majid, thanks that was interesting and well articulated.

my only question is about the relative breaking points of the different webbing tests. If there was only one test for each mode of failure then my science mind wonders how different the breaking forces really are. Dig? If you ran these tests maybe 3 or 4 times each what would the differences be in terms of percentage points rather than in terms of POF? Probably very different, but who knows until you do it?

As is, the percentages point to HUGE differences in failure points, but it wouldn't be totally absurd to imagine you had an outlier (in terms of data points) your first test.

But, well done over all. Its good to know that one shouldn't do something. But its even more useful to have the fullest information---like if you are need to bail in an emergency and are weighing all your options it helps to know exactly what type of risk you are taking if for some reason you HAD to tie webbing to a bolt hanger....thanks again

Both the webbing and cord were on rounded anchor points. As the cord drew tight, its loop narrowed and crimped the webbing where they joined. The cord broke at this joint at 1500 Pounds

thanks for your response majid.

the picture of the cord tests looked like it you used a double fishermans instead of a ring bend.

regarding the "rounded edge", in the text for test #5, you said:


i'm curious of the radius of these rounded anchors.


now, regarding "stretch caused by knot tightening"...this term is confusing. are you saying that cord contained within the knot stretched OUT of the knot? if the length of the cord changes to "at least 3X of the original size," well, this is pretty clearly PLASTIC DEFORMATION of the cord material, and has virtually NOTHING to do with the knot.

http://en.wikipedia.org/wiki/Deformation

based on what we noticed, the 6 mm utility cord even though smaller and weaker than the standard one inch webbing ( rated to 4000+- lbs), handles the tensions and forces gradually, compared to the 1 inch webbing, which was subject to suddenly snapping under the gradual load. This discrepancy was definitely due to the different elongation properties of the webbing and utility cord. Also, the doubled piece of webbing handled the forces slightly higher (by 200+ POF) than a single flat piece of webbing.

Our testing is unofficial and the accuracy of this test still is questionable however, based on what we noticed, the inner part of bolt hanger acts as a sharp knife and eventually will cut soft material under gradual tension or sudden impact. We think that the breaking forces will be significantly lower if we shock load the webbing and cord on the bolt hanger, though we have yet to do any tests.

majid, from your conclusions:


i generally disagree with your conclusions. what does it mean to "handle forces gradually?" webbing failed (worst case scenario) at 500 lbs higher than cord. i'm not sure you recognize that the cord DID fail, when it stretched so much that the testing limit of the equipment was reached. if the equipment was capable of reproducing a pull of 3 feet, the cord would have snapped. the material had failed.

also, you should not have stated what you THINK will happen (but did not test) in your CONCLUSIONS. you have NO BASIS for doing so.

but thanks for making the effort. nothing personal. i just dont like bad science.

EDIT:

the only conclusions that I could make, based upon your datum:

-In a SINGLE TRIAL, webbing broke at 500-800 lbs higher than cord, when loaded over a sharp edge.

-In a SINGLE TRIAL, cord failed first when tied to webbing.

preliminary results: loading over a sharp edge is not a good idea, but if you have to, use 1 inch webbing instead of 6 mm cord.

Majid,
thanks to you and Boku for running real, actual tests. Very interesting.

I take it you didn't test webbing ? I've encountered a few people who think this is OK (why, I have no idea), and while I'm very convinced it's not, I'd be interested in seeing my intuition/engineering background backed up with evidence.

Not long ago, I climbed on someone's top-rope at a local crag (a 3 bolt setup... I figured it was pretty hard to mess up), and as I was lowered I heard the conversation: they were asking someone else at the crag if girth hitching bolts was ok. I think that was 1/2 of 1 of my 9 lives....

I like seeing test results. Any test results are worth while. Corollary - there are no tests that can anticipate and answer all subsequent questions. This includes multiple repeats to give an idea of repeatability.

Having said that - It would be interesting to see if laying the tail of the webbing knot over the hanger - to pad the sharp edge - would improve breaking strength.
You made it clear that the webbing snapped suddenly (as we might expect a static material to do) while the cord (presumably, and clearly, more dynamic) broke after stretching. However, I don't see that this extrapolates to your assertion that a sudden load might produce significantly different tensile strengths than a slowly applied load. I dimly recall some MatSci expert saying that any impact slower than the speed of sound in the test material is effectively a slow load.

I guess you are not paying attention to what I said and yet you are still focusing on type of knot and or elongation which was not part of main test. We were not trying to see how much a webbing stretch or how far cord stretch before it breaks apart.

From what we observed, we (those who were involved) are satisfied with this test

look majid, apparently you don't understand what i'm saying...

you said:


first, you DID use two different knots, but said you didn't.

you don't recognize that the cord FAILED when it streched into the plastic zone. research "plastic deformation."


i'm happy for you. you have an interesting misconception of what you've done.

Thanks - that was a good contribution!

I would not call it definative research due to small sample size and controls, but good preliminary research - creates questions to answer.

Maybe the questions are answered elsewhere.

I was very interested in test 3 with the webbing "doubled". I could not see what you did, nor has it been explained. Was the webbing padded at the bolt by the tail of the knot, or did you do an all around pass if the hanger ?(kind of like how extendable cam slings are attached)

If you did an allaround pass, and it is actually stronger (more research needed) it may be a good tool to apply to tat rappel stations (assuming there is a solid metal rappel ring/mailon for the rope).

Not sure what the effect of that all around pass would be over time - would it resist abrasion from being held in place to the hanger more tightly, or become more abraded because wear is focused in a more specific place ...

More questions than answers - which is where knowledge grows.

B

In some ways, I agree with you, hardman, but in this case, the knots really don't matter. Both the webbing and the cord broke at the bend around the hanger, NOT at the knot. So long as the knot was not the weak point in the chain, it shouldn't really effect the test results.

What bugs me about this test is that it seems to suggest that webbing is better to tie through hangers than cord. After all, the test shows that the webbing tested out stronger than the cord, right? Well, 1" tubular webbing is rated to about 4000lbs whereas 6mm cord is rated to only about 1500lbs. A CONTROL test would have been very useful in adding to these limited test results. Still, if we just use the rated strengths of the materials, we can presume the following: Webbing tied directly onto a bolt hanger will lose a significantly greater percentage of its strength than perlon cord.

In all cases, the overshadowing moral of the story is obvious: Cord and webbing tied directly through bolt hangers lose a significant amount of their strength.

So from looking at the numbers, my conclusion is that 1 inch webbing breaks under a higher load than 6mm accessory cord.

Yes, cord that is rated to 4,000 lbs might break at a higher force than webbing rated to 4,000 lbs, but we call cord that is rated for 4,000 lbs a single lead line. Even 8mm accessory cord is only rated to 14kn which according to my calculations is equal to 3,150 lbs.

I am drawing my conclusion based on the amount of load that it took to break the materials.

What I don't understand (and I think several others as well) is why you (majid) are saying that the cord is a better choice when it took less weight (force) to break it.

So far the only answer that I can find from you on this is that because it fails more gradually, and because if fails gradually you will have time to know what's going on instead of being suprised by the webbing snapping. (that's not that good of a reason to use cord though IMO because you're not going to be able to see your anchor when your rapping and so I would rather have the strongest material possible.

IMO, this is suggesting that even small amount of internal stretch within the material could reduce the impact

We can’t compare a 6 mm cord to 1 inch webbing cause they are not rated the same

3X stretch (at 500-800 fewer pounds of force) is NOT a SMALL AMOUNT of internal stretch. a small amount of internal stretch would be something in the ELASTIC DEFORMATION range (see the stress/strain curve i sent you above). once you reach PLASTIC deformation, the material has failed.

its like pulling a piece of chewing gum out of your mouth, and pulling it apart 12 inches. then concluding the gum is absorbing the load gradually. the gum has already failed. you can DECREASE the amount of force on it, and it will continue to deform until it fractures.

do you understand this concept? i've brought it up several times, and you have not addressed it.

by the way, if i had the personal choice of using 8 mm nylon cord vs 1"webbing, i'd use the cord, even without the test. but i would use 1" webbing over 6 mm cord, because of your test results. so the test you've done is not without value.

what are your reasons behind choosing cord vs. webbing ( if it is going to be used on bolt hanger)

would you please acknowledge that you have some understanding of plastic deformation? please?

would you please acknowledge that you have some understanding of plastic deformation? please?

We can’t compare a 6 mm cord to 1 inch webbing cause they are not rated the same and the reason why we used 6 mm cord is because someone mentioned in ST and we decided to go with that.

Now, after what I saw from this test, I am convinced that If I had to do choose between building an anchor from two separate material meaning on is going to be standard one inch webbing which is rated to 4000+- lbs and let’s say another material like cord/rope which is also rated to 4000 lbs, I am going to pick cord vs. webbing cause IMO under shock load, I think cord handles the load better than webbing but we just have to do a test to find out.

hardman -- is there any chance we could get you to address these issues though PM with majid instead of tainting this otherwise intriguing thread. rc.com has enough of a problem producing useful threads as it is w/o having every other post containing some distracting personal issue you have with majid and this test. thank you.

I think your logic is flawed.

You should prefer cord because it will likely resist cutting by the bolt hanger better than the webbing.

Your test suggests that cord is much less susceptible to weakening over a sharp edge than webbing. In your test, the cord lost less than 50% of its rated strength whereas the webbing lost almost 75% of its rated strength (assuming no strength loss to the knots, which is acceptable since none of the tests broke at the knot).

Now if we assume this is true and the % strength loss to a bolt hanger is the same as with 6mm cord, 8mm cord (at 3100lbs) should out-test the webbing every time. Granted, because the relative radii of the hanger edge to the cord diameter is not the same as with 6mm cord, I would be willing to bet that there will be a greater % strength loss with 8mm cord than with 6mm cord. Still, I would bet that the 8mm cord would out-test the webbing.

On top of all of this, I would be curious as to how 5.5mm spectra cord would perform in the test.

is this turning in to interrogation of some sort ?

Do you want to put this 5.5 mm Spectra on the bolt hanger again or no just pull end to end ?

Make some suggestions for future work and turning out future numbers that are more robust, rather than slogging what has already been done and fixating on what holes exist in this impromptu and on-the-fly test.

you should not have stated what you THINK will happen (but did not test) in your CONCLUSIONS. you have NO BASIS for doing so.

Silascl -- the 'conclusions' proferred with majid's study simply state that:

a) running cord or webbing through a bolt hangar is a bad idea

b) it was noted that the 6mm cord was weaker in breaking strength than the webbing

and c) 6mm cord failed gradually when under stress, unlike webbing which suddenly cut when under stress.

What exactly 'makes no sense' regarding his conclusions, as you have stated?

In regards to accepting the study at face value, majid made of point of asking for comments and suggestions for a future test (which was included in his 'conclusions')-- and has been responsive to requests for different materials and setups, as well as for repeated testing to give results of higher confidence. What exactly are we being forced to swallow here? Do you have any suggestions for future work?

(Opinion) Some people appear to have taken issue with the fact that majid said he would make use or cord instead of webbing if one of the two materials had to be fixed to a hangar. I imagine he reached this conclusion due to the different methods by which the cord and webbing fail under stress (gradual vs. sudden), and not so much based on the absolute strength ratings arrived at by the test.

hardman -- I'm a practicing scientist myself, but there's a difference between being inquisitive and questioning the integrity of someone's conclusion and being an annoying devil's advocate or a nitpicky patty, which I feel is the role you have assumed. Make some suggestions for future work and turning out future numbers that are more robust, rather than slogging what has already been done and fixating on what holes exist in this impromptu and on-the-fly test.

...good tests, plain to see, take them for what they are.

Don't let the "mouse milkers" get to you Majid.... good tests, plain to see, take them for what they are.

Was there actually an end use or question these tests were for? I have read "rapping" a couple of times in here... was this an "is it safe to rap on webbing through a bolt hanger" thingy or something else?

I think, the guy was wondering instead of leaving a biner, webbing and rappelling ring, what if he just directly tied the cord or webbing to bolt anchor.

I knew that is was bad idea for anchor while leading cause you could shock load your entire life line if a leader fell but then it was questionable for rappelling so we thought about doing the test to see how bolt hangers handles soft material under tension. On rappelling, the load generally rest on the system therefore shock loading is at its lowest and chances of breaking the cord or webbing is almost near none but, then what if you were rappelling and somehow you got hit by a rock and fell full force to the end of the line( assuming you tie a safety knot).

Was that anchor strong enough to support the shock load?

That is how the idea of testing webbing and cord vs. bolt hanger came to our mind.

3X stretch (at 500-800 fewer pounds of force) is NOT a SMALL AMOUNT of internal stretch. a small amount of internal stretch would be something in the ELASTIC DEFORMATION range (see the stress/strain curve i sent you above). once you reach PLASTIC deformation, the material has failed.

its like pulling a piece of chewing gum out of your mouth, and pulling it apart 12 inches. then concluding the gum is absorbing the load gradually. the gum has already failed. you can DECREASE the amount of force on it, and it will continue to deform until it fractures.

do you understand this concept? i've brought it up several times, and you have not addressed it.

by the way, if i had the personal choice of using 8 mm nylon cord vs 1"webbing, i'd use the cord, even without the test. but i would use 1" webbing over 6 mm cord, because of your test results. so the test you've done is not without value.

The stress/strain plot you linked to does not indicate whether the deformation was plastic or elastic.

Besides, what kind of "source" is wikipedia? You should know to only site sources that are peer reviewed

Also, I am sure he eyeballed and guessed the "3X stretch",

Hardman, you must not understand deformation yourself. The stress/strain plot you linked to does not indicate whether the deformation was plastic or elastic. The determination of plastic deformation can only be made upon reversing the load on the material and measuring the resultant plastic strain, if there is any. Since the plot you linked to did not indicate any load reversal, it is impossible to come to the conclusion that plastic deformation occurred based on the shape of the stress/strain curve. After all, it could be a plot from non-linear elastic material - like plastics. The plot you linked to is fine for undergraduates working on homework problems involving metals, but for the problem at hand, namely deformation of a plastic (6mm cord) the plot in no way models the materials behavior. Besides, what kind of "source" is wikipedia? You should know to only site sources that are peer reviewed...

Also, the amount of stretch does not, in itself, indicate plastic deformation. Have you ever heard of a rubber band? 3X stretch for a rubber band certainly is small compared to the amount of stretch at failure and is still well within the elastic region. It's all relative to the materials tested - and he was testing plastic! Also, I am sure he eyeballed and guessed the "3X stretch", and by stretch he probably meant elongation of the loop of cord due to the knot tightening. Why don't you think about it a little and give him a break.

But I am sure you already knew all of this, seeing as how you review and write papers of your own, it just seems your comments would more likely come from a freshman student who took his first course in materials engineering than someone who reviews papers in the field. I'm glad you don't review any of the papers that I write...

And, thanks for the test results majid!

Tim

At now 3pp there are a lot of things to chase down ... .

1) The knots in the tape & cord were, resp., Water knot & Grapevine bend.

2) The significance of material feed from the compression of these knots
isn't zero even though the failure occurred away from them; esp. with so
short a test specimen, the amount of material drawn from the knots will
be relatively large--it's an absolute amount based on tension, and is a
high amount proportionately to the short loop. Therefore, there is some
effect of the material having lesser tension on the side with the knot.

Of course, if this is just the structure one is going to be using in practice,
then just these results are what is relevant. (I'd expect such imbalance
to be weakening.)

3) The folded tape isn't the same or so relevant to the question posed
about positioning a (knot's) end of tape against the hanger: in this
latter case, the nylon directly against the metal isn't tensioned but
simply compressed (by the loaded tape above it); that might make
for better results.

4) Comparing the cord to tape in terms of tensile strength might be
missing the point: more relevant to practical concerns, IMO, is the
equating of cost or weight--tape is per weight/cost much stronger.

5) The configuration used in testing differs from what would be used
in practice in anchoring the material away from the hanger with a
broad, STRAIGHT bolt: in practice, this will typically be a D-shaped
'biner, which itself will have deformation effects upon esp. 1" tape,
and that might affect how it delivers load over the hanger!?

6) Some other configurations to test:
-a. girth-hitching to hanger;
-b. basket-hitching (which doubles material all around);
-c. stopper attachment (i.e., the loaded material passes
through the hanger to a knot whose bulk holds it
(hanger opening might be so large as to make this pretty dubious));
-d. and the previously questioned loading of the tape over an
untentioned END for padding;
-e. anchoring tape with a 'biner vice straight bar; and
-f. smaller widths of tape.

(An e.g. for 6.c woud be the tape sling tied w/EDK
and the loop end poked through the hanger & loaded.)

7) I was surprised that the tape-to-cord saw the cord failing at even
lower force than cord-to-hanger (though it was different cord, so ...).
8) This business about the "gradual" failure of the cord I think is
misleading: there was graduated, incremental loading of the cord
on account of the pump action & cord stretch; in reaLife (tm), the
force would be continuous and "gradual" here suggests that it simply
began failing sooner than the resistant tape!

*knudeNoggin*