**4.3 Corroded steel wire fracture morphology analysis**

In the static tensile experiment of uncorroded steel wire, it was found that the fracture form is mainly manifested as cup-cone, the fracture form of specimens under stress-free conditions is also a cup-cone fracture (**Figure 8a**), but not as regular as the fracture of uncorroded specimens, the fracture form of specimens under static stress conditions is mainly milling cutter fracture (**Figure 8b**), individual specimen is cup-cone fracture, the fracture form of specimens under alternating stress conditions is mainly cleavage-milled fracture (**Figure 8c**), individual steel wire is miter-type fracture and their combined form, but there are very few cleavage fractures (**Figure 8d**) [15].

From **Figures 8a** and **b**, it can be seen that the cup-cone fracture has an obvious necking phenomenon, the milling cutter fracture also has the appearance of necking but not as obvious as the cup-cone fracture, this necking phenomenon indicates that the steel wire material has obvious plastic properties; while **Figure 8c** and **d**, there is no obvious necking phenomenon in cleavage fracture and cleavage-milled fracture, which shows the brittle properties of steel wire material, and the fracture time and location performance, the fracture time and location are sudden and uncertain. The probability of brittle fracture of the steel wire is the highest under the action of alternating stress and environmental coupling, corrosion fatigue is usually multisource fatigue, the fracture has more fuzzy fatigue striations or brittle fatigue cracks, and there are more secondary cracks in the fracture.

After the mechanical properties experiment on the corroded steel wire, the fracture of the corroded steel wire under alternating stress conditions was scanned with KYKY-2008B digital scanning electron microscope, and the microscopic morphology of the fracture is shown in **Figure 9a–d** [15].

*Corrosion Fatigue Behavior and Damage Mechanism of the Bridge Cable Structures DOI: http://dx.doi.org/10.5772/intechopen.109105*

### **Figure 8.**

*Fracture morphology of corroded steel wire under different stress conditions [15] (a) cup-cone fracture, (b) milling cutter fracture, and (c) cleavage-milled fracture (d) cleavage fracture.*

#### **Figure 9.**

*Morphology of the fracture under alternating stress [15] (a) 23 times, (b) 340 times, (c) 1700 times, and (d) 5000 times.*

After magnification, it can be found that a few sections of the edge of the fracture are neat and show a brittle fracture morphology, while other areas are destructive in shape, and individual areas appear in a river and mountain-like morphology. The

center of the fracture appears to have a tough nest-like morphology. When the fracture is magnified to 1700 times by microscope, a "fish-eye" morphology can be seen, and a shell-like fracture appears. This morphology is caused by fatigue, which indicates that the fracture is controlled by the main crack and gradually expands along the expansion of the main crack. As the primary crack grew and expanded, the secondary crack also expanded gradually, which accelerates the fracture of the steel wire. When the fracture is magnified to 5000 times by microscope, a large number of smooth "fish eye" shapes are distributed on the section, according to the shape of the fracture, it can be seen that corrosion of the steel wire is a brittle fracture. The experiment shows that corrosion of severe steel wire plastic properties reduced, brittly enhanced, and the time of fracture is uncertain, with abruptness under the alternating stress and environmental coupling.
