Fatigue Resistance Assessment Of Externally Impacted Railway EA4T Axle Steel

As a critical safety component of high-speed train,it is important for the safety and reliability of vehicle operation to ensure that the axle does not have destructive fracture accident during operation.For example,the axle is easy to be impacted by ballast,ice,snow and other foreign objects rolled up by the aerodynamic effect,which destroys the integrity of the axle surface to form typical foreign object damage(FOD)to greatly reduce the fatigue performance of axle.Therefore,the research on the failure mechanism and fatigue characteristics of FODed axle steel is helpful to explore the protection and life-extending technology of high-speed railway axles in extremely complex environment.In 2018,the state announced to start the construction of Sichuan-Tibet railway,the line of which passes through high mountains and deep valleys,and the terrain conditions are extremely complex.It is not only necessary to overcome the huge altitude and complex geological conditions,but also the technical problems such as super long tunnels and ultra-high bridge beams.Under such extreme conditions,the axle is easily damaged by the foreign object,so this research can provide a theoretical reference for the fatigue resistance assessment of the vehicle structure of Sichuan-Tibet railway.In this thesis,EA4T alloy steel axle in service was adopted to investigate the failure mechanism and fatigue performance assessment method of axle steel under the coulping effect of foreign object damage and shot peening(SP).The main results and conclusions obtained are as follows:Macro characteristics,micro damage and microstructure evolution of defect caused by foreign object damage.The macro shape and size of foreign object impact defect showed that the diameter and depth of the impact crater increased with the increase of the impact velocity.However,the impact craters of SPed specimens were smaller than those of un SPed.The micro damage characteristics of FOD included material folding,material loss,micro crack and micro notch,etc.The damage feature of SPed+FODed crater was mainly the continuous crater formed by the shot peening projectile.The microstructure of shot peening reinforced layer and the FODed defect analyzed by microstructure characterization technology showed that there was some evolution in the microstructure,including the dislocation density and grain boundary angle increasing,the grain size refining,the texture strength increasing and the hardness value changing gradient with the increase of depth.Residual stress field characteristics of damaged defect under the dual factors of foreign object damage and shot peening strengthening.The Johnson-Cook(J-C)constitutive model and failure model of EA4T axle steel were introduced to simulate the foreign object impact process,and the residual stress field of shot peening strengthening layer was obtained by setting the temperature field.The residual stress fields at the crater of un SPed+FODed,SPed+FODed,FODed+SPed specimens were reconstructed by combining the two methods.It was found that the residual tensile stress which promotes the fatigue crack initiation was formed at the edge of the impact crater.At the bottom of the crater,the residual compressive stress which can restrain the crack initiation was formed.Fatigue performance and reliability assessment of EA4T axle damaged by foreign object.It was found that the shot peening can improve the strength index and reduce the plasticity index of the axle specimen,and the effect of foreign object damage on the tensile strength was not obvious.The fatigue test results showed that shot peening can improve the fatigue resistance,especially the high cycle fatigue performance of FODed+SPed specimen was almost the same as that of the smoothed specimen.The fatigue limits of un SPed+FODed and SPed+FODed specimens decreased with the increase of the impact speed.The P-S-N curves with the survival rate of 97.5%were obtained by combing the mean value function?(?)of logarithmic fatigue life and the standard deviation function s(?),which can determine more reliable fatigue loading range to provide key data for reliability design of damaged axle.The Paris model,Donahue model and i LAPS model were adopted to analyze the fatigue crack propagation characteristics to obtain the crack propagation rate prediction model.The i LAPS model based on monotonic tensile property parameters can acquire more reliable prediction results of crack propagation rate.In addition,it was found that there existing a competition mechanism among stress concentration,residual stress,micro damage and microstructure in the fatigue crack initiation process through the fracture analysis of un SPed+FODed,SPed+FODed and FODed+SPed specimens,which was considered as the main failure mechanism of FODed axle steel.The fatigue damage-defect-life assessment diagram of EA4T axle damaged by foreign object.The stepwise fatigue assessment framework proposed by our research group was used to evaluate the service performance of the damaged EA4T railway axle,which provided an important scientific reference for assessing the effective service life and formulating reasonable defect detection period of the axle.On account of the axle fatigue performance was reduced after being impacted by foreign objects,the nominal stress and damage tolerance method were combined to propose the?area-??_w-N_w-m_wmodel based on the El-Haddad model and S-N curve.The model can predict two unknown parameters according to two known parameters,which avoids carrying out relevant test to greatly reduce the test cost.According to the modified Miner cumulative damage theory and load spectrum,the service life of the FODed axle was calculated by combining the critical damage value D_cri.It was found that service life calculated by the model prediction result was more reliable than that of the actual fatigue test results.Finally,the damage-defect-life service performance assessment diagram with 97.5%survival rate of foreign object damaged axle was established,by which the safe service limit of the damaged axle can be obtained.