The physics of rope bends

[pancake]

You guys are awesome. So much to digest. The exact information I was looking for. Thanks

 

[knudeNoggin]

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...Two circles of different radii have different circumferences. The fibers of a rope bent over form multiple circles with the inside being the smallest and the outside being the largest. The larger the diameter of rope or the smaller the bend radius the more the inside and outside circumference differs. If only half of a ropes fibers are bearing the load, the rope will break at half the strength....It is good to know the hows and whys of strength loss, but don't sweat over it.

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I think in the main this is on the right track, ...
[and] the basic idea is probably sound ... .

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Even though I wrote :
From some few specimens of broken ropes that I've
examined, it seems that rupture begins on the <u>inside</u>
of a bend, from compresion or friction--NOT on the
convex side, from elongation, as one can read being
speculated (or stronger) in some books.
This examination is of 3-strand/laid ropes, where principally
ONE (sometimes two) or the strands broke and one then can
follow search for where the strand lay at rupture point.

And then came, nevetheless, the counter assertions above
(with mere theory and no evidence for support.

Let me ask: According to the outer-fibers theory above re knot
weakening, where will the arborists' Strangle Noose hitch
(that misnamed "Fisherman's knot" etc., used to tie onto a 'biner
or ring--a noose formed with a Strangle knot on the mainline)
break?! The mainline runs through the Strangle knot,
turns rather sharply (roughly equal dia. rope &amp; metal) around
the ring, and then ties around itself in a "half a Dbl.Fish.
knot" to finish. !? Apply the theory here, and see what the
result is predicted to be.
The test is rather straightforward.
(And maybe Lazarus can give some details.)

*kN*

 

[davespencer]

I am surprised by the above statement about the rope breaking on the inside.

It has been my experience with testing and in field failures (of which I've witnessed a few) that the rope will break a couple of inches in front of the knot. I have no idea as to why it happens this way, I have never seen a rope break inside the knot.

 

[TheTreeSpyder]

i've seen his referance to this before; all i can figure is that the line would be more 'brittle' to compression than tension in some lines or lines worn more 'brittle' like this (from elements and use). That could then place the failure at the non supporting/compression part of the arc.