Page 194 - J. C. Turner - History and Science of Knots

184 History and Science of Knots

longer and thinner than at the start; hence, when reloaded, and provided all
loads are less than already imposed, they have a reduced elongation for a given
load but eventually show complete recovery from this elongation [20, p. 231].

Table 1. ELONGATION AND RELAXATION OF ROPE
Load 20% of breaking strength [20, p2381

12 mm diam 40 mm diam
sisal rope manila/sisal rope
Elongation
On reaching load 6.0% 14.2%
After 15 min holding load 6.4% 16.2%

Relaxation
Recovery after release of load
15 min 62.4% 47.5%
24 hr 70.3% 56.0%
1000 hr 76.6% 58.5%

B D

0
J

E F
Elongation
Fig. 2. Load/elongation curves for repeated loadings of a rope. The maximum
load imposed was 50% of the breaking strength; shaded areas correspond to the
energy absorption of the rope on the first and last cycle
When a rope is tensioned and stretches, the energy imparted to or ab-
sorbed by the rope is equal to the area under the load/elongation curve; it may
be expressed as metre-Newtons or Joules. As seen in Fig. 1, little energy is
absorbed by the rope when the load is light, that is, very little work is needed
for the early stretch, but as the load is increased, the energy absorbed or the
work needed increases more and more rapidly. Determination of the energy