Notch Torsional Fatigue Features of C250 Steel

The characteristics of fatigue fracture of C250 steel with different stress ratio and different cycle life were studied by stereoscopic microscope and scanning electron microscope. The results show that the macroscopic features of C250 steel notch torsion fatigue fracture can be broadly classified into four types of fracture morphology, such as inclined plane,ratchet type,sawtooth type and plane type. Bevel and ratchet fracture usually occur at R>0,the fracture of the loop cycle 10⁵ and above,sawtooth fracture usually occur at R<0、the fracture of the loop cycle 10⁵ and above;and the planar fracture usually occurs on the fracture of the loop cycle at 10⁴ and below. On the slant fracture and ratchet fracture, and on the axis is the expansion of the 45° incline to is a kind of cleavage fracture feature, fatigue stripe is typically seen in the microscopic characteristic. The vertical section of the axial plane is mainly to reverse the wear characteristics. Fracture zone is characterized by dimple fracture. The initiation and extension of the fatigue crack of inclined plane,ratchet type,sawtooth type and plane type are mainly affected by shear stress. The initiation and extension of planar fracture crack is mainly affected by plane tensile stress.