Study On Initiation Mechanism And Evolvement Of Surface Fatigue Cracks Of Wheel/Rail Materials

Surface rolling contact fatigue (RCF) damage of wheel/rail is dominating damage type in the high-speed railway rail due to low wear. The research is becoming one of the critical topics in the wheel/rail relations. Therefore, the study on the initiation mechanism and propagation evolvement of surface fatigue cracks of wheel/rail materials is helpful to prevent the surface fatigue damage and decrease various problems resulting from surface fatigue damage.The experiments on surface fatigue damage of wheel/rail materials were performed using a WR-1 wheel/rail rolling wear testing apparatus under dry condition. The influence of various factors on the initiation and propagation of surface fatigue cracks of wheel/rail materials was analyzed in detail by means of the electronic balance, hardness tester, digital microscope (DM), optical microscope (OM), laser confocal scanning microscope (LCSM) and scanning electron microscope (SEM). Furthermore, the formation mechanism and evolvement of surface RCF cracks of wheel/rail materials were explored.The main conclusions are as follows:(1) With an increase in the vertical force, lateral force and attack angle, the wear rates and the thickness of plastic deformation layer of wheel materials significantly increase. With the number of cycles increasing, the wear rates and surface hardness of wheel/rail rollers increase firstly and then remain stable, while the thickness of plastic deformation layer keeps increasing. The increase of vertical force and attack angle in the late stage of experiment leads to a rise in wear rates and the thickness of plastic deformation layer and a decline in the surface hardness on both wheel/rail rollers; opposite case occurs when they decrease.(2) The vertical force, lateral force and attack angle play an important role in the initiation and propagation of surface fatigue cracks of wheel materials. With the vertical force, lateral force and attack angle increasing, the surface fatigue cracks of wheel materials initiate and propagate more easily. Oblique surface fatigue peelings initiate and propagate obviously under large attack angle condition. However, the surface fatigue damage of wheel roller is not found under no attack angle condition.(3) Under the cyclic load, tiny cracks are observed firstly, further propagate to different surface fatigue morphologies. The resultant direction of shear force on the rail roller is perpendicular to the direction of propagation of surface fatigue cracks and the one on the wheel roller is parallel to the direction of peeling propagation.(4) Surface fatigue damage on both wheel and rail rollers alleviates when vertical force increases in the late stage of experiment. However, surface fatigue damage alleviates on the rail rollers but aggravates on the wheel rollers when vertical force decreases in the late stage of experiment. Increasing attack angle in the late stage of experiment alleviates the surface fatigue damage on both wheel and rail rollers; opposite case occurs when attack angle decreases in the late stage of experiment.(5) Fatigue cracks grow along a larger angle to depth on the rail rollers, but easily become parallel with the surface or turn towards the surface on the wheel rollers. Increasing vertical force in the late stage of experiment leads to more surface and subsurface cracks in the same plastic deformation area and those cracks easily join with each other. The change of attack angle in the late stage of experiment makes cracks propagate and turn repeatedly.(6) The size of wear debris increases firstly and then decreases with an increase in the number of cycles. The size of wear debris increases when the vertical force and attack angle increase in the late stage of experiment.