Page 193 - J. C. Turner - History and Science of Knots

Studies on the Behaviour of Knots 183

Rope Breaking Strength
There are several kinds of machine used for testing the strength of rope
and knots [15], all originally devised for testing engineering materials fairly
early in the industrial revolution. Older machines for the routine testing of
rope used either a constant rate of extension (CRE) or a constant rate of
loading (CRL), where one end of the rope was clamped and the other was
moved either at a constant speed (CRE) or so as to increase the tensile force
at a constant rate (CRL). Recent testing machines tend to use a constant rate
of traverse (CRT), where one end of the rope is moved at a constant speed
and the other responds by moving (more slowly) at a rate depending on the
load [15]. The different machines yield somewhat different results. The end
fastenings must be stronger than the rope; a wet eye splice usually suffices
for natural-fibre ropes, but synthetics have the ends clamped round a bollard.
Rates of extension are usually chosen to produce breakage within somewhere
around a minute or so. Most studies of the effects of varying the conditions of
the test have used the simple criterion of the breaking strength, the maximum
load applied at the time of failure.
Small changes of the rate of elongation around the standard rate make
little difference to the measured breaking strength, but at slower speeds, the
strength rapidly decreases [16], and increased speeds in the usual machines can
cause a 20% increase [20, p. 2171. Very low and very high speeds of elongation
will be considered later.
While the breaking load varies considerably with the rate of elongation,
the amount of elongation at break is nearly constant [20, p. 226] [25]. Similarly,
old used rope is weaker than new, but the elongation at break is little affected
[25]. However, wet natural-fibre ropes may show slightly greater elongation
than dry [20, p. 226].
There is a tendency for the breaking strength of rope in proportion to its
size (best measured as mass per unit length) to be greater for small sizes than
large [25], but inspection of some rope manufacturers' catalogues shows that
this effect is neither very big nor very regular.
No rope is truly elastic, the elongation is never strictly proportional to
the load; a constant heavy load produces an immediate elongation followed by
a slow continuing one (see below under Creep) and when the load is removed,
recovery is slow and incomplete, specially with new rope (Table 1). The smaller
the rope (Table 1), and the smaller the load as a fraction of the breaking
strength, the faster and more complete is the recovery. In another test [20,
p. 2381, 12 mm diam. manila rope loaded at 50% of its breaking strength
recovered 42% of the elongation in 24 hr, but loaded at 10%, it recovered
83%. On the other hand, a nylon rope recovered 78% and 85% respectively.
Hard-laid rope shows slower recovery than soft-laid, four-strand than three,
wet than dry. Ropes that have been repeatedly loaded are, when unloaded,