Fall forces & breaks or not, with HMPE vs. Nylon slings

[knudeNoggin]

This site's testing report was cited in rockclimbing.com

http://www.dmmclimbing.com/news.asp?nid=293&ngroup=1

It is another bit of evidence showing the severity of impact forces
with the static HMPE (Dyneema, Spectra) material. To my mind,
it also shows that the Fall Factor in this case is not determinant of
the impact force -- in that the 120cm sling (dropped 120cm) generated
roughly 50% greater force than the same FF-1 drop on a 60cm sling.

But someone is now taking issue w/me re this point, saying that the
difference is attributable to the stretch of (!) the test tower, drop
weight, and whatever rigid metal attachments. !? Huh? --seems
quite a difference to me, and that if so, it should be that these
metal parts are thus more stretchy than the fibre (which I don't buy
-- HMPE stretch at normal rupture is around 4%, btw).

(I've not yet watched the associated video of the drop-testing.)

*kN*

[knudeNoggin]

Here's the response I find hard to believe:

Its basic physic 101. FF isn't unique to climbing ropes it applies to every material. Steel cable or rubber bands. However, stretch in other components makes a difference.

knudenoggin wrote: So, of "At DMM's test-tower we took a look to see what implications ..." what sort of stretchy things do you think accounted for their data which shows the long (48" vs 24" (120cm/60cm)) fall having about 50% greater impact force (sufficient to break slings)? -- stretchy anchor pins and shackles?

Um, yes. Anchor pins, shackles, the drop weights, the tower itself, they all stretch. And their stretching makes a big difference.

By my very rough calculations based on impact force there was probably around 5cm of stretch aften the 60cm fall and 7.5cm of stretch after the 120cm fall. If there was only 20mm of stretch in the metal components then that would go a long way in explaining the difference.

Test with solid steel cable if you want. FF is what counts. Like I said its the other components in play that lead to different results. As falls get longer these influences become negligable.

[NZcaver]

Anchor pins, shackles, the drop weights, the tower itself, they all stretch. And their stretching makes a big difference.

A big difference?? What is this guy smoking? I find it very hard to believe there could be any perceptible "stretch" in these rigid components under regular FF testing. Unless you're using Jello drop weights or a tower made from Lego blocks, I don't think so. Slow-pull testing could yield slightly different results, but that's irrelevant to the drop test scenario.

You should ask this guy how he feels about getting on rope with stretchy carabiners.

[ek]

Well, I'm going to stand up for "this guy." I think he may be on to something.

NZcaver, this is not "regular FF testing." This is a drop test of an extremely static material. Static means non-shock-absorbing, not non-stretching. knudeNoggin, the Spectra may stretch 4% before failure (is that single-strand or double-strand, by the way), but that doesn't mean that there is necessarily any significant shock-absorption associated with that stretch.

To illustrate this point, consider 15 feet of steel cable in parallel with 10 feet of "bungee" cord (like the kind people use for ropewalkers). What happens when you shock-load this? The "bungee" cord stretches 5 feet almost instantly, and absorbs no energy in doing so. Then you slam down onto the steel cable, and all the energy gets dissipated in a very short time (and most of it is dissipated into you, becoming your deformation energy). It would be trivially easy to design a single piece of cordage that behaved like this.

The point is that there is no energy-absorption associated with that 33% stretch. It's not just the amount of stretch that matters. Stretching is an opportunity to absorb energy, but if the cordage offers no resistance while stretching, then no energy gets absorbed. To put this more concretely with an example: there are different dynamic ropes that are about equally stretchy but which yield radically different peak impact forces in UIAA drop tests.

I am currently unable to watch the video, since I'm in my Linux-based system, which does not have the appropriate plugin. (I can watch YouTube videos just fine on this system, so it's nothing so simple as me missing Flash. I'll figure it out later when I have more time.) Because of that, this analysis may be lacking. But you've said you haven't watched it either, knudeNoggin, so I presume we're on the same footing.

Besides the possibility that the Spectra is stretchier but nonetheless less shock-absorbing than the steel, it is worth noting that if you have a big, tall test tower, the compression in the tower could easily be significant compared to the stretch in the Spectra sling, in terms of energy absorption. The tower's material may be less energy-absorbing than the Spectra sling, but the tower is a lot taller. Furthermore, suppose the Spectra does absorb plenty of energy as it stretches. If the stretch is confined to a time interval that doesn't comprise the entire dynamic loading event (for example, if it's mostly at the beginning or mostly at the end), then even a much less energy-absorbing test tower and connectors would have plenty of opportunity to absorb energy.

Another totally neglected possibility is that all or most of the 4% stretch in the Spectra before failure occurs at forces higher than those produced in these tests!

Finally, and I think most plausibly, consider the dynamometer (the device that measures the forces). This is in series with the sling, and in order to measure a force, it has to stretch at least a little bit. When testing a caving or climbing rope, this stretch would be insignificant. But with a Spectra sling...perhaps it could be very significant.

Ultimately, I suspect that "this guy" is mistaken, at least in the sense that the Spectra sling probably is what's absorbing nearly all the energy after all. But as I hope I've demonstrated, it's not at all obvious that he's wrong. Furthermore, the principle of fall factor does apply to all materials. What I think "this guy" is probably missing is non-uniform characteristics in the Spectra slings that become significant because the Spectra slings are so static. Does it stretch differently near, or in, the stitching? Does it stretch differently as it enters a bend around the carabiners, and as it goes over that bend? Additionally, if the stitched and non-stitched sides stretch differently, is there energy-absorption associated with the friction of the Spectra (partially) equalizing tension by sliding around the carabiners? I agree with you (knudeNoggin and NZcaver) that the incongruity with a fall factor based prediction is due to characteristics of the Spectra and not the test rig, but I think that's because the Spectra sling itself constitutes multiple components placed together in series and in parallel. If you took single-strand Spectra rope or Spectra webbing, rigged in "tensionlessly," and performed fall factor 1 drop tests over very long distances (to eliminate contribution to energy-absorption from any other elements, including the tensionless interfaces at the ends of the Spectra cordage), I suspect that your results would be completely consistent with the theory of fall factors.

EDIT: I had neglected to consider the additional possibly of significant contribution to energy dissipation via sound. Is it loud when you do such a test? If so, that sound energy has to be coming from somewhere.

[knudeNoggin]

Well, I'm going to stand up for "this guy." I think he may be on to something.

Whereas I was thinking he was simply on something. YMMV.

knudeNoggin, the Spectra may stretch 4% before failure (is that single-strand or double-strand, by the way), ...

Whoa, think about this. The stretch is per tensile: double, treble,
... , whatever, per material you have a constant elasticity.
NOW, in this particular case of a sling around some pin,
there might be a diminution of breakpoint --if coming at the pin--
that implies that the material doesn't reach full rupture elasticity.

The point is that there is no energy-absorption associated with that 33% stretch.

Trust me that HMPE cordage does not stretch willingly, and I
believe that the plot is pretty linear vis-a-vis tension.

Another totally neglected possibility is that all or most of the 4% stretch in the Spectra before failure occurs at forces higher than those produced in these tests!

?? This isn't ignored; indeed, my point re the sling geometry and
the pin (creating a weak point) points to such lesser stretch in the
HMPE (which implies a greater stretch elsewhere).

But you've said you haven't watched it either, knudeNoggin, so I presume we're on the same footing.

True (library access had some issue; dial-up here IS an issue: darn
fool in video is rambling big-time about who knows what, on & on
& on ... ). BUT, I think I glimpsed enough of the gear to surmise
that one is going to be hard pressed to find much stretch, let alone
2cm of it, in the non-cordage bits. (The notion of the drop weight
itself stretching really tickles me!)

... the compression in the tower could easily be significant compared to the stretch in the Spectra sling, in terms of energy absorption.

Furthermore, the principle of fall factor does apply to all materials.

My problem is that I've heard Hook's Law explained as applicable
to materials roughly modeled as springs? -- and that this didn't
include everything. I should think that, contrary your assertion,
there are cases with so little force absorption that essentially what
you get is impact force = momentum, and momentum is greater
with longer fall, irrespective of FF. If one were to rig a test device
with a steel cable and broad slab of iron, say, I'll offer to sit on the
one dropped all of 4 inches on a 4"-long chain, while you and my
antagonist can have the ride of a lifetime on a 4-metre drop w/4m chain.
My strong surmise is that there are different ("enormously" so) impact forces.

Now, were the non-cordage materials to have some load absorption
analogous to a Screamers stitches, such that it had a set bit of force
to absorb and then it has shot its wad, I could see some sense to it:
both drops get this one "wad" of force reduction, but there is much
more force remaining from the greater momentum of the long drop.
But I still find it incredible.

*kN*

[PseudoFission]

Take a look at this study:

http://www.itrsonline.org/Papers/2001/W ... SPaper.pdf

Particularly the graphs on page 4/11 - You will immediately see the difference between dynamic and static material in relation to fall factors. Generally the more static the system (less energy absorption available) the less and less the fall factor equation applies. Kn also be aware of the limitation of using steel weights in extremely static test setups. Illustrated quite well in this fantastic study:

http://www.itrsonline.org/Papers/2001/W ... SPaper.pdf

Look at page 6/18 to see the extreme difference in a short drop on cable vs a live human being. I think you would be quite interested to read that full report!

-Andrew