Page 205 - J. C. Turner - History and Science of Knots

194 History and Science of Knots

authors listed knots in order of the magnitude of the curvature or nip, let alone
measured them, comparing them with measured efficiencies. And I have found
no description of a break actually occurring at the point of greatest curvature
or nip within a knot tied in rope. The two Alpine Club Committees [1, 2] said
that the rope always broke `at the knot'; others [5, p. 17][11, p. 30][16][20,
p. 243] [23, p. 64] that the break occurred where the rope enters the knot; I
have not noticed any account of which rope breaks in an unsymmetrical knot
like a Sheetbend. Ashley [5, p. 17] said that a rope is weakest just outside the
entrance to a knot, seemingly due to the rigidity of the knot; a knot, he said,
will be stronger if the nip is well within the structure. Himmelfarb [20, p. 243]
stated that as tension is applied to it, the rope is compacted, the compression
prevents internal movement of the fibres and the rope begins to act like a rigid
bar, and failure occurs where the rope enters the knot. Day [16] speculated
that the complex stresses and strains that operate in the rope where it enters
the knot are amplified by the rigidity with which the rope is held in place
at that point by the knot itself. In his tests on knots tied in gut or nylon
anglers' lines, Barnes found that the knots tied in gut broke just outside the
knot proper [7, p. 73], as with the fibre ropes discussed above. But if the knots
were tied in nylon monofilament, the break tended to be within the knot, at
the nip [7, p. 69], the place where a particular crossing first makes the knot
secure. Apparently the slippery surface of the nylon and the reduction of its
diameter under tension allowed a small fraction of the standing part to be
drawn out of the knot, the break occurring at the main nip within the knot [7,
p. 130]. Each knot tested was found to have its own specific point of breaking
[7, p. 78]. Barnes thought that the loss of strength of the line due to a knot
depended on how sharply the weakest coil was bent [7, p. 90]. There seems to
be no consistent explanation of where the break occurs at a knot.

Creep

We have all had the experience of tying a rope tightly between two points,
only to find a while later that the rope has loosened and requires retightening.
The rope has continued to stretch; this effect is measurable after only a few
minutes (Table 1). This slow stretch is known as `creep'.
If a constant load is applied to a rope, such as by suspending a fixed
weight, the rope continues to stretch slowly and, if the load is a substantial
fraction of the breaking strength of the rope determined as in the previous
section, the rope will eventually break. A manila rope given a load of 80-90%
of its breaking strength, breaks within a few minutes; with a load of 50% of
the breaking strength, the rope breaks within a few days [20, 24, 25]. Some
extrapolated figures suggest that, even with a load of 20% of the breaking
strength (listed as the safe working load for many industrial applications) the