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