Research On Fretting Fatigue Damage Mechanism Of EA4T Axle Wheel Seat

In order to reveal the fretting fatigue mechanism at the wheel seat of EA4 T axle.The rotating benging fretting fatigue test of the full-scale axle was conducted under high and low loads in this thesis.The high load was consistent with the fatigue limit of the wheel seat,while the low load was consistent with the operating load stress amplitude.The effect of fretting wear on the fretting fatigue strength at the wheel seat of the full-scale axle was analyzed;the mechanism of fretting wear and fretting crack initiation was determined.The results show that the fretting damage was caused by fretting wear and fretting corrosion,and the fretting wear was the main cause.The fretting damage zone can be divided into three zones.Zone I,at the edge of the contact area,was in the wear zone with the most severe damage,and the abrasive wear occurred.The severe damage was also observed in Zone II that was close to zone I.The red and black abrasive debris can be found in zone II and the abrasive wear,delamination wear and oxidative wear occurred.Zone III was in the stick-slip zone with the slight damage and the delamination wear and oxidative wear occurred.The width of the wear zone was consistent with the opening zone.The boundary of fretting damage zone was consistent with the stickslip boundary.The degree of fretting damage and the total width of the damaged zone increased with the increase in the load.The fretting fatigue crack initiated at the stick-slip zone and distributed along the circumferential direction of the axle.The orientation of the cracks was approximately 33° relative to the radial direction of the axle and they propagated in radial direction gradually.In order to clarify the effect of fretting wear on fretting fatigue at the wheel seat,the finite element simulation models were established based on the unworn contact surface observed before experiment and the worn contact surface observed after the experiment,respectively.The effect of fretting wear on the stress distribution and fretting parameters was studied.The SWT damage parameter and FS damage parameter were calculated by using the multi-axis fatigue critical plane method.The results show that the peaks of the contact pressure,the frictional shear stress,the fretting slip amplitude and the axial stress appeared at the contact edge for the unworn contact surface.These fretting parameters redistributed due to the fretting wear.The peaks of them moved toward the inside of the contact surface and appeared at the worn-unworn boundary.The axial stress(corresponding to the nominal stress of fatigue design)which controls the fatigue behavior changed from negative to positive in the wear zone.Fatigue crack initiated at the position where the maximum values of SWT damage parameter and FS damage parameter appeared.From the above results,the fretting fatigue mechanism at the wheel seat of the EA4 T axle was as follows:For the new axle,the stress concentration at the contact edge of the wheel seat was the most severe due to the combined action of the contact pressure,the frictional shear stress and the fretting slip amplitude.However,the axial stress was very small at the contact edge.With the increase of loading cycles,the fretting wear gradually appeared at the edge of wheel seat and the wear zone moved toward the inside of wheel seat.Meanwhile,the axial stress increased gradually.Fatigue crack appeared near the stick-slip zone(worn-unworn boundary)when the damage parameter increased to a certain value at the multi-axis fatigue critical plane.The crack propagation angle is consistent with the critical plane.Fretting wear changed the contact state at the contact area and increased the axial stress.Therefore,restraining fretting wear was the most fundamental method to restrain fretting fatigue.The research results of this thesis can provide a guidance for the anti-fatigue manufacturing of wheel seat and the anti-fatigue optimal design of the axle structure.