| The axle is an important load-bearing component to ensure the safe operation of high-speed EMUs.When the axle has defects or cracks due to surface damage during actual operation,whether the strength and service life of the axle meet the requirements needs further study.This article takes the S38 C axle widely used in the CRH2 EMU as the research object.Aiming at the structural characteristics of the spatial gradient distribution between the surface hardening layer of the axle and the core matrix structure,and the characteristics of the surface layer having large residual compressive stress due to the heat treatment process,The fatigue performance and crack growth characteristics of axle materials were tested with small specimens,and the fatigue life and crack growth behavior of axles with typical defects were simulated and analyzed.It provides a reference basis for the service life of axles and the formulation of maintenance cycles.The main research contents are as follows:(1)A sample is taken at the position of the axle body,and the material’s microhardness test is carried out from the surface along the radial direction.Samples are selected at the position near the core of the axle to test the tensile properties of the material and the rotating bending fatigue test of the smooth sample.The test results are compared with the surface material of the axle,and the changes in the hardness,tensile properties and fatigue properties of the material at different depths of the S38 C axle are analyzed.(2)According to the results of the defect investigation,a finite element model of the axle containing different sizes of indented,diamond-shaped and semi-elliptical defects was established.Based on the measured dynamic stress data of the S38 C axles of the CRH2 A EMU,the life cycle stress spectrum is extrapolated.Then the stress concentration factor and fatigue notch factor of each shape and size defect is calculated based on the full life cycle stress spectrum.Subsequently,the fatigue life curve of the smooth axle was revised according to the fatigue notch coefficient,and the fatigue life curve of the axle under different notches was obtained,and the crack initiation life of the defective axle was calculated from the perspective of damage.(3)A three-point bending sample containing the surface material of the axle was cut from the surface of the actual axle of the S38 C,and the crack growth test was carried out by visual inspection.The relationship between the fatigue crack growth rate of the core sample and the range of the stress intensity factor is obtained.Fit the Paris formula of different depth intervals according to the material gradient characteristics.The test results are compared with the axle surface materials,and the changes of axle crack growth rate at different depth layers are analyzed.Provide the basis and reference basis for the calculation of the actual axle life.(4)The finite element axle model is divided into layers according to the characteristics of the axle material showing a gradient structure.The influence of residual stress on the simulation results is analyzed and a shell-shaped crack is implanted at the inner arc position of the wheel seat.The high frequency is reproduced in the finite element model by combining the initial stress field method with unit pressure and the iterative approximation method.The distribution of surface residual compressive stress field produced by quenching.The stress is used to calculate the factor at the crack tip.The relationship between the crack depth and the stress intensity factor at the crack tip under the action of surface residual compressive stress was analyzed.Based on the crack growth rate model obtained from the experiment,the remaining life of the axle with different depth cracks is calculated.62 pictures,14 tables,68 references. |
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