Page 66 - Failure Analysis Case Studies II

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4000

3000

2000
d
a
P
0
Y
4 1000
&

0

__
-400 -200 0 200 400
Rope twist, R (/m)
Fig. 3. Results of series of torsional tests at constant tensile loads on 46 mm triangular strand Lang’s lay rope, from
Rebel [7].

a loop (or kink, but also termed a hockle) formed in the rope as a whole; and constructional
deformation involving hockling of individual strands or wires, or strands forming a ‘birdcage’
caused by local buckling. In either case, the instability can be prevented by maintaining adequate
tension. The most serious deformations occur at very low tension. Uncontrolled reloading of a
rope once it has developed a torsional instability is likely to lead to severe permanent deformation
which can reduce strength dramatically, and damage fatigue resistance catastrophically.
The overall, ‘whole rope’, hockle can be removed by skilled handling if spotted before reloading,
but the smaller scale, strand hockling cannot generally be removed without serious permanent
degradation. The tendency to overall or constructional deformation is a function of the bending
stiffness of the rope components, and the loading caused by imposed twist.
In a six stranded rope, untwisting whilst maintaining tensile load will eventually lead to local
de-stranding of the rope (Fig. 4) which, when tension is relaxed, will allow the torque carried by
the individual strands to cause local strand hockles (Fig. 5). If torque is imposed at low tension,
whether untwisting or twisting up, then the whole rope will hockle. However, six strand ropes can
sustain very severe torsional deformation, provided a minimum tension is maintained. Six strand
Lang’s lay mine hoist ropes have been reported [7] operating with lay length changes from - 30%
to + 70% without any major compromise to service life.
Torque balanced spiral strand and multi-strand ropes both tend to have outer elements with
diameters, and consequently bending stiffness, which are a much smaller proportion of rope
diameter than the six strand equivalent. These outer elements also have a helical sense that opposes
the layer underneath. Consequently, as already indicated, these ropes are more susceptible to