Re: The strength of a rope is more than the sum of the strengths of its separate

On Mon, 22 Sep 2008 08:00:00 -0700 (PDT), ed wolf <eduartwolf@xxxxxxx>
wrote:

On 22 Sep., 02:21, John Polasek <jpola...@xxxxxxxxxx> wrote:
On Sun, 21 Sep 2008 19:49:16 +0200, "minimus"

<spammergetthisem...@xxxxxxxxxx> wrote:
The strength of a rope is more than the sum of the strengths of its separate
strands.

Why?

What's your authority on that?
Look closely what you are dealing with, say 100 fibers. Are they 100
identical fibers? Not likely. They will have various breaking
strengths and each has a specific weak point.
When you bundle them as in good tight rope, every fiber's weak spots
will be supported by 99 or so strong ones, so it won't even be
stressed enough to fail.
You would probably find that the rope is as strong as 80 or 90 of the
strongest fiber.
John Polasek

Hi,
fibers say 12m long are twisted to a rope say 8 m long.
It will be 1.5 times thicker than the bundle of fibers you started
with.
Could it be 1.2 or even 1.5 times stronger?
Sounds quite reasonable,doesn´t it?
ed, slightly puzzled
I dont want to get too involved here, but...
Since you have shortened by 8/12 the area is 12/8 = 1.5 times, by
simple proportion. The 'screw' of the rope is at 42 deg m/l. (That
rope would want to unwind). sin 42 = 2/3; sec 42 = 1.33.
So for X pounds vertical, a slanted fiber has to take 1.33X pounds to
provide X vertical and some Y compression force of tan 42 or .9X.
Therefore the strength would be increased by only 1.5/1.33 = 9/8 .
John Polasek
.

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