Research On Damage Causes And Renovate Measures Of Small-radius Curve Rail

Because of the low design standard, long service life, bad geology and climate of existing railways in mountainous regions, also with the improving running speed, increases of gross rail load on axle and density of traffic flow, application of high-strength andgood-wear-resistance rail, shelling defects of rail head and horizontal cracks have become the main damages of small-radius curve tracks. When a train runs through a small-radius curve track with high speed,the lateral rail head will suffer from eccentric compression easily, and the sub-surface of the medial side will yield, both of which cause the growth of fatigue cracks.More seriously, the medial rail head will occursshelling defects, leading to irregular roll of wheels which will contribute to extension of the damages or even raildisjunction. Hence, this thesis takes the Nanning-Kunming Railway of Kumming Track Maintenance Division as an example to study and solve the rail damages caused by rolling wheels. This research makes a great sense to improve the safe operation of railways in mountainous regions.Take the400-meter small-radius curve track of Nanning-Kunming Railway ZHK707+732.4as research object, we first conduct a field investigation. Then combined with the daily maintenance data, summarize the causes and laws of development of rail head on the curve rails. With the parameters of small-radius curve tracks acquired from the scene, apply SIMPACK to conduct dynamics computation andoptimize the geometric parameters of the curve tracks. Then apply the extended finite element module of ABAQUS/XFEM to analyze the laws of growth and development of lateral cracks. Through this research, we can arrive at the following conclusions.(1) There is a causality between lateral cracks and shelling defects thatshelling defects of rails grow out of lateral cracks of rail heads. Hence, studying the lateral cracks of rail heads is the basis of shelling defects.(2) A lateral crack of the medial rail head is composed of several fatigue cracks which result from too much wheel-rail force andeccentric compression.(3) For the400-meter small-radius curve track of Nanning-Kunming Railway ZHK707+732.4, when a freight car with a gross weight of93tonnes runs through it at a limited speed of80km/h, the largest wheel-raillateral force is57.42KN, vertical force137.96KN and digression coefficient0.416. After optimizing the rail cant and elevation of outer rail, there will be a decrease of wheel-railforce,friction power and vermicular graphite rate, proving that the optimal design is rational.(4) When the wheel-rail forceacts on the center line of rail head’s surface, fatigue effect will only occur around the contact area and the stress of the medial rail head is small. When the medial rail head is subjected to eccentric compression, the peak of stresses and strains will appear on the area under6to12mm to its subsurface, where fatigue cracksoccurs.lt is obvious that the stresses on the crack tip will centralize when the lateral crack appears. And it also will lead to the expansion of the cracks.(5) The wheel-rail force has a different effect on crack tips from different areas of the rail head. The smaller distance from the surface, the more intensive the stresses of crack tips are.And the cracks are more easily for to expand.(6) The depth of the lateral crack has a certain effect on the stress of the crack tip. With crack expansion to the center of rail head, the maximum effective stress and strain will inverselydecrease with a small range.(7) Exerting a large enoughvertical and lateral wheel-rail force on the rail head, when the edge of wheel-rail contact surfaceis right above the crack tip and on ripping side, the lateral crack will expand to the top of the ramp and develop into oblique crack, even lead to nuclear damage and disjunction. Research conclusions are consistent with the results of field observation.(8) When the size of the lateral crack is small, the stresses and strains are large on the crack tip and small on other parts. Only when the crack develops into bigger size or depth, changing the effective section of the rail head, the rail breaks. Therefore, it is safe when small lateral crack appear on the rail head, but the crack propagation needs periodic observation. The maximum safe size of lateral crack and other forms should be studied and field tested combining with practice.