Fig.9 Macro fracture morphology; (a) P=50MPa, DP; (b) P=250MPa, DP; (c) P=150MPa, SP; σe=400MPa.
The fretting fatigue fracture of 35CrMoA steel basically includes three characteristic regions, named with the crack source zone (CSZ), crack propagation zone (CPZ) and instantaneous fracture zone (IFZ) (respectively I, II, III). When the contact stress is low, the proportion of CSZ and IFZ is large, and although CSZ is generated in the two corresponding fretting regions, there is always a CSZ that predominates during the crack propagation, which would make IFZ biased towards another CSZ; With the increase of contact stress, the fracture area occupied by CSZ and IFZ is reduced. In the whole fracture area, CPZ occupies the main parts, and the cracks generated by the two corresponding fretting regions both dominate, so that IFZ is located in the center of the fracture and always has a nearly symmetrical shape. When the contact stress is large, the amount of deformation generated by the sample and the stress concentration at the material damage caused by fretting are larger, so microcracks are easier to form and are more liable to propagate into main cracks under this action. Therefore, CSZ of fracture under high contact stress is small and shows good symmetry.
Compared with DP under the same conditions, there are multiple crack source regions in the fretting fatigue fracture under SP, but there is also only one main crack source region, and block spall of the material also occur in the fracture area. This is due to the fact that the main crack spreads to connecting with the microcracks that are not generated at the same level, which causes the material to fall off.
Micro fracture analysis
Fig.10 is the fretting fatigue fracture morphology under DP with equivalent stress of 400MPa and contact stress of 50MPa. The CSZ, CPZ and IFZ can be clearly distinguished from the figure. Within a certain number of cyclic cycles after crack initiation, it mainly expands in the cross section of the specimen. At this situation, although the specimen is also subjected to the combined effect of contact stress and equivalent cyclic stress, the microcrack is closer to the surface of the fretting zone. Therefore, it is mainly controlled by contact stress; When the crack grows to a certain length, which is far from the surface, the effect of cyclic stress on the crack growth gradually strengthens, so that the crack propagation appears along the longitudinal component, and as the crack further expands, the longitudinal rate of crack propagation increases gradually. Fretting fatigue fracture surface continuously produces stairs along the longitudinal direction.