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snoboy
Nov 13, 2009, 7:30 PM
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One of the benefits of webbing is that it is supposed to be stronger over smaller radii.
If you imagine a rope over a sharp edge, the inside fibers of the rope are bearing very little of the force, and the outer ones are bearing more. Webbing is constructed with the load bearing fibers next to each other, and so they all get bent about the same amount and thus bear the load more evenly. Does that make sense?
At some point I assume the edge becomes a knife though...
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knudenoggin
Nov 14, 2009, 2:42 AM
Post #52 of 71
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snoboy wrote: One of the benefits of webbing is that it is supposed to be stronger over smaller radii. If you imagine a rope over a sharp edge, the inside fibers of the rope are bearing very little of the force, ...
their butts kicked by being up against a *rock* with outer fibres "piling
on" ! Now, maybe in HMPE there is some different mechanism at work
given that fibre's low coefficient of friciton, but from what I have seen
in other stuff, the break comes on the inside of a bend, not outside.
*kN*
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snoboy
Nov 15, 2009, 11:20 PM
Post #53 of 71
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As I said, at some point the edge becomes a knife, I'm just not sure what that radius is for webbing or for rope.
I don't think our two 'images' are exclusive... The outer fibres are in tension, the inner fibres are in compression.
To clarify, I was referring to the tension when I said force in my OP.
(This post was edited by snoboy on Nov 15, 2009, 11:24 PM)
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ski.ninja
Nov 17, 2009, 12:15 AM
Post #54 of 71
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Back in the 80's when companies were competing to make lighter carabiners they found that anything with a rope bearing surface with a diameter of <10mm was more likely to cause the rope to break. I'm not sure about webbing, but I know that M_S did some parking lot testing of webbing slung through a bolt hanger. The results were not confidence inspiring.
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JimTitt
Nov 17, 2009, 8:23 AM
Post #55 of 71
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We have done some development on line lockers for slacklines which involved breaking loads of tape around various diameters.
With tubular tape the outer layer cuts the inner layer due to the pressure.
If you double a tape loop (such as when shortening a sling) and one layer lies over the other at the karabiner the sling is weaker than if used singly.
Woven tape stretches less on the outer edges than inside and rips from there inwards.
Around a 10mm pin you generally get about 60% of the single rated strength of the tape (though in a normal climbing application you are working with a loop so you will be up to ca 120% of the single tape rating).
After about 35% of the rated strength on a 10mm pin you start to see signs of pressure glazing.
To get near the rated strength you need to go up to a pin diameter of about 8-10 X the nominal tape thickness.
Curved pins like a karabiner make things worse, the wider the tape relative to the curve the more this effect appears.
Tape is rated around a large diameter split drum so the actual tape strength can be measured, from memory this is about 2" in diameter.
Jim
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knudenoggin
Nov 19, 2009, 6:52 AM
Post #56 of 71
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JimTitt wrote: If you double a tape loop (such as when shortening a sling) and one layer lies over the other at the karabiner the sling is weaker than if used singly.
!! Jim, pardon if I ask just 2 B sure of what you're saying (because it's
an eye-opener, for sure): fold a sling in half through a 'biner and clip
both bight-ends to a 2nd 'biner, thereby halving the length of the sling;
if the folded or bight-ends contact with the 'biner is stacked -- i.e.,
all strands evenly aligned above/below/atop each other,
then the sling will be weaker than an unfolded similar-material sling
(even though it has double the material!) ?!
What materials did you test this with? --HMPE & nylon, both?
I'll speculate that were one using a knotted sling, orienting an
overlap such as this so that the knotted side lay atop the other might
change the result at least a little, as the knotted side should lengthen
some from material feed from the knot, increasing shared tension on
the inner, compressed strands -- somewhat.
OTOH, for someone to know your finding and orient a double sling so
that it was spread wide & non-overlapping, there'd then be the problem
of such broad loading on the 'biner. -- damned if you do, ... don't!
In reply to: Woven tape stretches less on the outer edges than inside and rips from there inwards.
You see this when testing on a large pin? It certainly seems that in
many knotted situations that the sides are going to be asked to reach
longer, be stretched more, and so ... tear; but in a level bend around
a pin, the sides will still be first to go?
In reply to: Around a 10mm pin you generally get about 60% of the single rated strength of the tape
Is this just for tubular tape, or also for >>solid<< tape.
(Yes, I'm substituting "solid" for "flat" -- flat more or less is the
essential characteristic of all tape, no?! :-)
In reply to: (though in a normal climbing application you are working with a loop so you will be up to ca 120% of the single tape rating).
Hmmm, this seems odd: for it's generally reported that the Water knot
is about a 60% knot (and your results suggest that this is near enough
to perfect if joining 'biners); and yet that the Grapevine tests at a
higher strength -- and such tests I think are typically done using 'biners?
-- which implies that they see that higher load at the 'biner bends?!
Thanks much for these insights!
*kN*
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JimTitt
Nov 19, 2009, 10:10 AM
Post #57 of 71
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We noticed this first when we were using slings to join up the tester for some other tests and had to shorten them, then looked at what happens when one tape lies over another in a line locker. The outer layer cuts the inner one under pressure, sad but true.
We used both sewn and then after failure knotted slings, in all cases we kept the knot away from the ends and the slings failed at the karabiner not the knot.
I have only tested this with nylon but a gear tester from one of the major hardware manufacturers tells me this is also true of Dyneema (Spectra) but since the product in question is lower rated anyway it is of no concern in that application.
Even worse is with sewn slings where the cut and melted end of the tape pulls into the karabiner underneath the other layer, that melted end works just like a knife!
On the woven tape we had (two brands) the outermost longitudinal threads seemed to be effectively straight with the crossing threads wrapped around them whereas in the rest of the tape the longitudinal threads zigzag, presumably allowing the inner part of the tape to stretch more than the edges. Another type of woven tape didnīt seem to have these hard edges but this one failed because we doubled it so we donīt know for sure.
Mostly we are using 10mm dia pin shackles or we go up to 16mm to reduce the sling breakage (we arenīt testing slings, just using them to connect things together), we donīt normally use karabiners for safety and cost reasons.
The solid tape we used broke when testing other things so we didnīt check what percantage they got, only noticed they were much lower than the rated strength, they went in the bin and we changed to industrial lifting strops!
The tube tape however we specifically tested as this is what slackliners use. We tried lots (maybe 50) of different diameters and set-ups to try to get the failure load as high as possible. Around 16mm dia we started to get near the tape rating and 20mm was nearly perfect. Slackliners use extra layers of material between the tape layers to reduce the cutting effects and increase the diameter with some success. If you look at the way a BD Camalot sling is sewn they use this same idea.
There is quite a lot of testing been done on this in the slackline world which makes interesting reading.
Donīt quite understand the last point, maybe Iīm a bit slow though today!!
Jim
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knudenoggin
Nov 19, 2009, 8:02 PM
Post #58 of 71
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JimTitt wrote: *kN* wrote: Hmmm, this seems odd: for it's generally reported that the Water knot is about a 60% knot (and your results suggest that this is near enough to perfect if joining 'biners); and yet that the Grapevine tests at a higher strength -- and such tests I think are typically done using 'biners? -- which implies that they see that higher load at the 'biner bends?! Donīt quite understand the last point, maybe Iīm a bit slow though today!! Jim
Well, the remark might show my mis-assumption,
but what I'm saying is that if the 'biner bend is a 60% item,
and so too is a Water knot, then in a simple (and my presumed)
test configuration of testing that knot in a sling put around such
pins, there's a sort of race condition between the weakening
of the bend & knot to see which will actually be manifest -- that
they're roughly equally likely to fail. (And I'd not heard this.)
AND, that it has been reported that a Grapevine bend in tape is
significantly stronger than a Water knot. Now, I had presumed
--and maybe quite wrongly-- that Grapevine-knotted sling strength
would be tested the same way, pulled between attachment 'biners;
but if the bend at the 'biners is only 60%, one couldn't very well
see the greater strength of the Grapevine (other than concluding
it must be strong if it never is what breaks).
But, again, maybe my presumption of this testing configuration
is just wrong, and that slings were slung between beefier pins,
and so the knot indeed was the weak point, even with a strong
knot. (Though in this case it must be noted that if only ONE
side of the sling is knotted --the usual case, of course-- , then
that side likely gains material out of the compressed knot,
and friction at the pins inhibits equalization such that the
force will be higher on the unknotted side, and dividing the
overall sling strength by two to deduce knot strength will
exaggerate knot strength.)
In reply to: There is quite a lot of testing been done on this in the slackline world which makes interesting reading.
Anywhere one can get a hold of this interesting reading?
I and a few others would be happy to peruse the data & findings ... !
(and I esp. like pictures  )
Thanks,
--dl*
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JimTitt
Nov 19, 2009, 8:46 PM
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With you now, bit early for real thinking earlier!
Itīs for sure always going to be what fails first and as far as I can see if you use smaller pins the knot is always stronger (I only ever use a water knot). If you looks at Arics test setup heīs running what look like 12mm krabs which give the tape a significantly better chance.
As he says though he only used the manufacturers rating which might be very conservative in which case this throws the percentages off. The tape we used only just got its rated strength when we used a 30mm dia wedge system to clamp it but I know from the slackline tests some of the tape got easily over the rated strength.
I know what you mean about one side getting loaded more than the other with only one knot, I guess the only real way is to test a single length between two large dia clamp bobbins as is done industrially. This way you can get a true strength of the tape and of the knots without any possible interference. But there again thatīs also not `realīworld application either.
Iīll try and find the slackline stuff over the weekend, we decided the project wasnīt worthwhile commercially and so binned most of the files.
Somewhere there is also a test of a lot of quickdraws with plenty of busted tape that is relevant because of the karabiner shape and diameter. (Interesting because draws are not tested as a unit but each part tested seperately). That one was from the DAV I think but I couldnīt find it just now.
Plenty here for Aric to test, Iīm busy at the moment with something much more interesting and difficult!
Maybe next year.
Jim
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notapplicable
Aug 15, 2010, 6:20 AM
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adatesman wrote: It seems we have an internet connection here, so might as well post some results.... Since it was the smallest group of knots, I started with the knots for webbing. Nine different knots were evaluated, each tied as a loop consisting of two lengths of material with identical knots on both ends. The material used was 9/16" Blue Water climb-spec webbing with a manufacturer rating of 2300 pound-force; all cut from the same pair of 50 foot lengths of webbing (one red and one blue). For brevity's sake I'll only include pics of the structure of the knot, the front and back of the dressed knot and the loop ofter being broken. I placed marking threads tight up against the knot on all of the strands coming into and out of the knot to make it easy to see where and how much material moved while the knot was loaded. Click on the pic to get a larger version. I have a couple more pics of some of them if anyone's interested (how it was mounted on the puller, partial failure, etc.). All of the knots were dressed by hand, with each strand coming out of the knot(s) being tugged fairly strongly. Some loops were also worn by my nephew as a hat and possibly chewed on a bit. Sorry about that. Before getting to all the specifics, here's a quick summary of the results (click image for larger version): [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/webbing knot results.JPG[/image] And onto the specifics..... Double Fishermans Knot Fairly standard knot on cordage, not so frequently used on webbing. The three samples tested broke at 4039, 4104 and 4225 pound-force with minimal creep of the tails. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DFK0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DFK1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DFK1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DFK1break_sm.JPG[/image] Water Knot Very commonly used on webbing, also known as a Ring Bend when used on cordage. Creep of the ends is a known issue under cyclical loading. The three samples tested broke at 3426, 3244 and 3891 pound-force with minimal creep of the tails. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/WK0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/WK1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/WK1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/WK1break_sm.JPG[/image] knudeNoggin's Symmetric Water Knot A variation on the Water Knot that attempts to address the tail creep issue under cyclical loading by trapping both tails inside of the knot. The three samples tested broke at 3315, 3767 and 3874 pound-force with minimal creep of the tails. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWK0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWK1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWK1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWK1break_sm.JPG[/image] knudenoggin's Symmetric Water Knot with alternate tuck Another version of knudeNoggin's knot, with the tail tucked a bit differently. As with the first version, the intent here is to address the tail creep issue under cyclical loading. The three samples tested broke at 3499, 3381 and 3319 pound-force with minimal creep of the tails. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWKA0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWKA1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWKA1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/SWKA1break_sm.JPG[/image] knudeNoggin's Double Water Knot Some data knudeNoggin has seen suggests that creep in the Water Knot happens mainly with the piece of material on the outside of the knot. By using two water knots it is possible to have both pieces of material inside the knot and (in theory) address the creep issue. The three samples tested broke at 3797, 3655 and 3469 pound-force with no creep of the tails at all. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DWK0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DWK1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DWK1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/DWK1break_sm.JPG[/image] knudeNoggin's Tape Knot with 1 wrap An interesting knot which looks to address the usual reason for failure of webbing, which is an edge being pinched and the load not being applied evenly across the webbing. In this knot the standing part of the webbing folds flat across a turn rather than being squeezed into a circular cross section as with the water knot. The three samples tested broke at 2813, 2509 and 2668 pound-force and almost had the tails pull through. The structure changed significantly under load and ended up similar to a single fisherman knot. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN1W0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN1W1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN1W1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN1W1break_sm.JPG[/image] knudeNoggin's Tape Knot with 2 wraps Same knot as before, but with two wraps to give the loop more material to grip onto. The three samples tested broke at 3288, 2895 and 2822 pound-force and had some tail creep, but not as much distortion of the structure. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN2W0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN2W1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN2W1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KN2W1break_sm.JPG[/image] knudeNoggin's Tape Knot with 1 wrap and opposite tails A slightly different version of the earlier knot with the tails on opposite sides. The three samples tested broke at 1929, 1777 and 2055 pound-force and failed by the tails pulling though after severe distortion of the knot structure. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA1W0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA1W1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA1W1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA1W1break_sm.JPG[/image] knudeNoggin's Tape Knot with 2 wraps and opposite tails A slightly different version of the 2 wrap knot above with the tails on opposite sides. The three samples tested broke at 3088, 3140 and 2970 pound-force with some creep and a bit of distortion of the knot structure. Structure: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA2W0_sm.JPG[/image] The dressed knot: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA2W1front_sm.JPG[/image] [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA2W1back_sm.JPG[/image] The break: [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/KNA2W1break_sm.JPG[/image] -aric. |
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notapplicable
Aug 15, 2010, 6:20 AM
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adatesman wrote: Oh, forgot to mention something earlier... I realize 3 isn't anywhere near enough samples to do any meaningful analysis of strength, but thought it would be enough to weed out ones that were clearly inferior (like the 1 wrap alternate version of kN's tape knot, which pulled through on all three samples). Plus I only had 100 feet of webbing, so more samples would have meant testing fewer knots. -aric. |
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notapplicable
Aug 15, 2010, 6:20 AM
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adatesman wrote: You know, I didn't even notice that the blue broke more frequently. Interesting.... Both of the spools that the webbing came off and both were labeled with the same strength rating. Plus with such a small sample size and variation in dressing its probably just how it ended up. The puller I brought along was originally an engine hoist with an 8 ton hand pumped hydraulic jack on it, so yes, the load was applied in steps. I'm hoping to eventually find a used hydraulic power unit cheaply so I can get a smooth application of force, but it hasn't happened yet. In general there was probably an inch or so of material that came just from tightening of the knots (more in the DF, less in the WK due to differences in knot size and amount of material in the knots). The graph paper the pics were taken on has 5 squares per inch, so if you're interested it should be easy enough to tell just by looking at the larger version of the break pics (click on the image in the post to pull up a large version). (EDIT- Oops... Looks like I forgot to do the final find/replace on the links and it would just pull up the small picture. Its fixed now. Sorry about that.) I'm planning on starting on the loop knots tomorrow and have another 10' or so of the red 9/16" (picked it up a while back, so different spool) so I'll see about doing some up with the overhand / fig8 loops on the end. -a. |
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notapplicable
Aug 15, 2010, 6:20 AM
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adatesman wrote: Yeah, it looks a bit goofy in webbing since its so wide and can slide over itself. There's also 2 options for the last tuck... I rather arbitrarily chose to put the last tuck on the same side as the webbing started on because I thought it would dress nicer. Alternatively you could twist it around the other side. I don't know if one would be stronger than the other and I'm out of webbing, so maybe I'll give it a try (and MS's square knot as well) when I get home. -a. |
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notapplicable
Aug 15, 2010, 6:21 AM
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adatesman wrote: I just dug through the box of broken samples from this and tallied the results of which color webbing broke: Double Fisherman: 2 Red, 1 Blue Water Knot: 3 Blue (with additional ripping on 1 Red and 1 Blue) Symmetric Water Knot: 1 Red, 2 Blue (with additional ripping on 1 Red and 1 Blue) Symmetric Water Knot Alt.: 3 Blue Double Water Knot: 1 Red, 2 Blue kN's knot with 1 wrap: 1 Red, 2 Blue (with additional ripping on 1 Red) kN's knot with 2 wraps: 1 Red, 2 Blue (with additional ripping on 1 Blue) kN's knot Alt. with 1 wrap: no breaks, all pulled through kN's knot Alt. with 2 wraps: 1 Red, 2 Blue So in total, of the 24 breaks 7 were in the Red webbing and 17 were in the Blue webbing. Most likely this result is heavily biased by how I tied/set the knots.... For example, I tied all of Water Knots with the Red strand on the outside since the pictures came out better that way, and that knot typically breaks by the inner strand severing. So its not surprising it was the Blue one for all of those.... -a. |
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notapplicable
Aug 15, 2010, 6:21 AM
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adatesman wrote: knudenoggin wrote: 2. My Symmetrice Water Knot: ... wasn't all so symmetrically dressed, judging from the pics.... <snip> I thought I matched your diagram, but its quite possible I goofed somewhere. I'll get my hands on some more webbing and see if I can figure it out (I don't quite follow your description here...) knudenoggin wrote: 3. Note that the Grapevine breaks the material as it enters through the tight
turn, NOT within that hard turn!? <snip> I'll conjecture that some strength can be gained if the end is positioned as sort of a protective padding against the tight nip of the opposing SPart. I'll add that to the new list of knots to look at. knudenoggin wrote: And it would also be interesting to see what effect reorienting the Dbl.Overhand knot from a Strangle into an Anchor Bend had, as this would make the SParts wrap fully before u-turning at knot ends, and so might lessen the tight constriction at the entry point (though bulk rather oddly, otherwise). This one as well (if I can figure out what you have in mind... Sounds like you're suggesting a pair of anchor bends used in the same fashion as the pair of strangle/DF knots?) knudenoggin wrote: (And I concur in Aric's guess that the rated strength isn't matched to what his device reports--truly or nominally.) Actually, I found a better way to calibrate my load cells recently and (in theory) the results should be within 20 pounds of actual over the range of the load cells (10,000 lb-force). That said, chances are that the manufacturer rates their webbing conservatively (true strength a good bit over the rating) and the only reliable comparisons would be for samples tied from same spool(s). knudenoggin wrote: HOWEVER, I hold the ace of this design goal close to the vest, and I'll need to tie those knots myself, for starters, anyway. --either w/A-supplied like tapes, or presumed compatible locally. (What is the desired inner length of the specimen sling?) The pieces I used for my loops were somewhere between 18" and 24", as that seemed to be long enough to get a large enough loop for all of the knots and still have a bit of tail (I precut all the webbing). The puller can accommodate anything up to 5 feet or so, but that wouldn't exactly be material efficient....  knudenoggin wrote: ps: I concur in Majid's rec. for testing the SquaREef bend, as indeed it sees use in tapes (one can easily wrap & tuck ends back through center of knot, if slippage seems a problem). On the list. knudenoggin wrote: pps: Geesh, Aric, MASON LINE qua marker bits???! Are you devoid of simple thread (which sews in much more easily, surely)? That mason line can do some actual decent work (whipping rope ends, e.g.). [;) Well, what can I say.... I'm lazy. That line is what came with the sewing awl I used (easier to deal with than a regular needle and thread) and I didn't see the point of taking it all off the bobbin only to wind another type of thread on there when the original stuff would work just as well. Plus its not like the thing didn't come with a spool with several more miles of the stuff....  |
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notapplicable
Aug 15, 2010, 6:21 AM
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adatesman wrote: tradrenn wrote: Are you (adatesman) having red webbing on a hook or something similar and pulling on a blue webbing ? Actually, it was a biner on each end.... [image]http://www.shariconglobal.com/misc/pulltesting/knots/webbing/WK3pull_sm.JPG[/image] I suppose we could do some tests with identical knots with Blue on the outside on one set and Red on the outside on the other, but it would probably be easier to just ask one of the manufacturers whether they've seen a difference in strength by color. Given that they don't rate them differently, I suspect that there isn't one. -a. |
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notapplicable
Aug 15, 2010, 6:21 AM
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adatesman wrote: jeremy11 wrote: Great work! How about the figure 8 for webbing? I think that was on the list of things to be looked at last week (part of the loop knot list), but I only got to these webbing knots. If its not there I'll add it in as I'm still planning on working though the list as I have free time. -a. |
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notapplicable
Aug 15, 2010, 6:22 AM
Post #68 of 71
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adatesman wrote: jeremy11 wrote: how about water knots with the tails sewn down? I've done this on a few items (cam slings) just to keep the tails from creeping I don't know that it would effect the ultimate strength much (if at all), but it is on my yet-unformed list of things to try once I get the drop test fixture finished and take a look at creep due to repeated impacts. -a. |
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notapplicable
Aug 15, 2010, 6:22 AM
Post #69 of 71
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adatesman wrote: knudenoggin wrote: As he indicates, RATED STRENGTH is the basis for the % values. (How to measure its real strength, without big bollards?) Yup, I used rated strength. knudenoggin wrote: Incidentally, is it the case that device's total force is yet limited such that e.g. one might have trouble breaking 1" tubular tape? (double 2-ton loads) I haven't tried breaking a loop of 1" mil-spec yet, but I think the latest changes to the puller put it within reach. I'm sure we can now hit 6000 lb-f, and wouldn't be too surprised if it'll hit 8000 without anything breaking (on the puller, that is... ). This is actually the reason I did these tests with 9/16".... I was sure it wouldn't break the puller. If I get a chance this weekend I'll reassemble the puller and see what happens.... -a. |
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notapplicable
Aug 15, 2010, 6:22 AM
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adatesman wrote: Hey Randall, Sure, I'll add it to my list but will warn you it'll be a while until I get back to knot testing since the next couple months will be spent on anchor systems. Just in case Knudenoggin sees this- have you seen any data on Flemish bends in webbing? I don't recall seeing any, but you're more up on these things than I am..... I'm curious if they'd be less prone to creep than ring bends. That said your asymmetric ring bend for webbing looks promising and I apologize for not getting back to it. Sigh.... So many projects, so little time....  |
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notapplicable
Aug 15, 2010, 6:22 AM
Post #71 of 71
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adatesman wrote: Not necessarily.... There's a thread here in The Lab somewhere about this, but offhand I don't remember which one it is. Basically the issue is that with a loop tied with only a single knot the load will be shared between the side with the knot and the side without. Due to friction on the fixturing points and differences in stretch between the two sides of the loop (tightening of the knot being a big part of it) its quite difficult to determine the force that was actually applied to the knot, which will be something less than the force needed to break the loop. One way to avoid this issue is to avoid using a loop altogether and attach each side of the knot to a bollard. Unfortunately my equipment isn't set up to do this, so its not really an option. The 2 knot loop was suggested by Knudenoggin and looks to solve the problem fairly well. In theory both knots should have very similar breaking strengths (since they can be tied identically and are by definition from the same material) and somewhat similar stretch, the measured force will fall somewhere above the weaker knot and the stronger knot. Still not perfect, but better than the single knot loop method. A side benefit of this is that after one of the knots break you still have an identical example that was loaded close to its break point for later examination. At least, that's my understanding of the discussion last year as to the best way to do this testing.... |
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