The Bending of Spiral Strand and Armored Cables Close to Terminations

The problem of the bending of a spiral strand or armored cable is addressed, with particular reference to the “free bending” situation where pulleys or other restraints are absent. The analysis, based on a study of the properties of the layers of wires which form the strand (or armor), treats the “free field” bending, remote from the termination. A parallel treatment of the situation close to a termination is summarized. In the former case, limiting values for the effective bending stiffness of the strand are presented; in the latter case an examination of the behavior of an outer layer of wires sliding (with known frictional characteristics) over an inner core has led to predictions of the strains and movements between the individual wires and the inner core (as a function of wire position in the strand) which are reported in the paper. The results offer an explanation of some experimental observations from fatigue tests on a large (39mm) strand under combined steady axial load and lateral movements causing bending adjacent to the restrained termination. In particular the observed wire failures close to the socket, not at the extreme fiber positions but at the neutral axis, can be explained in terms of the much larger slip on the interlayer contact points there.