If you Search among these threads,
you'll find that Derek has tried using a car jack,
and one immediate & should be obvious problem
is its lack of distance/stroke --climbing ropes, among
many others, even, will require a fairly long span of
extension to break, esp. in a test plan that entails
two knots & some kind of anchorage.
Another problem with A-vs-B testing is the lack of information
it provides, unless perhaps one tests some 10 specimens per
case? Consider a 4:1 result : seems pretty lopsided, but in
fact the strengths might be close, and the next case could
make things 4:2, or by some chance a similar other-tester's
repeat of it would be 3:2. E.g., Lyon Equip. did testing
(quick-search : this might be the URL for the PDF of it:
www.hse.gov.uk/research/crr_pdf/2001/crr01364.pdf )
in which they tested A-v-B, A-v-A, B-v-B, and in some
cases the highest value was of the loser (well, the break
force value is accorded to the broken knot) of A-v-B !
There is a "Knot Wars" site with (monthly?) mock battles
of this sort though with knots getting individual break data
and then that being compared; they don't seem to have been
so wise in evaluation.
But how to get any sense of calibration & absolute measure?
... hoping that some knots would be stronger than the others,
and offer me the excuse to push the "delete" button.
To my mind, that is a poor evaluation criterion for knots
--or poor, in isolation.
Moreover, the testing should be tailored to practical circumstances
(unless your purpose, your business/need, is for some particular
knot-testing competition), and typical slow-pull testing is not a
great model of that. In some testing of sewn slings of either
nylon or HMPE, joined with a "girth hitch", one tester found that
in (nearly?) all cases, the nylon broke on slow-pull, whereas in
many the HMPE broke in drop tests (but all held at least one
severe (UIAA) drop). So, which material here is "stronger"?
Now, getting a way to mark cordage and learn where and thus
maybe how/why breaks occur, might be something more doable
(conceivably, it, too, will differ per circumstance). Unfortunately,
using marking pens is itself considered a risk on the material (and
some testing done recently for climbing cordage shows this) --but
perhaps some minor marking can be effective for research and not
sufficiently abundant to be significant in strength.
--dl*
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