Fatigue Assessment Method For High-speed Railwayaxles Due To Foreign Object Damage

Fatigue fracture is one of the key mechanical problems of high-speed train axles.In order to ensure the safe,reliable and efficient operation of high-speed trains,fatigue fracture accidents of axle must be prevented.With the increase in the operating speed of rail vehicles,the possibilities of foreign object damage(FOD)due to high speed impact of hard object are increasing.FOD destroys the integrity of axle structure and the surface protective coating,reduces the fatigue strength of the axle and exposes axle metal to atmospheric environment.Even though with allowable stress conditions,FOD may induce crack initiation and crack propagation to critical dimension which can lead axle fracture.Therefore,the traditional safety life design method can not keep the safety of FOD axle.It is necessary to use the combined method of nominal stress and fracture mechanic to accurately evaluate the damage evolution behaviour of FOD.In this paper,a comprehensive and systematic study was carried out mainly in the following aspects:1.The surface defects of the domestic active alloy steel grade and carbon steel axles were investigated and the types of axle surface defect,the proportions of different kind defects,and the size of defects were analyzed.Axle surface defects were mainly due to scratch and FOD which was nearly half of all the accounted defects.FOD mainly distributed in the middle of the axle.The observed maximum defect width was about 7700 ?m and the maximum defect depth was about 510 ?m.2.An compressed-gas gun device was used to perform FOD simulation experiment,and the macroscopic morphology and microscopic damage characteristics of FOD which include micronotch,microcrack,lamellar structure and pile-up of material were observed using a stereomicroscope and a scanning electron microscope.3.The fatigue experiments were performed with FOD specimens.Probabilistic method was used to analyze the fatigue experiment data and study the effect of FOD on the fatigue properties of the specimen.With the SEM,it can be seen that the stress concentration and microstructure damage resulted from FOD may provide favorable conditions for the initiation and propagation of fatigue crack and reduce the fatigue life of FOD specimens and components.4.The elastic-plastic finite element method was carried out to study the residual stress caused by FOD and compared with the residual stress data of X-ray diffraction experiment.It was proved that the numerical simulation method was correct and the influence of residual stress on crack initiation and propagation were analyzed.There generally was residual compressive stress in the root area of the defect,which can inhibit the initiation and propagation of the crack.The part area of FOD rim was residual tensile stress which can provide favorable conditions for crack initiation and propagation together with the microcrack and other damage characteristics.5.Considering the influence of FOD,geometry,surface quality,and loading kind on the fatigue limit of the component,the Kitagawa-Takahashi diagram which was applied to short crack was improved and applied to the fatigue evaluation of the FOD axle.A more reasonable and reliable fatigue loading safety region and conservative evaluation result can be obtained.