Fatigue Crack Initiation Mechanism Of Rail Under Hardness Gradient Change

With the booms of China's railway industry,the problem of fatigue damage of rails is increasingly serious,which brings great hidden dangers to driving safety.Therefore,many experts and scholars focus on how to predict the initiation life and initiation position of rail fatigue cracks more accurately.The rails are usually treated as homogeneous materials during the study of rail fatigue cracks.However,that is inconsistent with the facts.The rail heads will form a cap-shaped heat-treated hardness gradient layer when went through heat treatment.After a period of time,there would form a work hardening hardness gradient layer at the rail head.On the grounds of the theory of hardness-yield strength conversion relationship,combined with the secondary development of USDFLD,the non-homogeneous treatment of the rail can be realized.Based on this,the fatigue gradient of the rail is studied.The impact of reasonable matching of wheel and rail hardness,the specific research results are as follows:(1)The hardness-strength conversion relationship for the wheel steel CL6 O steel and the rail steel U78 CrV is computed based on the theory of hardness-yield strength conversion relationship by the least square method.The cap type heat treatment hardness gradient layer and the work hardening hardness gradient layer of the rail are relatively accurately restored in the rail with the hardness-yield strength conversion relationship and the secondary development of USDFLD.(2)The results indicate that the heat treatment hardness gradient and the work hardening hardness gradient rail mainly affect the maximum Mises stress,equivalent plastic strain and the fatigue crack initiation life of the rail.The heat treatment hardness gradient will reduce the maximum Mises stress inside the rail.However,since the internal yield strength of the rail head is smaller than the homogeneous rail,the equivalent plastic strain increases and the crack initiation life decreases.The work hardening hardness gradient will significantly increase the maximum rail length.Mises stress.Nonetheless,as the yield strength in the rail head is greater than the homogenous rail,the original stress level of the rail does not yield the rail,so the equivalent plastic strain inside the rail becomes 0 and the crack initiation life increases greatly.(3)The optimal correction effect of the heat treatment hardness gradient on the reasonable matching of the wheel-rail hardness under the new wheel new rail condition and the work hardening effect of the wheel and the rail on the wheel-rail are studied from the perspective of the fatigue crack initiation life and the initiation position of the wheel.The results show that the heat treatment hardness gradient will reduce the reasonable matching range of the Wheel-rail hardness ratio by about 0.05 under the new wheel new rail condition.The work hardening effect of the wheel and the rail has a great influence on the reasonable matching range of the Wheel-rail hardness ratio.In the new wheel old rail state,it is recommended to select 0.75~0.85 for the reasonable matching range of Wheel-rail hardness ratio.In the old rail new rail state,the 1.00~1.12 for the reasonable matching range of Wheel-rail hardness ratio is advised.In the old wheel old rail state,it is recommended to choose 0.76~0.86 as for the Wheel-rail hardness ratio.