Individual yarns within ropes can be subject to axial compression even though the rope
as a whole is under tension. This leads to buckling in sharp kinks and then to failure by
axial compression fatigue after repeated cycling. An existing elastic theory, which
applies to heated pipelines subject to lateral and axial restraint, predicts alternative
modes of either continuous buckling or intermittent buckled zones alternating with slip
zones. The mechanics of axially compressed yarns within ropes are similar, but the
theory has been extended to cover plastic deformation at hinge points. The predicted
form of groups of saw-tooth buckles, which curve at the ends of the zones into the slip
lengths, is in agreement with observed effects. Numerical calculation gives quantitative
predictions in agreement with experimental results, despite uncertainty about the
correct values for bending stiffness and plastic yield moment, depending on whether the
yarns act as solid rods or freely slipping fibre assemblies.
1 INTRODUCTION
Fibre and wire ropes are primarily intended for service as tensile elements, but failure may
result from axial compression of parts of the cross-section, namely individual fibres, yarns or
strands, while the bulk of the rope is safely in tension. Similar effects may occur in other
textile structures, such as carpets or industrial woven fabrics. If the axial compression leads
to a mild rounded buckling, as in an elastic deformation, there will be little damage, but if, as
often happens, plastic yielding leads to sharp kinks, then fibres will fail in repeated cycling.
In ropes, axial compression of individual components can arise from a number of causes
including:
C Bending. If a rope under tension passes over a sheave (pulley) or is taken round any
other solid object with too small a radius, components inside the curve may be put
into compression. Ropes may also buckle into bent forms or, at low or zero tension,
be forced to bend by transverse forces.
Rope twisting. If a parallel assembly of fibres is twisted in either direction at constant
length, the outer layers are forced into longer paths and so develop tension. If the
overall rope tension falls below the value developed in this way, the rope will contract
and the central fibres will be put into compression. In a simple twisted structure, an
increase of twist will cause the central straight components to go into compression,
whereas a decrease of twist will compress the outer components. In more
complicated rope structures, with twist at several levels, the precise effects will
depend on the geometry, but twisting will always force some components into axial
compression in the absence of sufficient overall rope tension
Rope twisting. If a parallel assembly of fibres is twisted in either direction at constant
length, the outer layers are forced into longer paths and so develop tension. If the
overall rope tension falls below the value developed in this way, the rope will contract
and the central fibres will be put into compression. In a simple twisted structure, an
increase of twist will cause the central straight components to go into compression,
whereas a decrease of twist will compress the outer components. In more
complicated rope structures, with twist at several levels, the precise effects will
depend on the geometry, but twisting will always force some components into axial
compression in the absence of sufficient overall rope tension
Kinking due to axial compression is a phenomenon that occurs on many scales from
mountain ranges to oriented polymer molecules. In fibres, the effects at the molecular level
are shown by the presence of kinkbands, which run across the fibres at about 45E, when
fibres are uniformly compressed, or, more commonly, on the inside of bends. Repeated
flexing of fibres leads to failure, either due to breakdown along kinkbands or to axial splitting
from the accompanying shear stresses. As described by Hearle et al (1998), these forms of
failure have been observed in laboratory flex tests and in ropes and carpets after cyclic
loading. In typical test conditions, failure may occur in around 1000 cycles in aramid fibres,
Hearle and Wong (1977), but polyester and nylon fibres would last longer, Hearle and
Miraftab (1991). Data from yarn buckling tests carried out for FIBRE TETHERS 2000
(1995), show severe strength loss in aramid yarns after 20,000 cycles, in HMPE yarns after
200,000 cycles and in polyester yarns after 1,000,000 cycles.
The first reported engineering failure in an aramid rope due to axial compression fatigue was