Page 195 - J. C. Turner - History and Science of Knots

Studies on the Behaviour of Knots 185

absorption capacity of a rope requires accurate measurements at all parts of
the load/elongation curve (see Fig. 2).
A new rope subjected to a large load (e.g., 50% of the breaking strength)
shows a curve like AB in Fig 2. The elongation is AJ1 and the energy absorbed
by the rope is the area ABJ1. If the load is then released, the elongation at
zero load after a short time corresponds to point C. The rope is then subjected
to a similar load, and the steeper curve CD is obtained, with less elongation
and less energy absorption. If the load is again released, all the elongation is
not recovered, and the rope returns to point E. This process can be repeated,
with less and less elongation and energy absorption until eventually (usually
more than 10 cycles) a stable curve is found, such as FG, and the rope may
have about 50% of its original energy absorption capacity FGJ2 [30].

Knot Strength

The relative knot strength, also called knot strength efficiency or knot effi-
ciency, is the breaking strength of a knotted rope expressed as a percentage
of the breaking strength of the same rope without a knot. Some of the many
measurements that have been made are summarised in Tables 2 to 7. I have
entered in my tables only those data for which I am reasonably certain of the
identity of the knot (I have given the Ashley [5] number, when available), and
either at least two studies have been made on a knot, so that different esti-
mates may be compared (Tables 2, 7) or one person has made measurements
on different kinds or sizes of rope, again allowing comparison (Tables 3 to 6).
Several published knot names are ambiguous in the absence of an illustration;
I have ignored these data.
The first impression of the tables is the wide range of values for some
of the knots. In Table 2, for example, the efficiency of the Fisherman's Knot
varies from 39 to 81%, of the Bowline from 52 to 78%. In trying to explain this
variability we need to look at two things: how reliable the actual values are
and whether the different investigators were really looking at the same things.