Just some quick comments … :
- My “testing” amounts to putting what I regard as
“serious force” onto knots tied in real rope, so as to
gain some idea of what they do --what geometry
is obtained, whether it jams, or whatever. (And,
re jamming, I work up to “serious” force by steps,
as I do not care to have to labor to untie a really
jammed knot! So, those getting pulley treatment
must look half-decent by lesser loadings.)
.:. I’ve not done break-testing by my own means,
very much. (But I could do so --and have done–
for plastic binding tape (such as are ubiquitous around
copier-paper boxes and the like), and fishline or some
other tiny stuff.)
.:. Well, I’ve not break-tested intentionally, but have
had (a) some specimen (or two?) break accidentally,
and (b) the pulley line break (aided by running over
the pulley wheel or side --so, “cut” in a sense), and
these events weren’t terrible. They’d be less so were
they anticipated!
-
I don’t think that you need be put off by fears of
physical consequences of break-testing small stuff
–e.g., up to 5/16" utility line. You can take measures
to guard things and keep parts from flying all over;
the break forces should be, I’ll estimate, at about
half of the pulley’s rating (and esp. if that’s a safe
working-load rating, you’re fine!).
-
You might arrange more of a test bed with the
pull being horizontal --giving you easy, ground-level
access to all parts–, maybe pulling some small weight
(20#?) as dead-weight resistance to keep things from
flying apart on rupture, assuming you’re using rope
in the pulley that isn’t terribly elastic.
And assuming that your test method is intended in large
part to explore how/where knots break, you can avoid
having to somehow come up with calibration. Here,
having two knots in the specimen ensures that you
have a survivor (presumably the same) that you can
examine in comparison with the broken one, in an
effort to understand where & why breakage came.
I marked some specimens sent away for real testing
with threads : white at points where all limbs (i.e.,
eye legs, tail, & SPart) left the nube; pink at a point
where I guess rupture would occur, and gold at a
point downstream of the pink (which point often
was moved in knot compaction to where pink had
been). Knots were pulley-set by me and tied in
HMPE, so I’d the advantage of getting little change
in position by material elongation --just further
compaction and then slippage if any (nope!)–;
my 500#? force was remote to breaking forces
of about 2 tons (4_000#)! With more elastic material
–just about anything else–, one might adjust markings
of rupture-point guess, or add a 3rd. --simple stitching
through surface fibres, pretty much. (Maybe Sharpie
marker is okay, despite concerns about potential
influence of the marking itself on strength --some
bright dots here and there aren’t likely to upset things
much (on white/yellow line, in red/black/blue ink).
Breaks should start at insides (concave part)
of bends, not the sometimes-mused outer (convex)
part --where maybe fibres need to stretch more,
but CAN, while inner ones are hard-pressed to move
with loads of force upon them.
Or maybe by you aim to do testing A-vs-B for some
general ordering of strengths (although beware that
this takes repetitions and can be misleading if only a
few tests are made; consider : result of 4 vs. 2 seems
consequential, but if 7th test made it 4 vs. 3, then
would you think the same thing?!).
The obvious set up with two blocks is a 2:1
loading the other 2:1 for 4:1 advantage --if these
are single-line blocks.
A camera is good, for pre-testing, and maybe some
shot during testing when you are safely shy of the
rupture force --I think you’ll have an idea of when
you’ve got a “serious” load on them but that they
shouldn’t be near rupture (and parts flying from
such a rupture should go away from you if at that
point). It might well be that most of the geometry
is set by this point, beyond some turns getting
tighter and so on.
–dl*
