Article,
INTERWIRE SLIPPAGE AND FATIGUE PREDICTION IN STRANDED CABLES FOR TLP TETHERS
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Abstract
Analytical work backed by experimental studies on a 6 m long, 38 mm diameter, 91 wire strand with a nominal
brea.'<ing load of 1.3 MN is reported.
The theoretical work extends the classical twisted rod theories for the behaviour of helically laid wires to
include a stronger set of kinematic compatibility conditions. This theory is compared with the results of a novel
treatment of the layers of wires in a strand as a series of orthotropic sheets. The stiffnesses of each sheet in its
principal directions are defined by reference to contact stress theory incluuing, most importantly, shear deformation.
Transformed into the direction corresponding to the strand axis, results are obtained for the tangent axial stiffness
of the strand as a function of load level, and, via contact stress theory, for contact forces and slippage.
Full slip histories are predictable, from the micro-slips in the periphery of contact patches at low loads, to
gross slip at higher loads and even beyond. In addition, the hysteresis in the strand under cyclic loading regimes
is predicted.
The experimental work concentrates on measurements of wire stress, wire-to-wire (centre line) slip, and overall
hysteresis and is in substantial agreement with the theory.
It is concluded that theoretical predictions of the interwire forces and slippage in large strands such as
those envisaged as tension leg members in buoyant platforms are feasible, and this information is of obvious value
as an input to a fracture mechanics analysis of the fatigue.behaviour of the strand away from its terminations.
