Research On Fatigue Property And Reliability Of Railway Axle Steels In Giga-Cycle Regime

With the development of high speed of railway transportation, the alternating load acting on railway axles will be more than 10~9 times in the service. According to the past researches, fatigue limit of steel materials is thought to be the point where the fatigue life is about 10~6 cycles. Therefore, the S-N curve of material designed in 10~7 cycles will be guaranteeing the safety of structural components. However, in recent ten years it is discovered that internal fatigue failure caused by internal defects and casting defects occurs in high strength alloy steels,surface treatment steels and cast iron etc. in the life cycle region of 10~7～10~9 and surface fatigue failure caused by very low speed crack growth occurs in low carbon steels which doesn't have defects. Railway axle is an important component for the safe running of rolling stock. Once fatigue failure occurs in axles, accidents such as train derailment etc. will happen during transportation. Therefore, explanation for the fatigue property of railway axle steel in ultra-long life is the foundation for guaranteeing the safety of railway axle in the service.This paper investigated the fatigue property and reliability assessment method of two kinds of axle steels, 50 medium carbon steel and 35Cr-Mo alloy steel. The research content includes: (1) investigate the fatigue failure mechanism and property of the steels, (2) explore a reliability assessment method under the condition of a small fatigue test data, (3) compare the difference of the fatigue property between the two steels and give a scientific basis for axle design. Based on the above purposes, fatigue tests of the steels in the ultra-long fatigue life (10~7～10~9) were performed by four-axle cantilever-type rotary bending fatigue machine. Research results obtained by the fatigue property and reliability assessment of the fatigue test data were as follows:Two railway axle steels have fatigue limit and also have the same fatigue failure mechanism, which is surface crack initiation mechanism. The fatigue limit of Cr-Mo alloy steel is higher than medium carbon steel's, but the dispersion of Cr-Mo alloy steel is larger than medium carbon steel's. The fatigue crack initiation-growth properties of Cr-Mo alloy steel are basically in accordance with medium carbon steel.Two reliability assessment methods of fatigue limit were proposed in this paper. Basing on equivalent damage assumption and S-N data conversion method, fatigue limit distribution can be obtained by the methods. The methods are respectively fit for the reliability assessment of fatigue limit of medium carbon steel and Cr-Mo alloy steel under the condition of small sample fatigue test data.The differences of the fatigue property between medium carbon and Cr-Mo alloy steel showed that the mean fatigue limit of Cr-Mo alloy steel is 1.49 times of medium carbon steel's. The ratio of standard deviation and mean value of fatigue limit is 4.42 times of medium carbon steel's. Therefore, medium carbon steel is suggested for the material of railway axle when the steel can meet the requirement of strength design.