Rope for knot testing: What type?

[Mobius]

I know very little about rope types. I do know that trialling a knot by purchasing what is advertised as 'climbing rope' is financially unacceptable to me (~$450 Australian dollars for 60m in some cases).

I have been using some hardware store bought 8mm Polyester rope rated at 1200kg bf. It seems quite nice to tie and untie with, though I wonder if I should be using something else for trialling with.

I am putting together a home built test bench that could (in theory) provide 3000kg+ of tension. I am not initially interested in testing a knot's breaking point since (quite frankly) I do not want to be anywhere too close to a rope that breaks under that sort of tension in a home setting. What I was thinking of doing was taking a rope to (say) 500kg of tension and seeing whether the knot distorts/collapses/slips and whether I can untie the knot by hand after such a load.

I was thinking that 8mm Polypropylene (not braided) rope might be a good choice: It is strong and slippery from what I have gathered about it. Also, the 8mm size might give me some reasonable hope of untying the rope by hand after use, whereas I suspect smaller diameter ropes would make that very hard. Perhaps braided Polypropylene is a better choice?

Any advice is appreciated,

Cheers,

mobius

[Dan_Lehman]

Your test results will apply to what you test,
and beyond that, by only some reasonable
guesstimation to anything else.  What happens
in one rope might not happen in another.

If you can break rope, you can protect yourself and
things around the test bed, surely --though you are
right to raise the issue!  And, now, in testing rupture,
rather than anguishing over how to calibrate things
for comparable results, and how many repetitions
of tests to do for the Statistics God, you will make
a better contribution by learning where rupture
occurs (by some clever marking of the test specimen).
IMO,
--dl*
====

[Mobius]

Your test results will apply to what you test,
and beyond that, by only some reasonable
guesstimation to anything else.  What happens
in one rope might not happen in another.

If you can break rope, you can protect yourself and
things around the test bed, surely --though you are
right to raise the issue!  And, now, in testing rupture,
rather than anguishing over how to calibrate things
for comparable results, and how many repetitions
of tests to do for the Statistics God, you will make
a better contribution by learning where rupture
occurs (by some clever marking of the test specimen).
IMO,
--dl*
====

Thanks for the feedback 

I think I need to see a rope break under tension and not have things start flying around my garage to have some confidence about testing a knot to breaking point, by design    Admittedly, I am also interested in seeing where a knot ruptures and at what % of the rope's rated strength.

Cheers,

mobius

[DerekSmith]

Hi Mobius,  here is a suggstion.  Get hold of a length of cheap 3mm polyester braid and use your test righ to take a knot to destruction.

The amount of energy stored in the rope as it is stretched is crazy and when it eventually breaks it might shock you as to just how much energy is released in that one moment.  Even the bits of 3mm cord fly away at crazy speeds.

Hopefully it will put you off taking larger loads to destruction and also make you respectful of taking any large diameter rope anywhere near is breaking point.

As for interpretation, I totally concure with Dan, whole families of knots behave differently in different cords and with loads applied in different ways - but hey - have fun, but please be SAFE.

[Mobius]

Hi Mobius,  here is a suggstion.  Get hold of a length of cheap 3mm polyester braid and use your test righ to take a knot to destruction.

The amount of energy stored in the rope as it is stretched is crazy and when it eventually breaks it might shock you as to just how much energy is released in that one moment.  Even the bits of 3mm cord fly away at crazy speeds.

Hopefully it will put you off taking larger loads to destruction and also make you respectful of taking any large diameter rope anywhere near is breaking point.

As for interpretation, I totally concure with Dan, whole families of knots behave differently in different cords and with loads applied in different ways - but hey - have fun, but please be SAFE.

Thank you Derek.

Your thoughts echoed my own niggling worries about playing around with some pretty large loads in an uncontrolled situation. I am confident my rig (when it is built) will be strong enough to handle the forces, however I have to be close by to get it to work and my proximity to it when a rope breaks concerns me.

I might still use larger diameter rope and make sure I go nowhere near the rope's breaking strength. Finding out if a knot slips, or jams, is still useful to me.

As for breaking a rope, I think a small diameter rope is the way to go. From what you said I think a face visor would be a quite a good investment too.

Cheers,

mobius

[xarax]

  We are not sure that the thin and the thick ropes behave in a similar way ! On the contrary, we have seen that ropes and cords of smaller diameters may present higher relative strengths, the points of rupture may move inside the knot, etc.
  No, I believe that, if one wants to study a knot which is going to serve for rescue purposes, for example, he should test it on the diameters used in this application - and, of course, with the same kind of material.
  To cover the tensioned specimen with something, which can also be transparent to enable you to watch what is happening, so you will be protected from the pieces of the material during rupture ( or even before it...), is an easy thing. The difficult thing is to get an idea of the process of the rupture, which takes place in milliseconds - you will need a veeery expensive high speed camera to do this ! 
 
 

[Dan_Lehman]

I think I need to see a rope break under tension and not have things start flying around ...

One can cover the knot with a rag, and tie off the
line on either side of the knot with small cord to
arrest parts' hasty departure.

But thoughts of rupture remind me to suggest using
laid/twisted rope, as at least in some samples of that
that I've had broken --1/4" (6mm) nylon-- the line did not
separate completely, but broke usually in one, sometimes
two of the three strands (and then the test device stopped).
AND with this partial break, one had a better hope of placing
the point of rupture --seeing how the strands were turning
into the likely rupture place and which strand was where.

Xarax writes :

We are not sure that the thin and the thick ropes behave in a similar way !
On the contrary, we have seen that ropes and cords of smaller diameters may
present higher relative strengths, the points of rupture may move inside the knot, etc.

Yes, as I mentioned in another post --re how very slippery
the thin HMPE material that EStar used--, I think that
"surface" conditions have some depth which doesn't
scale with size --i.e., that will have greater influence in
small diameters.

...  the points of rupture may move inside the knot, etc.

Well of course they're inside the knot (or were just)
--that is where friction occurs, et cetera.  Assertions of
breaks occurring outside of the knot I think are simply
failing to take into account all that has transpired during
the loading, if not simply mistaking the (initial) break point.
(But I tihink I've heard some that are just hard to understand
or dismiss as I do above --that are so far outside (inches, in
a not-stretchy material, such as webbing, e.g.).  Still, ... !?)


--dl*
====