Studies On The Fatigue,Crack Propagation Characteristics And Reliability Of EMU High Speed S38C Axle

ABSTRACT: S38C axle as one of the main axle types of modern high speed railway in China, is wildly used on many kinds of EMU train as CRH2A, CRH2B, CRH2C type one and CRH2E. However, due to the particularity of S38C axle import and the material processing, the research of this kind of axle is just at the beginning stage in our country,and the content is less at present. This paper does some studies about fatigue and propagation characteristics of S38C axle specimens and real axle. And crack stress intensity factor and crack propagation simulation of hollow axle are studied. On the other side, evaluation of axle crack remaining life time and axle crack using reliability are also did in the paper. These studies are aimed at providing basis of establishing maintenance cycle and management method of S38C axle. The main contents are as follows:1. Select specimens from the surface of the axle, and the microstructure and hardness of the axle steel were measured along the radial direction. Make three point bending fatigue specimen. According to the delamination change of S38C surface material. The hardened layer and transition layer are thin. According to the real structure and hardness of the axle, the high frequency quenching process of S38C axle steel was simulated by thermal simulator. Then using self-made quenching device for batch processing. A small specimen with the same material properties as the actual axle surface is obtained. This work solved the problem of simulating the surface heat treatment processes of S38C axle steel materials at a small sample level. The fatigue test of processed specimens are used to obtain the S-N curve and fatigue limit of the axle steel material. And use the MM rule to modify the S-N curve below the fatigue limit. Make crater defect and drilling defect specimens. The corresponding S-N curve and fatigue limit are obtained. Effect of three different heat treatment processes such as induction heating quenching, quenching bath furnace and muffle furnace heating quenching on the fatigue properties of axle steel are studied. Effect of pre-torsion on fatigue performance of axle steel are studied. According to damages of axle, artificial defects are made on surface of axle, the defects are ring grooves and saw grooves. The processed axle is placed on the cantilever beam axle fatigue test bench for the fatigue test. Fatigue test results are analyzed. The results show that the stress concentration maximum nominal stress results of small specimens and notched axles are very close. Maintenance regulations of "scratches cannot be greater than 0.1mm,impact injury cannot be greater than 0.3mm" is probably on the conservative side.2. The samples are directly get from the actual surface of the axle, and the microstructure and the hardness gradient of axle the surface are maintained. Make artificial defects on the surface side of specimen, and crack growth data are obtained by experiment. According to the change of hardness and microstructure at different depths of axle surface, a method of delamination considering crack propagation is proposed. The results are divided into three groups: 0-2mm, 2-4mm and 4-8mm according to the change of microstructure and hardness of axle surface. The Paris formulas and Forman formulas of three groups are calculated separately. Then make comparison of Paris formulas and Forman formulas. The first crack growth test of S38C axle in China was carried out.Crack propagation tests were carried out using two axles. The cracks on the axle surface were monitored by using strain gauge, magnetic particle inspection method and ARAMIS crack monitoring system respectively. The artificial defects are made on the surface of axle, then make crack on axle fatigue test bench and the crack growth test is also carried out. Axle surface high frequency quenching hardening layer is clearly visible according observation of axle section. And then the Paris formula is obtained by using test data. The Paris formula of the small sample and the Paris formula of the real axle are compared and analyzed. From the point of view of crack propagation characteristics, maintenance regulations of "scratches cannot be greater than 0.1mm, impact injury cannot be greater than 0.3mm" is probably on the conservative side.3. The geometry model of surface crack on a hollow axle is built, and the analytic formula of the stress intensity factor of hollow axle surface crack is obtained by modifying considering effect of inner surface on stress intensity factors of hollow axle.The semi-elliptical crack with different depth and different shape on the axle surface is established. And the finite element results are compared with analytic solution results. On the basis of stress intensity factor analytical formula of hollow axle surface crack, an analytical model considering residual compressive stress is put forward. At the same time,the finite element model considering residual stress is established. Compared solutions before and after residual stresses, which proves the inhibitory effect of residual compressive stress on crack propagation. The initial residual compressive stress is introduced by the ABAQUS subroutine written in Fortran language. Using XFEM and VCCT simulation techniques to study the effect of residual compressive stress on crack propagation. It is found that residual compressive stress has a significant inhibitory effect on crack propagation. Based on the Zencrack simulation software, the crack propagation parameters of real axles are used to do simulation of axle crack propagation. The crack propagation life of the axle is calculated by using simulation results.4. Based on measured data of the line, the measured stress spectrum of the S38C axle is obtained by the rain flow counting. Based on the fatigue characteristics of the axle steel obtained from the test, the equivalent stress of the axle is calculated by the Miner linear fatigue cumulative damage rule. The crack growth characteristics of the axle steel and the stress intensity factor of different shape ratios were used to predict the critical crack size and residual life of the axle under severe conditions. Finally, on the basis of probability fracture mechanics, the stress intensity factor of the crack front is chosen as the "stress",equivalent stress of axle is chosen as the random variable of "stress",and the fracture toughness of the axle is chosen as the "strength". The equivalent stress and fracture toughness are used as random variables to evaluate the reliability of the crack propagation life of the axle under fixed ultimate crack and considering random character of crack propagation.