Free bending fatique of axially pre-loaded spiral strands

Abstract

The response of a large diameter and a multi-layered spiral strand to an applied moment is considered in some detail for a given mean axial load. Carefully conducted large-scale experiments have cast some light on an interesting phenomenon observed in previously reported bending fatigue experiments The first wire to fail was invariably the wire which entered the socket on the bending neutral axis rather than the wires in the ‘extreme fibre’ positions, as might usually be expected. Using a series of electrical resistance strain gauges placed on the individual wires at the mouth of the socket, the previously reported theoretical predictions that interwire slippage is greatest at the neutral axis (in terms of simple beam bending theory), and least at the extreme fibre positions, have been confirmed. In line with the theoretical predictions reported elsewhere, the test results show that, in general, the onset of deviations from no-slip interlayer shear interaction occurs at rather small levels of bending movements near the socket. Based on the theoretical and experimental findings, which identify interlayer fretting as the primary cause of wire fractures, a newly developed parameter capable of predicting spiral strand free bending fatigue life has been proposed. Unlike the traditional extreme fibre maximum direct stress approaches, the proposed parameter takes the interwire/interlayer fretting phenomenon into account.