Page 192 - J. C. Turner - History and Science of Knots

182 History and Science of Knots

core and sheath ropes and for monofilament lines, tensile pull is accompanied
by some degree of torsion. There is internal friction between constituents,
fibre against fibre, yarn against yarn and so on. There is external friction, of
the outside of the rope against capstans, spars, rocks or the like. Often these
forces fluctuate as the rope flexes round curves, accelerating, jerking, chafing
against surfaces, perhaps reversing direction [20, p. 213]. Knot failures are
often due to the action of these forces. A fundamental property of rope is
that it stretches when loaded. This is best measured by subjecting a known
length of the rope to a steadily increasing load, and plotting the load against
the resulting elongation. Fig. 1 shows typical curves. Quite a small load
causes substantial initial elongation, but later a large load would be needed
to produce a similar elongation. Laid nylon rope shows much more elongation
compared with natural fibre rope at the same load, and it is also significantly
stronger.
The initial flat part of the load/elongation curve (Fig. 1) is mainly due
to internal movements of the rope components, the later steep part to the
actual stretching of the fibres [20, p. 223] A sheath and core rope with a
fairly loose sheath and a core of untwisted fibres will behave more like the
pure fibres, with a nearly straight load/elongation curve; a hard-laid rope will
have a more nearly straight curve than a soft-laid one. The rate of elongation
affects this behaviour.

m
0
J

Elongation

Fig. 1 . Typical load/elongation curves for manila and nylon rope, taken up to the
breaking point
Slow rates allow more time for the components of the rope to overcome their
frictional binding so that the load/elongation curve is more concave, with a
prolonged early nearly flat phase. Fast rates of loading, specially the jerk of
holding a falling object, may give a slightly' convex curve with some complex
kernmantle climbing ropes [4]. Consequently for any work comparing different
ropes, or comparing rope with and without knots, the testing methods must
be carefully controlled.