Torsion tests on large spiral strands

Abstract

Carefully conducted experimental studies of the torsional characteristics of substantial structural strands are reported. The match between experimental results for an old and fully bedded-in 39 mm diameter, 91 wire spiral strand and some theoretical predictions is very encouraging. The theory, which has been reported elsewhere, treats the individual layers of wires in a strand as orthotropic sheets and, via established results in contact stress theory, takes full account of the frictional interactions between wires. Static and dynamic torsional stiffness and hysteresis data are presented for axially-preloaded strands. Strain gauging of individual wires showed significant scatter, but the match between theory and the mean of the experimental data for a position remote from the termination was good, using the same assumed value of the friction coefficient for all tests. For an old strand, random loading can significantly increase the level of torsional hysteresis above that found for regular cyclic loading, provided that the interwire force changes are large enough to overcome the contact friction. Tests on a new 41 mm diameter strand indicated that hysteresis measurements on newly-manufactured strands could be misleading for long term applications.