Study On Fatigue Performance Of Impact Damage On S38C Axle

High speed railway axles will inevitably be subjected to foreign object impact damage during the actual operating conditions.The scratch or impact grooves will be produced in axle surface when flying rocks or metal fragments hitting the axle which seriously threatens the safety of the train.Although existing axle damage preparation methods can provide certain theoretical support,they have not been able to simulate actual axle damage.Therefore,it is of great engineering significance to develop a new method of axle damage preparation and to explore the fatigue fracture mechanism of axle impact damage.The results as follows:(1)The tensile strength,radial microhardness and microstructure of the induction hardened layer on the surface of the S38 C axle are tested and analyzed.The fatigue limits of the four-point bending standard specimens of the axle are tested by the lifting method and the step-loading method.The data were calculated by different statistical methods.Step-loading method fatigue limit is about 10% higher than the lifting method.(2)Impact projectiles(shaped as balls,cubes,made of bearing steels,and tungsten steels)were used for gas gun to create damage defects.The macroscopic morphology and cross-sectional microstructure of the defects were statistically analyzed.The variation of spherical defects produced under different impact velocities is discussed.When the impact velocity reaches 300m/s or more,adiabatic shear bands are produced.The reasons for the formation of adiabatic shear bands were analyzed.The elastic modulus(254.0 GPa)and nanohardness(11.83 GPa)of adiabatic shear bands were higher than those of smooth samples and flaw samples by 4.35% and 46.96%,respectively.The adiabatic shear band of the secondary surface of the defect is more harmful and his hardness and grain size mismatch can cause inconsistent deformation.(3)There are two kinds of defects in the external body damage of cubes: scratch defects and grooves defects.The scratches caused by 300-500m/s velocity impact of the bearing cube are similar to the actual axle scratches,and the secondary surface has no obvious change;grooves defects can be generated when the tungsten cube is used for 100-300m/s velocity impact.The actual axle groove defect parameters can be simulated at an impact speed of 100 m/s.(4)The surface of the axle of the S38 C is very sensitive to stress concentration.Spherical defect fatigue fracture mechanism is dominated by stress concentration.As the impact speed increases,the fatigue fracture mechanism gradually evolves into stress concentration,adiabatic shear band failure cracks,missing materials,and multiple crack propagation.Scratch defect fatigue performance only slightly decreased.The groove defects include foreign object inlay,stress concentration,secondary surface adiabatic shear band and its long cracks caused by failure,and the fatigue performance is obviously reduced.The adiabatic shear band is generated at this speed,which means that the impact damage of the sharp hard object cannot be the sole criterion for evaluating the impact speed.