## 2 Surface and Fracture Morphology of the Damaged Steel Wire  
At the outset of design, it was assumed that the suspender components, being insulated by the sheath, would experience minimal or no corrosion damage to the internal steel wires during service. However, during the service life of the bridge, factors such as alternating vehicle loads, changes in environmental temperature, and aging of the suspender sheath can lead to damage to the sheath. This damage allows environmental corrosive agents to infiltrate the interior of the suspender, leading to corrosion of the steel wires. With the passage of time, the surface of the steel wires transitions from uniform corrosion to the formation of pits. As the number of pits increases, individual, relatively independent corrosion pits connect with each other, forming larger pits. The shape and quantity of the pits on the steel wire cannot be predicted. Researchers conduct quantitative studies of different damage characteristics through statistical methods, dividing the degree of corrosion of the steel wires into different grades. The surface morphology of the damaged steel wires can be observed using a digital microscope and a field emission scanning electron microscope, as shown in Figure 1.  
The corrosion fatigue performance of this batch of steel wires has been studied in the experiments detailed in reference [18]. Through an artificially accelerated corrosion test on 7 mm galvanized high-strength steel wire, the number and three-dimensional dimensions of the pits on the surface of the wire were found to vary unpredictably across different corrosion grades, showing weak regularity. The positions of the pits also appear randomly. However, these pits can generally be categorized into the following typical shapes based on their depth: groove-shaped, semi-ellipsoidal, hemispherical, and composite, as shown in Table 1.  
The corrosion process of steel wires inside arch bridge suspenders lasts for a very long time, with the number and morphology of pits differing in each stage. The morphology of pits changes as time progresses, and there is a certain regularity in this change. In the early stages of corrosion, micro-cracks will appear on the surface of the wire matrix, as shown in Figure 2a. In the mid-term of wire corrosion, relatively dispersed and independent corrosion pits will appear on the wire matrix, as shown in Figure 2b. As the corrosion progresses, in the mid-term of wire corrosion, some independent corrosion pits will coalesce into a larger area corrosion pit. The appearance of corrosion pits greatly reduces the effective cross-sectional area of the wire, and the appearance of cracks seriously affects the performance and service life of the wire. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Groove-Shaped** & **Semi-Ellipsoidal** & **Hemispherical** & **Composite** \\ \hline \multirow{3}{*}{Pitting morphology} & & & & \\  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \hline Pit depth & h \(\geq\) 0.25 mm & h \(\geq\) 0.15 mm & h \(\geq\) 0.10 mm & h \(\geq\) 0.20 mm \\ \hline \hline \end{tabular} Note: The white circular areas indicate the locations of typical corrosion pits.  
\end{table} 
Table 1: Typical pit morphology characteristics. Figure 1: Surface defects in the wire. (**a**) Digital microscope. (**b**) Field emission scanning electron microscope. Figure 2: Surface defects in the wire. (**a**) Surface cracks. (**b**) Cracks near corrosion pits. Figure 3 illustrates the typical fatigue fracture characteristics of steel wires with three levels of corrosion severity at a stress amplitude of 450 MPa, corresponding to mild, moderate, and severe corrosion states. The fracture surfaces of the three steel wires are generally similar, consisting of smooth and flat fatigue propagation regions and rough transient crack initiation zones.