Author Topic: Break testing of the Alpine Butterfly Knot  (Read 12421 times)

Tex

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Re: Break testing of the Alpine Butterfly Knot
« Reply #15 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.
« Last Edit: May 01, 2015, 02:02:30 AM by Tex »

xarax

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Re: Break testing of the Alpine Butterfly Knot
« Reply #16 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 ...
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Tex

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Re: Break testing of the Alpine Butterfly Knot
« Reply #17 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). 
« Last Edit: May 01, 2015, 02:20:52 AM by Tex »

Tex

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Re: Break testing of the Alpine Butterfly Knot
« Reply #18 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.
« Last Edit: May 01, 2015, 05:16:40 AM by Tex »

xarax

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Re: Break testing of the Alpine Butterfly Knot
« Reply #19 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.
« Last Edit: May 01, 2015, 06:06:24 AM by xarax »
This is not a knot.

Tex

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Re: Break testing of the Alpine Butterfly Knot
« Reply #20 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.

xarax

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Re: Break testing of the Alpine Butterfly Knot
« Reply #21 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.

This is not a knot.

Tex

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Re: Break testing of the Alpine Butterfly Knot
« Reply #22 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.

Dan_Lehman

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Re: Break testing of the Alpine Butterfly Knot
« Reply #23 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
  [ 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*
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Dan_Lehman

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Re: Break testing of the Alpine Butterfly Knot
« Reply #24 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.

 :)

xarax

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Re: Break testing of the Alpine Butterfly Knot
« Reply #25 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.   
« Last Edit: May 02, 2015, 02:08:13 PM by xarax »
This is not a knot.

knot rigger

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Re: Break testing of the Alpine Butterfly Knot
« Reply #26 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

Dan_Lehman

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Re: Break testing of the Alpine Butterfly Knot
« Reply #27 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*
====

Dan_Lehman

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Re: Break testing of the Alpine Butterfly Knot
« Reply #28 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*
====

xarax

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Re: Break testing of the Alpine Butterfly Knot
« Reply #29 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 )
« Last Edit: May 03, 2015, 09:15:38 AM by xarax »
This is not a knot.