...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*