The Study On The Fatigue Crack Nitiation And Propagation Life Prediction Of Rail

Since the reform and opening-up policy began, the economy of China has grown rapidly. To meet the needs of economy development, China railway transportation is also developing fast toward high speed and heavy haul. The opening to traffic of Beijing-Tianjin line and Wuhan-Guangzhou line symbolized the high-speed railway construction in China has entered a stage of development by leaps and bounds. With the background, this paper analyzed and discussed the fatigue phenomenon of rail steel, which encountered in construction of high-speed railway. At first, this work analyzed the complex stress intensity in rail. Then, the mathematical model of predicting rail's fatigue crack initiation and propagation life were established by the Fatigue and Fracture Mechanics Theory. Finally, rail's fatigue crack initiation and propagation life were estimated by the prediction model. Furthermore, the effects of related factors on the fatigue life of rail were analyzed in detail. The results provided a theoretical basis for the maintenance of rail and development of rail maintenance cycle. According to the calculation and analysis, the following conclusions were obtained.1. On the base of static calculation about continunous oundation-beam, the dynamic bending stress of rail was analyzed under the conditions of the proposed rail line. From the results, we got those conclusions:when the elasticity coefficient of rail basis was constant, the circulating levels of dynamic rail bending stress would increase with the increasing of the train velocity coefficient. However, when the train velocity coefficient was constant, the circulating levels of dynamic rail bending stress would decrease with the increasing of the elasticity coefficient of rail basis.2. Based on Carter two-dimensional Rolling Contact Thepry, the internal stress field of 60kg/m rail head was calculated when CRH2 train-set's wheels contacted on it. According to the results, we found that:when the friction coefficientμ≤0.3, the relative maximum shear stress in Z-axis increased and then decreased with the depth growing, the value reached to the maximum in the depth of z/a=0.6 under the rail surface. However, when the friction coefficientμ>0.3, the value of relative maximum shear stress in Z-axis reached to the maximum on the rail surface, then it would decrease with the increasing of depth.3. Results of rail fatigue crack initiation life prediction showed that:intensity limit can be extended to improve rail's fatigue crack initiation life, but the effect was not obvious. When the temperature stress was compressive stress, the larger the temperature stress the longer the fatigue crack initiation life of rail. However, it was just the opposite when the temperature stress was tensile stress. From the results, we also found that:the crack initiation life of rail decreased with the increasing of the train velocity coefficient and increased with the elasticity coefficient of rail basis.4. The prediction model of propagation life of rail surface oblique crack was established by the fracture mechanics theory. The results showed that the propagation life of rail surface oblique crack would decrease with the increasing of friction coefficient between wheel and rail. With the increasing of inclination angle, the propagation life of rail surface oblique crack would increase. However, it would decrease when the inclination angle was larger than a particular value. The propagation life of rail surface oblique crack would increase slowly with the increasing of wear rate. When the wear rate reached to a particular value, it would increase rapidly.5. The prediction model of propagation life of nuclear damage was established by the Fracture Mechanics Theory, too. The results showed that the propagation life of nuclear damage of rail that made of U71Mn material was longer than that of PD3 under the same operation condition. With the increasing of train velocity coefficient, the propagation life of nuclear damage would decrease. However, it would increase with the increasing of the elasticity coefficient of rail basis.