Bending fatigue performance of high-strength seven-wire monostrand

Large-amplitude cable vibrations are remarkably common in cable-stayed bridges subjected to wind and mechanical loading due to low inherent damping of these structures. Although the high slenderness ratio of cables means that bending moments are not normally significant over their free length, local effects near the cable anchorage induce non-negligible bending stress variation under dynamic loading, which could lead to fatigue failure of stay cables. In modern parallel strand stay cables, the fatigue performance depends on the fatigue properties of the individual seven-wire monostrands which together make up the cable. We introduce a new bending fatigue testing device for seven-wire monostrand which can control the fatigue loading in the axial and transverse directions of the cable independently. The fatigue endurance of monostrand with guide deviators is significantly longer than without guide deviators present, especially for smaller bending fatigue amplitudes. We found that bending fatigue life of monostrand is also reduced by increased constant or dynamic axial stress, but over-tensioning of the monostrand (e.g. during construction) had no consistently adverse effect on fatigue life. For monostrand loaded in severe bending, fatigue occurs due to the bending stress variation caused by the combination of bending moments and axial forces at the extreme edges of the monostrand cross-section, rather than due to relative movement and fretting between adjacent wires within the monostrand.