13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Influence of Surface Rolling Time on Short Fatigue Crack Behavior of LZ50 Axle Steel Bing Yang*, YongXiang Zhao State Key Laboratory of Traction Power, Southwest Jiaotong University, 610031, China * Corresponding author: yb@home.swjtu.edu.cn Abstract Based on the mean fatigue life of LZ50 axle steel specimens which were unrolled before testing, five surface rolling times were determined according to the fatigue life fraction, i.e., f=0.0, 0.3, 0.5, 0.6, and 0.7. Five groups of smooth hourglass shaped specimens which were turned and rolled at above surface rolling times were tested by a replication technique. Results show that with a given dominant short crack size, crack growth rate after surface rolling occurs much slower than that before rolling. However, the more prolonged the surface treatment is performed, the greater the growth rate occurs at the transition point between the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage. Furthermore, influenced by the change of surface hardness and residual compressive stress, the effective short crack density in all the specimens, which is the average number of short cracks per unit area, decreases significantly after rolling than before. Focusing on the density after surface rolling, it is evident that the highest effective short crack density for the five studied groups of specimens increases from 662 mm-2 to 941 mm-2 with postponed rolling time. On the other hand, the average fatigue life for each group of specimens decreases with postponed surface treatment time. The average life of the initial rolled specimens was 882,562 cycles, while that of specimens turned and rolled at a 0.7 fatigue life fraction is 618,640 cycles. Therefore, the surface rolling procedure can improve the fatigue performance characteristics of the material. The choice of rolling time may affect the short fatigue crack behavior greatly. The earlier the surface treatment is performed, the better the collective effect of short cracks can be restrained as well as the longer the fatigue life of the material will be. Keywords Fatigue, short crack, surface rolling time, LZ50 axle steel 1. Introduction With the development of the high speed and heavy haul railway in China, the service load conditions for structures and components of railway vehicles are much poorer than before[1]. As one of the important parts for vehicle running gear, axle bears complicated alternate loadings in service, and is the component with the highest loading frequency and the most complex failure modes[2]. If the axle failure caused by fatigue damage is uncontrollable, vehicles are likely to derail, and railway operation safety will be gravely affected. For components with smooth surface under alternate loadings, short fatigue crack (SFC) initiation, coalescence and propagation normally occupy more than 70% the fatigue life[3]. For example, with a overhaul cycle of 100,000 km and a reliability of 0.999, the critical size of semi-elliptical crack on the load relieving groove of RD2 axle is 1.23 mm, and the size of circumferential crack is only 0.94 mm[4, 5]. Thus it can be seen that the fatigue damage process of axle belongs primarily to SFC stage. The maintenance strategy for load relieving groove of RD2 axle, for which the final processing method is turning, is finish turning and surface rolling. However, the impact of surface treatment time choice on maintenance effect, i.e., how the surface rolling time affects the short fatigue crack behavior, is still an on-going research issue. LZ50 axle steel is one of the widely applied axle materials in Chinese railway manufacturing
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