The mooring of floating wind turbines is an important challenge for the offshore industry. It is an important part of the design engineering and, often, a critical point for the fatigue life assessment. A solution that could improve the fatigue life is to directly connect the mooring rope to the floating platform, without an intermediate chain. However this solution can generate fretting fatigue near the connexion, because of the local combined loading: tension and bending. This study investigates the fretting fatigue endurance of crossed steel wire contacts (σ_Y = 1800 MPa et σ_U = 2085 MPa), representative of steel wire ropes used as mooring ropes. The experimental campaign focuses on the influence of contact displacement amplitude under constant fatigue stresses and normal load. A characteristic “U”-shaped fatigue life curve is observed under dry air condition, with a minimum at the transition between partial and gross slip regimes. This usual behavior can be explained by the fact that, under partial slip conditions, increasing the displacement amplitude increases the tangential loading and therefore reduces the fatigue life. However, once the gross slip transition is overpassed, wear becomes the dominant mechanism (while the tangential load remains constant). This wear simultaneously leads to an expansion of the contact area, which reduces the fretting loads (cyclic shear), and to the removal of microcracks initiated at the surface. These two combined effects result in a rapid increase in fatigue life.

 To account for the effect of seawater associated with this offshore application, the same experimental study was carried out in a seawater-immersed environment. Unlike contact in dry air, the evolution of endurance now exhibits a “U–L shape.” In this case, the presence of seawater significantly complicates the contact response due to two competing effects: on one hand, a positive lubricating effect of the seawater, and on the other hand, the detrimental influence of corrosion phenomena.

 For partial slip conditions and the beginning of the gross slip domain up to a displacement amplitude of ±150 µm (the “U” part of the endurance curve), the positive lubricating effect of seawater dominates, resulting in a decrease in the partial slip/full slip transition and, more importantly, in longer lifetimes under seawater conditions compared to dry contacts. Conversely, for larger sliding amplitudes (> ±150 µm), the relative displacements favors the circulation of the corrosive medium within the interface, which enhances corrosion processes and ultimately reduces the fatigue life. A further drop in endurance is then observed (the “L” part of the endurance curve), with lifetimes significantly lower than those obtained in dry air.

This shows that the sliding amplitude can strongly influence the impact of seawater on fretting fatigue life—either by promoting the beneficial process of interface lubrication, leading to a reduction in mechanical loading, or by activating detrimental corrosion processes that significantly decrease the lifetime. To better understand these effects, a mechanical simulation is proposed to formalize the positive influence of lubrication and, by comparison with experimental results, to quantify the negative effect of corrosion.