Fig.10 Micro fracture morphology under DP, P=50MPa, σe=400MPa.
The amplified morphologies of areas I, II, and III in Fig.10 are shown in Fig.11;
There are obvious cracks in the CSZ, and due to the combined effect of axial stress and torsional stress, the cracks are tortuous, as shown in Fig.11(a). In addition to the main crack, there are multiple secondary cracks around the main crack. The CPZ is between the CSZ and IFZ, and fatigue striation is the main feature due to the repeated open and closure of the cracks, therefore many grooves are formed here, as shown in Fig.11(b). The IFZ is formed due to the instability propagation of the crack. Since the formation of this characteristic region is instantaneous, stairs are often formed along the crack propagation direction on the fracture surface, as shown in Fig.11(c).