## 2 Experimental Study on Fatigue Crack Propagation of High-Strength Steel Wire
### Specimen and Load Setting  
Steel wire for bridge cables in service is subjected to the fatigue loads and environmental corrosion. Corrosive medium is easy to form initial defects on the surface of steel wire. A large number of tests show that the initial defects on the surface of the old bridge cable wire caused by chlorine salt are mostly semi-ellipsoidal pits [22; 23], which involves a complex electrochemical reaction process. In order to simplify the forming process of pits on steel wire surface, in this paper machine-cut notches are used to replace the pits caused by electrochemical inhomogeneity, and the effects of different notches size and morphology on fatigue crack propagation law were studied. A standard high-strength steel wire specimen with a diameter of 7 mm and a tensile strength of 1670 MPa specified in Chinese specification-GB/T 17,101 [24] was selected as the test specimen used in this paper, and the length of steel wire was taken to be 1100 mm. The steel grade of high-strength steel wire is S82B, and its chemical composition is shown in Table 1. Zn99.99 is the zinc used in the steel wire galvanized layer, and its chemical composition is shown in Table 2. The fabrication technology processing of steel wire includespickling, phosphating, drawing, cleaning, hot-dip galvanizing, and stabilization, among which the stabilization process involves the heat treatment process. Domestic Stelmor air-cooled wire rod is used for bridge cables. The hot rolling temperature of austenitization is 950-850 \({}^{\circ}\)C, and the quenching temperature of the air-cooled wire used is 620-680 \({}^{\circ}\)C. As the heat capacity of air is low, and the thermal stability is weak, the transformation rate of pearlite is not stable during quenching. GB/T 17,101 is the specification of hot-dip galvanized steel wire for bridge cables in China, and all information about the chemical composition and fabrication technology processing of steel wire is contained in Section 7.1 of this specification. This specification is generally in line with the requirements of BS ISO 19203:2018, and a description of the steel grade and chemical composition can be obtained from Section 7 of BS ISO 19203:2018. The schematic diagram of defects on the surface of steel wire is shown in Figure 1, and the photo of the steel wire specimen is shown in Figure 2.  
In the traditional fatigue test, the fatigue test machine (FTM) is directly used to apply the fatigue load to one specimen at a time. When the number of test specimens is large, the test will take a lot of time. In addition, due to the size limitation between the two clamps of the FTM and the vertical placement of the specimen, the corrosive medium is not easy to act on the steel wire, so the corrosion fatigue test is difficult to be realized. In view of the above reasons, a new test device is designed in this paper. The schematic diagram of the device is shown in Figure 3, consisting of two main parts. One is composed of the FTM and the hydraulic jack with large range, HJ1 for short, which are responsible for providing the total fatigue loads. HJ1 is placed between two clamps of FTM, and the oil amount of HJ1 is provided by hydraulic hand pump with oil storage capacity of 2700 cc. The FTM adopted in this test is INSTRON 8802, which is shown in Figure 4. The other part is the fatigue loading device for wire specimens, as shown in Figure 5, including 40#C channel steel, braces with diameter of 20 mm, bolts, multiple hydraulic jacks with small range, HJ2s for short, anchors. HJ1 drives a series of HJ2s through tubing and diverter valve to apply fatigue loads to multiple steel wires. Data acquisition is carried out through data acquisition equipment--National Instruments BNC-211, hollow shaft type annular force sensor and computer. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**C** & **Si** & **Mn** & **P** & **S** & **Cu** \\ \hline
0.80–0.85\% & 0.12\(\sim\)0.32\% & 0.60–0.90\% & \(<\)0.025\% & \(<\)0.025\% & \(<\)0.2\% \\ \hline \hline \end{tabular}
\end{table}