Modelling The Fatigue S-N And Crack Growth Of Railway Vehicle Structural Materials

With the rapid development of railway transportation system,the running distance and operating speed have changed from short distance and low speed to the long distance and high speed,leading to the poor in-service condition for critical structures and the degraded fatigue performance.Therefore,it is of vital significance to evaluate the service safety and reliability of critical components as well as the fatigue crack growth rate(FCG)of defect-containing materials.The main research contents are as follows:In order to improve the fitting accuracy of P-S-N curves obtained from small sample fatigue data controlled by the traditional nominal stress,an improved backward statistical inference approach(ISIA)is proposed by optimizing the search method of the standard deviation(STD),giving a more reasonable scatter in fatigue life.The accuracy of the fatigue P-S-N curves for full-scaled structures is also improved by considering the fatigue life of small-sized samples.The P-S-N curves of two kinds of railway axle steels are fitted with test results by the ISIA.The fatigue life is also predited by a combination of the P-S-N curves and the load spectrum.Moreover,the fitted fatigue P-S-N curves of 14 kinds of welded alloys are compared with the IIW recommendations.It is found that the ISIA presents the accurate STD and conservative fatigue life with the load spectrum.However,the above theoretical system ignores the defects of vehicle structural materials leading to unreasonable design.Therefore,the damage tolerance design related with fracture mechanics is introduced to predict the fatigue crack growth(FCG)rate.The influence factors on the fatigue crack growth rate are summarized,including the service temperature,corrosive environment,loading frequency,residual stresses and crack closure effects.The FCG rate models are summarized accordingly.In order to evaluate the crack closure effect,the crack closure factor U is taken into account.Furthermore,to unify the calculation method of U,the models are subjected to the quadratic fitting,giving the two expressions under the plane stress and plane strain conditions.Each formula has the applicable range of stress ratios.Based on the FCG based on a linear damage accumulation(FCG-LDA)model with simple calculation and the long and physically short(LAPS)crack model useful for physically short crack,the uniaxial tensile properties parameters affecting the low cycle fatigue(LCF)performance and the effect of crack closure are summarized.The relationship between the uniaxial tensile properties and LCF is obtained based on the experimental data and the newlyproposed i LAPS model with the uniaxial tensile properties is also proposed.In the case of 11 kinds of engineering materials,the predicted FCG rate obtained from i LAPS model is in well agreement with the experimental data.Compared with the traditional LAPS and FCG-LDA models,the newly-proposed i LAPS model presents a higher accuracy.