Study On The Crack Initiation And Early Propagation Of Gigacycle Rolling Contact Fatigue Cracks Of Axle Box Bearings

The axle box bearing is the key traveling part of the EMU.Its complicated service environment and harsh working conditions have led to many forms of axlebox bearings in the service process.Shelling on the raceway of the axle box bearing is the main failure type which is more than 70%of total failures.In this paper,crack initiation and early propagation of the gigacycle rolling contact fatigue of the bearing steel and axle box bearing was investigated by the simulation analysis and experiment methods.First,very high cycle fatigue tests of bearing steel were conducted by a rotating bending fatigue test machine.The S-N curve of bearing steel was obtained,indicating that fatigue failure still occurred when load cycles is beyond 10~7,which is very high cycle fatigue behavior.The observation of fracture surface in very high cycle fatigue showed that crack initiated from the interior non-metallic inclusions,which is different from low-cycle and high-cycle fatigue.The ultra-high cycle fatigue caused by inclusions in the bearing steel is closely related to the raceway shelling of the axle box bearing.Then,the mechanism of interior crack initiation and early propagation in the axle box bearing of EMU was investigated.The sheeling on the raceway of the bearings presented the typical fatigue characteristicssuch as a center crack initiation site and shell-shaped fatigue crack arc.Inclusions,early butterfly cracks and large-scale cracks were observed on the profile of the bearing raceway.The mechanism of crack initiation and early propagation of gigacycle rolling contact fatigue in the axlebox bearing raceway was proposed,i.e.,with the repeated fatigue loads during the bearing service process,dispersed small crack initiated around the defects,cavities,non-metallic inclusions within the range of 0.5～1.5mm from the raceway surface of the bearing outer ring along with.grain refinement.With the following fatigue loadings,tthese small cracks coalesced to form early butterfly cracks.As the crack grows,it eventually expands to the bearing raceway surface to shell.Finally,the interior crack propagation of the bearing was conducted by simulation method.The ABAQUS software has used to perform dynamic and static stress simulation analysis on the EMU axlebox bearings.The results show that the stress distribution between the roller and the raceway surface of the bearing conforms to the linear elastic Hertz contact stress distribution.The maximum stress area of??the bearing is concentrated within the range of 0.5 to 1.5 mm from the raceway surface.There are complex stress states in this area,causing fatigue cracks to belong to compound fatigue cracks.Use Franc3D software to perform fatigue crack propagation simulation analysis on the bearing finite element equivalent model.The simiation results is in agreement with the actual failure case,which verify the effectiveness of the simulation model.The influences of the inclusion size and position on the gigacycle rolling contact fatigue life of bearings were conducted.With the increase of the inclusion size,the life rapidly reduced,and with the increase of inclusion depth,the fatigue life increases.