## 3 Analysis of Macroscopic and Microcosmic Morphologies of Fatigue Fracture of Notched Wire  
The analysis of fatigue fracture morphology is an important method to study the fatigue process and analyze the reasons of fatigue failure. The corrosion fatigue damage of high-strength steel wire is similar to the fracture mode of other metal materials. The fracture retains the traces of the whole fracture process and records many information occurred during the fracture process, which has obvious morphological characteristics. Therefore, macroscopic and microcosmic analysis of fatigue fracture of steel wire can accurately predict its fatigue fracture mechanism and understand its fatigue crack propagation. Figure 11 shows the macroscopic morphology of typical corrosion fatigue fracture of high-strength steel wire used in bridge cables. It can be seen that fatigue cracks generally start at a certain surface defect or stress concentration and slowly extend into the steel wire in a semi-circular until the steel wire suddenly breaks. The whole fatigue fracture surface can be roughly divided into three regions: slow extending region (Region A), fast extending region (Region B) and instantaneous fracture region (Region C). From Figure 11, it can be seen that slow extending region is relatively smooth, fatigue cracks extend inward in circular arc. This is mainly because the stress intensity factor (SIF) at the crack tip is relatively small at the initial crack propagation stage. Each time the steel wire undergoes a cyclic load, the crack tip advances a tiny distance, so the law of crack propagation at this stage is relatively stable. Thus, this stage is commonly referred to as the steady-state extension stage. With the continuous application of cyclic load, the stress intensity factor at the crack tip increases as the crack grows into the material, and the crack propagation rate changes from slow to fast, so the cracks enter the fast extending region. At this stage, the crack propagation depth under each cyclic load is greater than that in the steady-state region, which is the transition region between the steady-state region and the instantaneous fracture region. When the cracks grow to a certain depth, the effective cross-sectional area of the steel wire will be reduced to a critical value. At this time, the fracture toughness cannot resist the stress intensity at the crack tip, so the cracks enter the unsteady-state extension stage, and the steel wire breaks instantly. As can be seen from the macroscopic morphology of the steel wire fracture surface, the instantaneous fracture region is relatively rough with obvious radial fibers. Figure 12 show the microcosmic morphology of the fatigue fracture of steel wire with slow extending region. Under the continuous application of cyclic load, cracks begin to form at a certain defect position or stress concentration point of the steel wire, which become the origin points of fatigue cracks. As the load increases, the cracks extend into the material. When the cracks are in the slow extending region, the speed of extension is relatively slow, and the fatigue stripes can be seen, which is perpendicular to the crack propagation direction, as shown in Figure 12. The crack propagation of the steel wire in the slow extending region is transgranular fracture, as shown in Figure 13. When  
Figure 11: Macroscopic morphology of fatigue crack propagation. the cracks enter the fast extending region, the speed of fatigue crack propagation is relatively fast and transgranular fracture (same as the slow extending region). The crack propagation morphology showed circular arcs along the crack propagation direction. The microcosmic morphology of this stage shows that the cross section is relatively rough. As the fatigue cracks extend further, the bearing capacity of the specimen continues to decrease. Because the area of the effective bearing surface decreases constantly, the stress of the crack front increases.