ek wrote: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*