Study On The Very-high-cycle Fatigue Behavior Of Railway Wheels And Fatigue Life Assessment Method

Due to the increasing engineering significance for service reliability of metal materials applied in wheel/rail system,the very-high-cycle fatigue(VHCF)behavior and fatigue life evaluation method of railway wheels and wheel steels under rolling contact loading were studied by theoretical analysis and experimental methods in this paper.Initially,for the case of fatigue property of wheel steel,fatigue tests including high-cycle fatigue(HCF)and VHCF were conducted on the ER8 wheel steel.The observations of fracture surfaces in HCF indicated that crack initiated from surface for most of the specimens,and interior for few specimens with large inclusions.The maximum size of the inclusions causing crack initiation is about 200?m.Meanwhile,it presented that crack initiated from interior,fish-eye morphology was observed and Fine Granular Area(FGA)was not clear from microscopic analysis of VHCF fracture of wheel steel.Hence,interior crack initiation caused by inclusions occurred in HCF and VHCF,indicating that the failure type of interior crack initiation in railway wheels should be investigated.In engineering,failure caused by interior crack initiation during service of wheels should be paid more attention.Furthermore,for the case of railway failure caused by fatigue,the shattered rim and shelling of railway wheels in the past ten years were statistically analyzed.It was demonstrated that the shattered rim usually caused by the crack initiation from the inclusion located in the depth of 10 to 25 mm from the tread,and shelling was attributed to the subsurface crack in the depth of less than 10 mm from the tread.The service mileages for shattered rim and shelling wheels were beyond one hundred thousand kilometers or even more than one million kilometers.The corresponding fatigue lives were within 107 cycles and 109 cycles which belonged to the VHCF regime.Therefore,the essence of shattered rim and shelling is the VHCF under rolling contact load.Three typical features were defined in the fracture surfaces of shattered rim and shelling typically:inclusion,fish-eye and crack propagation zone,which were similar to that of the VHCF under uniaxial loading.Unique VHCF features were also observed,i.e.,three-dimensional crack morphology,uniform distribution of bands in the crack propagation region,but no FGA in crack initiation region.In addition,the investigation of microstructural evolution around the crack surface indicated that a layer of ferrite nanograins with the grain size of about 20nm was located at the crack initiation region of the shattered rim wheel.Typical pearlite microstructure was found under the nanograins layer.The original pearlite switched to the ferrite nanograins with VHCF loading.No nanograins were found in the VHCF crack initiation region of wheel steel under uniaxial load,which was all original pearlite microstructure.Microstructure characterizations of VHCF crack propagation region of shattered rim presented a large number of ferrite nanograins.The hardness of this region decreased.Secondary cracks and carbon segregation occurred.This region could be the weak areas that easily lead to crack initiation.However,no nanograins were observed in VHCF crack propagation region of wheel steel.Different fatigue loading,rolling contact loading for shattered rim and axial loading for the VHCF of wheel steel was responsible for the difference of microstructures.Finally,damage tolerance and remain fatigue life of railway wheels were investigated.The contact stress distribution of wheel/rail with inclusions was analyzed by ABAQUS.Then the stress results were imported into FRANC3D to simulate the crack propagation.The simulation results were in good agreement with that of the actual shattered rim,which proves availability of the simulation.Based on the crack propagation curves obtained by simulation,the rolling contact fatigue damage tolerance analysis and remain fatigue life assessment method of wheels were proposed.Using this method,for the initial inclusion radius is 0.1 mm the total fatigue life of the wheel is 4.28 million kilometers.It is known that the wheel can operate safely in the whole service life.Furthermore,for the initial inclusion radius is 0.3 mm the total fatigue life is 890,000 km and might fail during the service.