps reduced to 1.


Divide by the average if you want to write it as a percent and call it a 1-sigma fractional uncertainty.  If you were publishing in a real journal this would be enough.  You could stop there. Congratulations.

But usually if you're seeing nothing, you want to set a limit on the range of likely values so you google a z table and how to use it.  You find that 2.5% of the area of a gaussian is above mean+2sigma, so 5% is is above or below mean plus or minus 2sigma. So you multiply your answer by two and say that with a 95% confidence level the values are at least that close together.

This does all assume Gaussian distributions, especially the z table bit.  The other parts work reasonably well with any reasonable distribution though.  With this little data it would be hard to prove you didn't have Gaussian distributions .  The process is an estimate of what you don't know so more if and buts really aren't useful. These assumptions are so standard they often go unstated as do the steps in the calculations.


Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on April 30, 2015, 01:54:06 PM
P.S. if you want to just make a big improvement and aren't comfortable making statements to back up such numbers, you can just state your standard deviations in place of the range.  The standard deviation won't get better as you take more data (the standard error will), but it least it won't get worse.  The problem with the range is, it actually gets worse with more data and doesn't really describe the distribution.  Someone who knows how can at least quickly use the SD to calculate what I wrote.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on April 30, 2015, 04:34:39 PM
  Yes, 2s should be enough, for practical knots, IMHO. However, I think that when you examine data coming from destructive tests of elastic materials, you do not get a Gaussian distribution - you probably get a Weibull distribution ( but that is what I had only read somewhere - I have no real knowledge or experience with those kinds of things...). Also, I think that one should have a method to find out which data he should ignore, the "outliers" - and I do not know if different distributions are related to different methods of determining the outliers, and different numbers of outliers, and, if they do, how they are related...

Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on April 30, 2015, 06:59:00 PM
Tex

Thanks for the statistics lesson.  I'll work on the data analysis some more, and let you know how it goes.

Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on May 01, 2015, 01:40:50 AM
xarax.  Yes one should probably throw out outliers.  I would certainly hope that in climbing rope (which maybe this isn't?), there are no outliers.  If there are any it should be from mis-tied knots or such, but I haven't looked

You are right that nothing with a strict limit of zero can have a true gaussian distribution. 
Gaussian is much easier to work with, and is probably close enough.  It is also commonly used as an approximation for poisson distributions.  There are a few cases where the real distribution can be know at least out to many sigma, but for most things like this, it cannot. kr mentioned two breaking mechanisms.  This could even very easily produce a lopsided or even bimodal distribution, which has a much bigger impact that the Weibull vs Gaussian issue.

For finding the "true" mean, it doesn't even really matter if your estimate is biased though, because the true mean is not important for its own sake here (there is no true rope).  Any well defined measure of central tendency will probably do just as well.

As for the 2S you should realize that this has less to do with the distribution than you think.  The "S" at that point is not the standard deviation of the breaking distribution.  It is the standard deviation of repeated whole experiments, which is why 2 of these , let's call it 2e instead, equals 0.75 of the distribution SD.    The distribution of the means is not the same distribution) does not have the same shape) as that of the data itself.  This 95% confidence estimate has nothing to do with how confident we are about a particular rope breaking of course.  It only has to do with how confident we are that the distribution of breaking points is not affected by this knot orientation.   Your particular rope still has a 95% chance (well, let's just say high) to break within 2S, not 2e, but for this estimate, we CANNOT remove outliers.  Outliers will kill you. For digging into what we can learn about the underlying details, we can remove them.

Nobody can know the exact distribution.  In truth standard methods are used because they are standard, and they give pretty useful results that tend to work quite well (and I didn't say 100% confidence).  This result will be MUCH more reasonable than just taking the difference in the means for instance, and that's the main point.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 01, 2015, 02:01:34 AM
kr mentioned two breaking mechanisms.  This could even very easily produce a lopsided or even bimodal distribution

   Right !  :) I had not thought of that ...
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on May 01, 2015, 02:14:48 AM
But a bimodal distribution does not produce a bimodal distribution of means.  The mean is still the mean and is and fluctautions around it from one data set to the next are still random, with no preference (spikes, modalities) for any other false mean.  A different single-data distribution will have some impact on the width of the distribution of means, but the effect is probably not as strong as one might think (but  the distribution of what people might think tends to have many outliers). 
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on May 01, 2015, 04:57:58 AM
By the way, I don't have much experience with weibull distributions except for one  special case of them, but I'm not that impressed with them for this.  In the most general definition(not the one on wikipedia) it seems they can describe darn near any(maybe actually any, requires more thought) unimodal distribution that hits the origin, but I think the philosophical point behind them that makes them attractive is deceptive.  We are just measuring the distribution of strengths of particular knots tied on particular ropes.  Unlike time till failure the chance that a particular knot might die early doesn't exactly limit its chance to die late.  That configuration at that moment is as strong as it is and it will die where it dies.  I can speculate on many reasons for distributions of strength within the ropes and some may produce slightly different curves than others, some more weibully inspired than others (and maybe others know more about the physical defects and how they collude), but it's still speculation.

I do think at a given near-failure tension, there should be a very simple weibul distrubtion of survival probability vs rope length.  This is about probability of defects vs legth, and that's very 0-power weibullish, which  has its own special name; it's called exponentinal.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 01, 2015, 06:01:50 AM
   My (rudimental ) understanding is that, since Weibull distribution is the proper tool in situations involving a weakest link, it will be able to describe the rupture of ropes under tension, which is local. Of course, if ropes not only break, but also melt, and the heat generated by friction in one area can be transferred, along the fibres, to another, and act there, the whole argument in favour of its utilization becomes less convincing.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on May 01, 2015, 07:38:02 AM
Well, like I said at the end, if you are in fact talking failure vs lenght, linking together segements of length, then you are  exactly right, it is a weak link problem, and for a FIXED tension, it's the simplest 0 power (k=1) weibull distribution in length.

But that's not what's being tested here.  He's not testing a series of a links of rope.  He's testing one or two specific points on the rope that are under the most stress. There might be a microscopic model of deformity, that, well never mind, or there might not, it might be meter scale variances from chemical mixtures, or whatever or just small variations in knot dressing. What's clear, even for a very pure material you eventually get to a point where you simply exceed the strength per area of the fundamental chemical bonds involved.  The underlying probability function is then certainly not power-law like or increasing, there is no expected delta tension left before failure, however small, it is simply reaches 100.000% failure probability (exactly instantaneous if this were a time-like weibull problem).  In a couple of ways this doesn't seem to fit at all with the weibull concept.  Anyway, this is way off on a tangent of a tangent.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 01, 2015, 10:49:54 AM
... He's not testing a series of a links of rope. 
... exceed the strength per area of the fundamental chemical bonds involved.

   What I was talking about is this -simplistic, most probably - mechanical model of the individual fibres of the rope, where the molecular bonds, arranged in a row, play the role of the links of the chain. In the microscopic scale, because of the slightly different orientations of the bonds relatively to the axis of the loading, and probably also because of the thermal energy generated by friction ( dan Lehman says that heat may also be generated by local compression ), the distance between them varies, so the ability of each of them to retain its integrity under a particular tension varies, too. It is not that the strength exceeds the strength of a bond, it is that bonds are not, regarding strength, behaving the same way, and the weakest of them will break.
   Anyway, that is the simplest thing the na?ve mind of this poor ignorant knot tyer has to have, in order to have a model in the first place, because, as you said, human mind adores simple things, and hates the messy real complex things.  :)
   I would be glad if somebody would tell me how scientist believe things "really" are, and why this or that probability distribution is the one which describes the behaviour of the individual parts of their model.

Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Tex on May 01, 2015, 03:54:27 PM
xarax:
Quote
the distance between them varies, so the ability of each of them to retain its integrity under a particular tension varies, too.
.. which all sounds extremely Gaussian, not weibull.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 02, 2015, 07:47:37 AM
The breaking point in the figure 8 knot was inside the body of the knot.
Where the stading part takes its sharpest bend, at the loop side of the body of the knot.
I did tie each fig 8 in it's "strong" form intentionally.

You can see in the video how the fig 8 reacted to the high load, the standing part slips down
and under it's twin (the tail) at the loop end of the body of the knot.  ... I've also seen this
deformation on heavily loaded fig 8 knots in larger rope, "in the field".

As an aside, I'm not convinced that there is a stronger and weaker way to tie the figure 8 loop knot.
I've seen it said in one source that there is, and I've seen it said in another source that they are equal.
Most sources don't mention a distinction at all.

Perhaps that will be my next break testing experiment (tex i may need your help with the math)
Thanks.
I got "strong form" from assertions made by Rob Chisnall's
Ontario Rock Climbing (-ers?) Association Ref. Manual
(I might've mis-remembered this title, egadz!), which in
at least recognizing that there is a difference (clearly, in one
loading the SParts beats into its twin, in the other it pulls
away!) gave some credence to the assertion.  (Most folks
don't seem to have a clue about this.  So often, the knot
is presented without indication of SPart/tail !!)  His assertion
was a difference of 10%-points (that's "percentage points")
--unless I've also mistakenly recalled this.  (60 vs 70 and
not 60 vs 66, e.g.)

(You don't say what your sources are; I'm curious.  Now I'll
name further ... .)

Dave Merchant, in Life on a Line, asserts a similar
difference (I'll be similarly unsure of %-pt.s etc  :-\ ) in
advocating for the not-so-easily made form that Xarax
nicely presents here (in two posts):
igkt.net/sm/index.php?topic=5268.msg34654#msg34654 (http://igkt.net/sm/index.php?topic=5268.msg34654#msg34654)
  [ Fig.8 IMAGES of Perfect & D.Merchant's & other dressing ]
My guess is that OnRope1.com's Bruce Smith's "Mythbusters"
rebuke is directed at Merchant (though he doesn't name names
--and maybe is reacting to heard-thru-grapevine hearsay  ;) ),
but fails miserably in citing the incredibly stupid, perfectly
impossibly "flat" image of a fig.8 in which the artist gets off
easy by just tracing one '8', neverminding that round-crossectional
material cannot maintain such orientation under force !!
This image is echoed all over, no one the wiser.  It's supposed
to show a wider vs. tighter "first bend", but that's nonsense.
.:.  If Bruce cannot see this, to heck with what follows.
Presumably, Merchant, who could illustrate his orientation,
has test data to support his claim --though I think his main
one was for easier untying?!  --and he didn't lean hard on
the difference in strength.

In the field, one can readily discern (a) much slop in the
knot, and many cases of "perfect form" loaded "weakly"
in eye knots : the turn first going around the eye legs
being slack, quite obviously so, and the other drawn
tightly into the body.
In the end-2-end knot (I avoid "bend", yes), the "strong
form" shows by crunching --from opp. ends-- the body
into a sort of 45-degree angle to the axis of tension;
whereas the "weak form" (and I see these names mostly
now as just labels, not necessarily meaning as they sound)
the knot looks more parallel, aligned with this axis.

(My "Lehman8" was designed based on the assumption
derived from Chisnall's assertion, aiming for the curvature
got by the 'bearing against the twin part", which occurs
before the U-turn around the legs.  Maybe some test of
cordage with marked differences in surface friction would
show the slicker one weaker or at least breaking later,
at the u-turn, whereas the frictive one would gain what
is hoped, of off-loading force on the bearing-against part,
and so be less stressed at the U-turn.
ALL OF WHICH EXAMINATION/testing/study is done mainly
for edification about knot/cordage mechanics,
and not to write better rockclimbing recommendations
--which might favor ease of (un)tying over (slight) strength
differences!!

As for core breaking and then sheath : I've seen the opposite,
and esp. perhaps in ropes with hi-mod but slippery cores
(esp. HMPE), where slippage of the core might leave the
sheath taking the load.  (Sometimes, it has been reported
that the core then pulled out of the knot!)


--dl*
====

Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 02, 2015, 07:52:13 AM
   To knot rigger :

   When you will perform your next series of tests, using 1/2 inch climbing/rescue ropes this time  :), do not forget, please, to measure ( using a vernier scale ) the external width of the ( loaded by 100% of the total load ) first curve of each knot, after rupture.
   We have to settle this issue, and see if the first curves differ, when the ABK are loaded by different ends.
After rupture???  --there will be nothing left to measure
(or it will have relaxed into a different form)!

In specimens I had tested, though, I did tie duplicate
(eye) knots (one at each end), expressly to have a
non-ruptured-but-near-100%-loaded survivor,
albeit no longer under tension.  I do think that it
was close enough to rupture state --esp. in HPME,
which has so little elasticity-- to assess the position
of in-rope marking threads vis-a-vis the ruptured
other knot.

 :)
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 02, 2015, 02:05:47 PM
After rupture???  --there will be nothing left to measure
(or it will have relaxed into a different form)!

  I have seen that the "broken" knots remain in one piece, even after the rupture - and this is especially true if the rupture lies outside the knot, as it happens very often. The one end may be "uprooted" from the nub, but the nub itself remains in the same form it had before the rupture - perhaps because, in complex enough knots, and after such heavy loadings, all parts have been "glued" with the parts next to them, and they form an integrated tangle, which is not loosened, even if one part is cut off.
  However, it could be better if we measure the first curves just before the rupture, or during the rupture ( which takes place in milliseconds !  :) ). But, since we can not measure them during the rupture, how long before it ? Also, I would nt ask knot rigger to go near to a knot which may break the next millisecond !  :) That is why I had written that "after"... Of course, if it will be established, from those tests, that the nubs of those knots explode, or simply get more voluminous, after the rupture, we will have to measure them before the rupture, at, say -5% of the average maximum strength - which, probably, will still be lower of the minimum of all the recorded rupture strengths.   
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on May 02, 2015, 10:48:00 PM
Quote
You don't say what your sources are; I'm curious

I thought, when I originally posted, that I should mention the sources, but I didn't have them on hand at the time to quote accurately...  As you probably guessed, my two sources are the same as yours.  Merchant's Life on a Line proposing that there is a stronger and weaker way, and Smith and Padgett's On Rope stating that there is no difference.  In hindsight, I'm happy that I neglected to cite my references, as you gave me a few further sources on the topic.  Any idea where I can find a copy of Rob Chisnall's Reference manual? 

Merchant does indeed state that the "weaker" method "can" reduce the breaking strength by 10%, but he doesn't cite any particular study or data. He also says of this 10% difference in strength: "in tests it can be difficult to prove this reliably" How about that for a caveat! He does say that the "weaker" version is harder to untie than the "stronger" version.  I disagree with his picture showing the "correct" dressing of the knot.  Other than disagreeing with him in small ways, I think that Dr. Merchant's book is the best text available on the subject of rope rescue, and in the top five texts for rigging with knots.

http://igkt.net/sm/index.php?topic=2198
Xarax's version C is what Merchant shows as the "correct" dressing, whereas I believe Xarax's version A is the "correct" dressing of the figure 8 loop knot.  (thanks for the pics Xarax)

Smith and Padgett site Neil Montgomery's Single Rope Techniques as saying that it is possible to tie the fig 8 loop incorrectly, and that the weaker version is 10% weaker (same as Merchant) but they go on to discredit this notion. They site "knot destruction tests and Ashley" as sources.  I can't find anything in the ABOK on the topic. (ABOK #1047)  I will say that I'm fairly knowledgeable about rope access and rescue rigging, and this notion that there is a 10% weaker version of the fig 8 is NOT widely known, or taught.  The common wisdom is that there is no important difference in the two ways of tying the fig 8 loop.  Personally, I agree with the common wisdom, but I have been tying 8s Merchant's way since I read his book.  His way certainly can't be worse!

BTW the ABOK is on google books:
http://books.google.com/books/about/The_Ashley_Book_of_Knots.html?id=aN58gdigmy4C

Last thought about the figure 8:  In the 4th edition of the CMC rope rescue field guide (I just own the field guide, currently not the full text) they state that the efficiency of the figure 8 bend is 51%, and they state this 51% is much lower efficiency that in their previous tests (which I believe are widely cited by other sources).  There are certainly stronger bends out there (like the double fisherman) and also more jam-resistant bends (like the zeppelin bend).

Since I mentioned it, here are my top five texts for working with knots (in no particular order)

On Rope by Bruce Smith and Allen Paggett
Life on a Line by Dr. Dave Merchant
Knots & Ropes for Climbers by Duane Raleigh
Knots for Boaters by Brion Toss
Knots at Work by Jeff Jepson

how 'bout that for drifting my own thread off topic?

Last thought (more thread drift!):  On Class 2 double braids (HMPE core, conventional synthetic jacket).  On a show I worked on we used a class two double braid as guy lines, which were tensioned, and then subjected to cyclical dynamic loading (swinging of a trapeze).  We found that the core would bird cage, causing bumps in the line, and eventually break at those bumps!  We solved this issue by adding extra slack in the jacket (the lines were spliced either end).  We think the friction of the jacket on the core was leading to the issue of the bird caging.  On another show (i didn't work on) class 2 line moved a trolley.  They were heavily loaded,  under constant tension, and dynamically loaded, but the D:d ratio was large.  They broke-test a set of used lines and they broke at 50%! of their published breaking strength!  My conclusion is that we (all of us) are still learning the behavior of these "new" HMPE ropes.

cheers
andy
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 03, 2015, 06:37:26 AM
References, ok, type error analysis in amazon.com.
Choose the first book that comes up.
It's actually a great one (older editions are cheaper though).
The next 10 or so all look good too though.
This is dubiously helpful, in hoping that the state of
the world (or that vast part of it called "Amazon"  ;) )
is stable.  But taking this recipe, I find ...
Quote
An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements
Aug 1, 1996
by John R. Taylor
Paperback
$16.80 to rent
$37.44 to buy
... and marvel at the "TO RENT" option, which is new to me!?  :o

--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 03, 2015, 07:44:06 AM

I thought, when I originally posted, that I should mention the sources, but I didn't have them on hand at the time to quote accurately...  As you probably guessed, my two sources are the same as yours.  Merchant's Life on a Line proposing that there is a stronger and weaker way, and Smith and Padgett's On Rope stating that there is no difference.
You're being ambiguous,
except by content of assertion --which pins both
claims I think to 2nd versions : On Rope, 1st ed.
carries the assertion of difference (and the incredibly
stupid illustration (that "flat" impossible '8') with
guidance to "dress & set" but how the devil is one
to know how to do that, devoid of specifics?!
(typical rubbish, I'm afraid)
And Merchant I think had an earlier version that was
publicly available for free and which has less interesting
knotty information --I think.

And, while it's on my mind, noting that you must be
with OR-2nd, let me point out to you its mistaken
presentation --and to some danger, IMO-- of those
"3- / 4- / 5-coil Prusik [hitches]" : the 2nd edition
turned them upside-down, and it's possible that they
will not grip at all, and otherwise do so poorly --for
the single-turn half then being on "top" will press
down upon the multi-coils beneath, and cause the
hitch to slide (just as Ashley remarks for the rolling
hitch
) !!  (I see that "after Thrun" is cited, and
IIRC Bob & I --or him alone?-- tried to contact Bruce
Smith about this (or else whom?), w/o acknowledgement
for the effort.  There had to be some matching alteration
of the wording, which I recall discussing --and had thought
it pretty simple--; and that should belie a claim of pure
innocence for whoever made the unjustified change :
i.e., if you confront an assertion of needing coils on TOP
and you're putting them on the bottom, you must know
that you're going against the rationale!  And I believe
that the words ARE changed.
[Perhaps Bob can chime in re this.]

Quote
Any idea where I can find a copy of Rob Chisnall's Reference manual? 
Not of any official source.  It's dated (ca. 1984, from memory?),
and in full might be about a $60 copying fee alone (for much
more than just knots, of course).  It didn't as I recall have
anything beyond the "10%" claim; not sure where I might
have gotten the thought that ORCA did their own testing.

Quote
Merchant does indeed state that the "weaker" method "can" reduce the breaking strength by 10%, but he doesn't cite any particular study or data. He also says of this 10% difference in strength: "in tests it can be difficult to prove this reliably" How about that for a caveat!
The first thing to ask about such claims is what the
"10%" (or whatever) means --"10 %-points" or purely
"10%".  It won't amount to a huge difference, but, still,
one should speak clearly.  The easy and IMO more useful
value is %-points --what one can see between test values,
and which works in *both directions* (if I'm 50% bigger than
you --by your weight, x 1.5--, you're not 50% but 33%
smaller than I by my weight  :: the two "*directions*").

Again, with Smith unable to get a decent image of the
orientations, I have no confidence that he knows what
he's talking about.  AND, to those at least with the right
orientations, recall my thoughts on setting --i.e.,
to set with hard force on the TAILS, to try to give that
curvature in them against which the SPart will bear,
and have them tight so that they don't (so easily) get
pushed out of the way (and allow the SPart to straighten).

Quote
I think that Dr. Merchant's book is the best text available on the subject
of rope rescue, and in the top five texts for rigging with knots.
I can take him to task for dismissing the existence of
mid-line/directional "fig.9 knots", but such things
are the natives of minds like mine or Xarax and hardly
commonplace.  There are actually a batch of directional
eye knots to be played with!

Quote
http://igkt.net/sm/index.php?topic=2198
Xarax's version C is what Merchant shows as the "correct" dressing, whereas I believe Xarax's version A is the "correct" dressing of the figure 8 loop knot.  (thanks for the pics Xarax)
X. posted some images more recently, too --adding
the version-A after I remarked of the missing "perfect form".
But here's another ambiguity (your 3rd : back to the bench!),
for X. doesn't give loading.  To match D.M., it would be white
on right, orange left, of version-C.

Quote
Smith and Padgett site Neil Montgomery's Single Rope Techniques as saying that it is possible to tie the fig 8 loop incorrectly, and that the weaker version is 10% weaker (same as Merchant) but they go on to discredit this notion.
 They [c]ite "knot destruction tests and Ashley" as sources.
Which begs the question of how that really "discredits"
anything --as you note re Ashley, and I re their understanding
of the actual orientations (as they cannot illustrate it).
I don't know of Montgomery's work, and so cannot comment
on what he might show (or verbally illustrate) as dressings.
Do you have that?

Quote
... this notion that there is a 10% weaker version of the fig 8 is NOT widely known, or taught.
The common wisdom is that there is no important difference in the two ways of tying the fig 8 loop.
??  Common wisdom?  I don't see evidence of what I'd
call "wisdom" --it would have to be inferred by absence of
counter statements--, but rather a common ignorance
of the difference : mostly, there is no discussion of it,
and many times there is no indication of which end to
load, for the eye knot (and maybe also the end-2-ender,
which is less frequent in presentation, the grapevine
having much of that knotting domain).
And note that we are discussing more than "two ways"
of that knot : there are two loadings --i.e., choice of
Which End?-- per images Xarax gives, so that's six.

Quote
Personally, I agree with the common wisdom, but I have been tying 8s
Merchant's way since I read his book.  His way certainly can't be worse!
Why not?
But as he notes the differences (in contrast to Smith & P),
one can give to him (as for Chisnall) some hope of credibility!
We might note simply that "ropes don't break at knots"
and that the vast usage of all sorts of slop suggests at least
their practical safety.   Still, for our finer understanding,
we'd like intelligent testing!  (Here is a good point to remark
at the testing of both "tied in the bight" and "rewoven"
fig.8 eyeknots by both CMC & the no-longer-easily
available Dave Richards testing (which, alas, Smith got
booted off of its host, on account of some simply explained
inconsistencies (misreading data sets between two of the
three ropes tested --i.e., reading A's for B and vice versa).

Quote
Last thought about the figure 8:  In the 4th edition of the CMC rope rescue field guide
(I just own the field guide, currently not the full text) they state that the efficiency of
the figure 8 bend is 51%, and they state this 51% is much lower efficiency that in their
previous tests (which I believe are widely cited by other sources).  There are certainly
stronger bends out there (like the double fisherman) and also more jam-resistant bends
(like the zeppelin bend).
Now you scare me : how can CMC come out with such
nonsense?!!!  I mean that in the sense that LOTS of
testing has put its strength well higher than 50% !!
--i.p., their own, as pub'd in their 3rd version : it gives
the fig.8 end-2-end knot (of some loading (they
use the easy-for-artist-but-impossible-for-rope image))
as 81% which is stronger than their grapevine!!
How do they explain this drastic revisionism?!?!?
(Oh, <groan>, they don't actually say '81%' : no, they
indulge the nonsense of talking about "strength lost",
so write '19%' and leave the needed arithmetic for the
reader --bugs me, for its the other value that one uses
in figuring systems and so on, grrrr.)
Do they, e.g., claim to have been doing "youthful
indiscretions" / "taking drugs" when they put out the
test results in the 3rd edition??  (I forget, but think that
Dave Richards didn't test the end-2-ender, but only the
eye knot (tied both ways!).)


Quote
Since I mentioned it, here are my top five texts for working with knots (in no particular order)
I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...  ;D ) as a fresh, different treatment.


Quote
Last thought (more thread drift!):  On Class 2 double braids (HMPE core, conventional synthetic jacket).  On a show I worked on we used a class two double braid as guy lines, which were tensioned, and then subjected to cyclical dynamic loading (swinging of a trapeze).  We found that the core would bird cage, causing bumps in the line, and eventually break at those bumps!  We solved this issue by adding extra slack in the jacket (the lines were spliced either end).  We think the friction of the jacket on the core was leading to the issue of the bird caging.  On another show (i didn't work on) class 2 line moved a trolley.  They were heavily loaded,  under constant tension, and dynamically loaded, but the D:d ratio was large.  They broke-test a set of used lines and they broke at 50%! of their published breaking strength!  My conclusion is that we (all of us) are still learning the behavior of these "new" HMPE ropes.
Okay, not an acronym, but ... what is "birdcage"
qua verb?!  And "trapeze" as in the usual thing,
with people swinging?  --and then "would eventually
BREAK" ??!  What's with the swinger, then?!
As for friction of jacket on HMPE core : I'd think that
there'd be not much --less than for other then HMPE,
anyway.
One can question strength loss, or published accuracy, eh!?
(Angling line seems to often come with WAY understated
tensile strength, so knotted strength is high enough.)


--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 03, 2015, 09:05:35 AM
X. posted some images more recently, too --adding the version-A after I remarked of the missing "perfect form".

   " adding the version A after I..." :) :) :)
  Your ability to distort facts and history is remarkable, indeed !
  It was exactly the other way around !  :)
  I had posted two, out of the 5 (FIVE), in total, symmetric versions, as a proposal for a tricky poll with the aim to prove that knot tyers ( and especially those who work their material self in indoor gyms !  :) ), can not say which is the "perfect" version and which is not - but the trick was that, in fact, none was ! (1) You suspected that, but you were not very sure, so you replied by a faint-hearted "I am thinking that..." - while you should had shouted or mocked at me, as you always do  :) (2).
   Then, in the next post I had revealed the trick (3), and pointed out that your sight does not improve, otherwise you would nt had replied at all, or you would had replied in your usual way.
   Now, I have to inform the readers that I had posted pictures of those 5 symmetric variations 5 years ago ! !  - along with all the numerous non-symmetric ones (4). And, of course, I was always speaking of three musketeers, because I had always considered the A as THE perfect fig.8 bend, that is, as the D'Artagnan of the company ( D'Artagnan was NOT one of the three musketeers : Athos, Porthos and Aramis ).
   So, why 5 ? Because there is yet another symmetric dressing of the same knot ( or, perhaps, another topologically equivalent knot ), which I had called the Ring bend". See (5), and the attached pictures. 
     
1. http://igkt.net/sm/index.php?topic=5268.msg34637#msg34637
2. http://igkt.net/sm/index.php?topic=5268.msg34646#msg34646
3. http://igkt.net/sm/index.php?topic=5268.msg34648#msg34648
4. http://igkt.net/sm/index.php?topic=2198
5. http://igkt.net/sm/index.php?topic=2198.msg16675#msg16675

   I am sick and tired to repeat the same things over and over again ! The last time you had been pretending you do not understand, like you do now, I had deleted my relevant posts - and only later I had posted the pictures, and the pictures only, again...
   So 5 ( FIVE : one, two, three, four, five ! ) are the symmetric variations of this fig.8 bend, of which one is the perfect form, three are not-so-perfect ( but as we have seen, even experienced knot tyers often can not distinguish them...), and one, the last ( which I have not seen anywhere ), the Ring bend, is, well, the odd man out, indeed.

( Note : I am talking about 5, because I count only the geometrical "loose", and not the actual, loaded variations, where the pair of loaded ends varies )
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 03, 2015, 06:50:52 PM
X. posted some images more recently, too --adding the version-A after I remarked of the missing "perfect form".

   " adding the version A after I..." :) :) :)
  Your ability to distort facts and history is remarkable, indeed !
  It was exactly the other way around !  :)
Nooope, it's your penchant for taking offense,
for seeking it in everything, and so mis-taking
the meaning intended : simply, in that particular thread
you (kindly & quickly) supplied that image (and
that thread is moREcent than the others, yes).

Quote
I am sick and tired to repeat the same things over and over again !
Unless it's the raving about a hinge in the zeppelin,
or ... .

--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: xarax on May 03, 2015, 07:21:15 PM
in that particular thread you (kindly & quickly) supplied that image

   Aha ! I see ! You have been swallowed by a wormhole, and you are now at the other side !  :)
   Because I HAD SUPPLIED THIS SAME IMAGE 5 ( five ; one, two, three, four, five : you can use your fingers to count, you know  :)) years ago ( ago, before, in the past, then, etc : PAST tense...)

1. http://igkt.net/sm/index.php?topic=2198.msg15419#msg15419
2. http://igkt.net/sm/index.php?topic=2198.msg15419#msg15419
3. http://igkt.net/sm/index.php?topic=2198.msg15419#msg15419
4. http://igkt.net/sm/index.php?topic=2198.msg15419#msg15419
5. http://igkt.net/sm/index.php?topic=2198.msg15419#msg15419

(Repetition is the mother of knowledge
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on May 04, 2015, 12:54:09 AM
Quote
You're being ambiguous

If I was ambiguous, it was on accident.  I was merely making observations on the question of the two versions of the figure 8 loop.  As I said, I'm not convinced there is a difference in strength, it is an unanswered question to me.  (ie, up for debate)  The question is important to me, as the figure 8 loop is part of the foundation of rope's access work, and much of what I do.  I, and my co-workers, rely on a figure 8 loop knots that I tied every day, and you can see how their strength may be of some interest to me. 

Quote
which pins both claims I think to 2nd versions

correct, I have 2nd editions of both texts.  If you'd like me to scan and send you the pertinent pages of Life on a Line I would be happy to.

Quote
Any idea where I can find a copy of Rob Chisnall's Reference manual? 
Not of any official source

I would be interested in obtaining a copy, if anyone knows where I can.

Quote
10 %-points" or purely "10%".

I agree that Merchant is could be more specific here.  He just says 10%

Given that he states this can be difficult to show with breaking tests, I am dubious of performing these test, but I may proceed anyway if I feel I may obtain meaningful results.  It would be an interesting challenge, if nothing else.

Quote
Do you have [Neil Montgomery's Single Rope Techniques]?

Not yet, I ordered a copy on amazon.  I'll share what I find.

Quote
??  Common wisdom?  I don't see evidence of what I'd
call "wisdom" --it would have to be inferred by absence of
counter statements--, but rather a common ignorance
of the difference

Indeed, it may be common ignorance.. but ignorance by persons, who as a group, trust this knot with their lives, and the lives of others. 

Quote
Personally, I agree with the common wisdom, but I have been tying 8s
Merchant's way since I read his book.  His way certainly can't be worse!

Why not?

Well, that is a good question.  One, an appeal to the greater wisdom of the industry at large.  Two, if there is a difference, it is only 10% or less (according to the "experts") and a fig 8 knot is still one of the strongest, secure, and easily verifiable knots for the purpose.  Three, Dr. Merchants reasoning is sound IMO, that his "stronger" version has a larger radius first bend in the load bearing line.  It stands to reason that a wider bend is stronger than a tighter bend.  The question, to me, is if this slight difference has large, or important influence in how the knot performs.

Quote
Now you scare me : how can CMC come out with such
nonsense?!!!  I mean that in the sense that LOTS of
testing has put its strength well higher than 50% !!

I had no intention of giving you a fright!  ;)  It seems clear to me that CMC revised there assertions based on the presence of new data.

Quote
the no-longer-easily
available Dave Richards testing

If anyone could provide me Mr. Richards testing, I would appreciate it.

Quote
I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...  ;D ) as a fresh, different treatment.

Thanks for the tip, I'll pick up a copy

Quote
Okay, not an acronym, but ... what is "birdcage"
qua verb?!  And "trapeze" as in the usual thing,
with people swinging?  --and then "would eventually
BREAK" ??!  What's with the swinger, then?!
As for friction of jacket on HMPE core : I'd think that
there'd be not much --less than for other then HMPE,
anyway.

Okay, "birdcage" is a terms usually applied to a deformation of wire rope, usually due to shock loading, where the strands gain a permanent deformation, or twist, that allows you to see daylight through the lay of the line.  It looks like a little metal birdcage.  I used the term to describe a similar deformation, or bump, in the 12 strand core of the rope I describe.

The rig I describe has a "crane bar", (a metal bar from which the trapeze is suspended).  The crane bar is held up by two chain hoists, and has four guy wires to hold it's position.  The crane bar is first flown in (down) slightly below its final position, the slack guy lines are a then attached to achors on the ground, and the crane bar is flown out (up) to its final position, thus tensioning the guy lines.  Then the artist swings on the trapeze for her act, which cyclically and dynamically further loads the guy lines.

The guy line rope is constructed of a vectran core, with a polyester jacket.  The guy lines had spliced terminations on each end.  The original construction of the guy lines (the ones that birdcaged) was that the jacket was stretched equally tight as the core during fabrication.  The eventual failure of these lines occurred at one of these bumps.  After failure the external appearance of the line was hourglasses at the failure point, indicating that the core had ruptured, while the jacket was still intact.  Cutting open the line, to inspect the core, showed the rupture point to be very fuzzy and extremely elongated, signs of a failure due to abrasion.  This failure mode occurred on multiple lines within a few days of each other.  We initially replaced the lines with new ones fabricated the same way: with the jacket tight to the core, and the replacement lines began showing the same bird-caging within weeks.  I then fabricated a new set of line, but with the jacket very slack.  Even under tension, the jacket could be easily milked along the line.  These new lines never exhibited any of the bird-caging the previous lines did.  Also, the new lines had a much softer hand to them than the old ones.  The old lines were very stiff, and coiling them was similar to wire rope, or very stiff kernmantle.  The new lines felt like a soft class one double braid.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 04, 2015, 10:12:51 PM
correct, I have 2nd editions of both texts.
If you'd like me to scan and send you the pertinent pages of Life on a Line
I would be happy to.
Thanks for confirmation & offer.  I have the latter,
but might ask particular questions re On Rope, 2nd ed..


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10 %-points" or purely "10%".

I agree that Merchant is could be more specific here.  He just says 10%
As do so many others, alas.

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Given that he states this can be difficult to show with breaking tests,
I am dubious=>[doubtful] of performing these tests, but I may proceed anyway
if I feel I may obtain meaningful results.
It would be an interesting challenge, if nothing else.
I would tie specimen knots in both ends.  For this test,
since you have a seemingly well-calibrated device, you
could make them same-orientation,
or otherwise you have --in addition to what the device
might record, in absolute values-- A-vs-B testing.  In
either case, with eye knots at both ends, you get one
that survives intact, for examination.  Now, were the
results so pure that dressing-XYZ was always breaking
and leaving dressing-PQR the survivor, you might feel
that you'd found a pattern, BUT would want to get a
surviving weaker dressing and so need to double up
on that, at least for this point of record.

And given your relatively not-so-strong material (if you
are indeed continuing to use something like that),
you should be able to get a good firm set as I specify
with pulling on tails, to try for that, *baked-in*, twin-part
path-shaping for the SPart to bear against.  At least, this
is my theory and the best sense I can make of how that
"stronger form" (my label) could in fact be stronger
(for the turn around the eye legs seems rather sharp,
arguably worse than using the other end qua SPart).

But, another aspect of this "perfect form" (not the
Layhands form, mind)  fig.8 is that maybe with
the "weak form" one gets less hard constriction of
the eye legs and therefore they bring more gripping
effect where they collar the SPart and that is
what gives strength --some frictional off-loading of
force at the entry point!?  --Theory-2, which, note,
puts significance to effects coming before any hard
radius of turning is encountered.  ;)
(In any case, it is interesting to note that the eye knots
are often found stronger than the end-2-end knot!?
(E.g., one fellow used a truck to do A-vs-B end-2-end knot
testing, and all specimens were anchored w/fig.8 eyeknots
none of which ever broke (even vs. a case where effectively
the same structure was there, in the "twin fig.8s" joint
which is essentially two eyes sharing the eye strands!?).)
(CMC 3rd ed. has end-2-end & eyes equally at 80-81%.)

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Do you have [Neil Montgomery's Single Rope Techniques]?

Not yet, I ordered a copy on Amazon.  I'll share what I find.
Great, thanks much!   :)

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Indeed, it may be common ignorance.. but ignorance by persons,
who as a group, trust this knot with their lives, and the lives of others.
Which could be a point to worry about!
I recall Tom Moyer lamenting that despite his efforts to warn
some group about the dangers of the offset fig.8 end-2-end knot
(aka "fig.8 EDK"), they seemed unconcerned about it.

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Now you scare me : how can CMC come out with such
nonsense?!!!  I mean that in the sense that LOTS of
testing has put its strength well higher than 50% !!
It seems clear to me that CMC revised there assertions based on the presence of new data.
Yes, fine, but also in the face of solid other data
--or did they admit to having indulged "youthful indiscretions"
at prior data gathering/interpreting/recording?!?   ::)

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If anyone could provide me Mr. Richards testing, I would appreciate it.
Ah, it came out nicely, via Agent_Smith's hosting --voici
 www.paci.com.au/downloads_public/knots/03_Cordage_Institute_Tests.pdf (http://www.paci.com.au/downloads_public/knots/03_Cordage_Institute_Tests.pdf)
And here are remarks about the mistakes in the labeling
of data --a simple case of A vice B & vice versa : soooo
simply noted (even w/o correction), but Smith it seems
has found levels of "embeddiing" of error upon error,
sadly w/o a sane reviewer to set straight.  And, so it has
languished in Purgatory of non-presented status, waiting
for Godot.  Thanks again to Agent_Smith for doing better.
www.forums.caves.org/viewtopic.php?f=5&t=12907 (http://www.forums.caves.org/viewtopic.php?f=5&t=12907)

NB : knots are identified nominally; we do NOT see images
asserted to show geometry!

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I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...  ;D ) as a fresh, different treatment.

Thanks for the tip, I'll pick up a copy
NB: the "square fisherman's" got mis-represented in the
photo --it there is actually a, um, "thief fisherman's" : yeah,
tied a thief knot backed with strangles (not how the
climbing community names things, but ... .  (Clyde protested
that it couldn't be, for he doesn't know how to tie the thief;
I infer that, yes, he doesn't, but tied what he intended (and
missed) by flipping his ropes around to tie off with the strangle
and --these being short pieces-- confused tail w/SPart !
OTOH, arguably, this shown is better, easier to untie
(just make sure that the strangles stay tied --which,
if they're set snug to the thief, they have grounds to
be more secure, as slippage will snug them up further!)
.:.  a play in double-edgedness!
(I saw photos too late for corrections to be made.)

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Okay, "birdcage" is a terms usually applied to a deformation of wire rope,
usually due to shock loading, where the strands gain a permanent deformation,
or twist, that allows you to see daylight through the lay of the line.
It looks like a little metal birdcage.
That was my surmise, except I've trouble with ...
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  I used the term to describe a similar deformation, or bump,
in the 12 strand core of the rope I describe.
... how one sees ANYthing in the core?!?!
What's happening with the sheath, to enable this?
(If it's in postmortem cut-&-see, well, okay; but it
still seems odd for the core to have such strength
to birdcage within a close-fit sheath, IMO.
--and more problems with Vectran, hmmm.
(Used on one of rockclimbing gear-seller's hi-mod
lines (5.5mm) and discontinued, for dubious results.)

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The guy line rope is constructed of a vectran core, with a polyester jacket.  The guy lines had spliced terminations on each end.  The original construction of the guy lines (the ones that birdcaged) was that the jacket was stretched equally tight as the core during fabrication.
//
I then fabricated a new set of line, but with the jacket very slack.  Even under tension, the jacket could be easily milked along the line.
I'm struck by "fabricated" : implying one's own making,
and not a (complete) "store-bought" product. !?

I'm stumped for an explanation, other than musing that
somehow there was a segment (or few) where the original
sheath was less married to core and slipped back'n'forth
over it, and the Vectan suffered not the polyester (though
the former is much better with heat, but not so, abrasion?!)!?
With a relatively tight sheath, it could be load bearing.
But with a slack sheath, the core didn't have such *oppression*.

And the failure was incomplete, not catastrophic --the sheath
was working, the guy still stabilized , the problem was seen
(before the hourglass ran out of time).  --and yet, in just
some few days (or was it a longer period per line, and that
each of the quartet of guys came to grief in short order?)!


--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on May 05, 2015, 10:27:32 AM
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I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...   ) as a fresh, different treatment.

Thanks for the tip, I'll pick up a copy

Dan, take a closer look at picture 7.21 in outdoor knots. 
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: Dan_Lehman on May 05, 2015, 09:45:28 PM
Quote
I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...   ) as a fresh, different treatment.

Thanks for the tip, I'll pick up a copy

Dan, take a closer look at picture 7.21 in outdoor knots.
:o  Oooops!   ;)  (must be the muscle-building version!)

Good eye (and I don't recall noticing this).

Now, putting it to test/check with 12.5# barbell metal,
I conclude that "muscle-building" is it (the right way,
i.e.) !  --for which the answer might be that there is
a further part of the hauling system which gives some
MA (even "TTAMA" --"Tex's Theoretically Aware MA").


--dl*
====
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on May 07, 2015, 02:28:09 AM
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Dan, take a closer look at picture 7.21 in [Outdoor Knots by Clyde Soles..

There is a great article by Dan Chisnall in Knotting Matters 97 (i think it's 97, it's not in front of me) about carabiner hitches with is the best treatment of the alpine clutch I've seen in print.  Mr. Soles picture is mislabeled, but also is anchored into a webbing sling without a girth hitch.  The girth hitch makes the AC much more stable.

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particular questions re On Rope, 2nd ed.

I imagine the page about the figure 8 loop, and the page about asymmetrical prussics would be of interest to you.  Send me a private message with any requests and I'll scan and send to you.  I'd rather not post copyrighted material, but I consider private sharing "fair use"

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If anyone could provide me Mr. Richards testing, I would appreciate it.
Ah, it came out nicely, via Agent_Smith's hosting --voici

Ahh.. I have seen this before, I'm sure I saw it from a link here before.  My testing rig isn't nearly so fancy as this set up.  And, I've determined from the manufacture that using this style of dnyamometer for break testing is a no-no.  I'm trying to see if my work will buy the correct type, but for now break testing is on hold.

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I'm struck by "fabricated" : implying one's own making,
and not a (complete) "store-bought" product. !?

On the topic of the trapeze guy lines,  Yes, we did the splices in-house.. so we (I) fabricated them.  Here is a quick timeline of the saga as far a I remember from a few years back.  The original lines we're quite old (a couple years?) when they started to fail.  We had two fail within one week of each other, but it wasn't catastrophic.  One was found outside of a show when the measured tensions in the system we're too low.  And another was found by visual inspection.  We replaced all the lines with ones fabricated in the same way, with a tight cover, (tension balanced with the core at time of splicing).  This 2nd set showed the same bumpy birdcages within a few weeks, and we knew they we'er destine to fail like the first set.  We then made a 3rd set with the cover intentionally very loose, and they lasted a long time, over a year until I left the show after that, and they we're still doing fine.  For all I know, they may still be in use some 3 or 4 years later.

Two things may have been going on.  Vectran hates a tight bend, and in handling of these lines (especially the stiff handed first two sets) there may have been kinks that we're put in the core, that then became weak points where the rope eventually failed.  The other thought is that there was some friction between the core and cover that slowly wore through the weak points at the bumpy birdcages.  We we're using the vectran core due to it's low stretch, but then there was the polyester jacket that has a different way or stretching.  Perhaps the cyclical loading of the system was causing the jacket to stretch and rub over the core, which moved less that the jacket.  I'm not 100% on the cause of the failure, but I am 100% sure that putting slack in the cover made the bumps go away.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: alpineer on May 07, 2015, 03:12:17 AM
On the topic of the trapeze guy lines,  Yes, we did the splices in-house.. so we (I) fabricated them.  Here is a quick timeline of the saga as far a I remember from a few years back.  The original lines we're quite old (a couple years?) when they started to fail.  We had two fail within one week of each other, but it wasn't catastrophic.  One was found outside of a show when the measured tensions in the system we're too low.  And another was found by visual inspection.  We replaced all the lines with ones fabricated in the same way, with a tight cover, (tension balanced with the core at time of splicing).  This 2nd set showed the same bumpy birdcages within a few weeks, and we knew they we'er destine to fail like the first set.  We then made a 3rd set with the cover intentionally very loose, and they lasted a long time, over a year until I left the show after that, and they we're still doing fine.  For all I know, they may still be in use some 3 or 4 years later.

Two things may have been going on.  Vectran hates a tight bend, and in handling of these lines (especially the stiff handed first two sets) there may have been kinks that we're put in the core, that then became weak points where the rope eventually failed.  The other thought is that there was some friction between the core and cover that slowly wore through the weak points at the bumpy birdcages.  We we're using the vectran core due to it's low stretch, but then there was the polyester jacket that has a different way or stretching.  Perhaps the cyclical loading of the system was causing the jacket to stretch and rub over the core, which moved less that the jacket.  I'm not 100% on the cause of the failure, but I am 100% sure that putting slack in the cover made the bumps go away.

That's interesting kr. Rope manufacturers might be interested in hearing of this.
Title: Re: Break testing of the Alpine Butterfly Knot
Post by: knot rigger on May 07, 2015, 05:42:40 AM
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That's interesting kr. Rope manufacturers might be interested in hearing of this.

Hmm... It never occurred to me to tell the manufacture about it, we we're focused solely on solving the problem.  maybe I'll drop them a line.. too bad it was so long ago, if I had thought of it at the time I could have sent the broken bits into them.  alas

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I could suggest Outdoor Knots by Clyde Soles
(and w/help from ...   ) as a fresh, different treatment.

Dan, more bad news about Outdoor Knots I'm afraid :( re-read the section titled "coiling basics"  the paragraph beginning with "When coiling laid rope..."  (sorry I can't provide a page number, I have an ebook version.  BTW I should know better, knot books are always better in print, they always suck as an ebook)  So far the errata count is up to three items, and I haven't gotten half way through the book!

I do really like his section about rope construction, material choices, etc.  It's well put together.