THE PREDICTION OF AXIAL DAMPING IN SPIRAL STRANDS

This paper addresses the problem of predicting the axial hysteresis in multi-layered helical strands from first principles. Using previously reported orthotropic sheet theory for helical strands with wire compliances derived from results in available contact stress theories, normal forces on the interwire/interlayer contact patches have been estimated. Using such data, the full slip histories on the contact patches from the micro-slips on the periphery at low loads, to the onset of gross slip at higher loads and beyond are predicted. It is shown that cable overall axial hysteresis is dominated by the frictional energy dissipation over the line-contact patches within individual layers. Strand overall hysteresis is predicted by two rather different theoretical methods which provide an analytical double check. In the first method, the overall load-deflection curve, as derived by a previously reported orthotropic sheet model, is used to follow the loading and subsequent unloading response of an axially pre-loaded strand with its ends fixed against rotation. The calculated enclosed hysteresis loop area gives an estimate of strand damping which should closely match alternative predictions based on the summation of energy dissipation over the individual contact patches throughout the strand. The match between these two alternative predictions and available experimental data on some fully bedded-in (old) and substantial 39 mm spiral strand is very encouraging. The practical limitations of the proposed analytical models are critically addressed in the light of recent experimental findings.