Research On Fatigue Strength Evaluation And Structure Optimization Of Heavy Haul General Purpose Gondola Car Body

With the development of heavy haul transportation in China,the wagon structural fatigue strength problems have become increasingly prominent during applicatioa Until now,the total number of general purpose gondola cars is about 436,000 in China,accounting for more than 46%oftte wagons,total number.Because loading conditions and operating conditions are limited,the local structure of the general purpose gondola car body is asymmetrical,resulting in different degrees of stiffness mutations at the local structure.Fatigue cracks appear at the position which is welded joints with stiffness mutation on some car body,and its life is much lower than the design life.Therefore,it is necessary to research on the fatigue strength evaluation and structural optimization design of the general purpose gondola car body,and establishing a more reasonable fatigue strength evaluation method and structural optimization design method,which provide theoretical support for anti-fatigue design for car body.The ultimate goal is to improve the fatigue reliability of car body and reduce maintenance costs from the source of design.Now,the fatigue strength evaluation of wagon car body mainly includes two aspects.On the one hand,the AAR load spectrum and the Daqin line load spectrum are used as input load.On the other hand,the fatigue performance parameters of welded joints mainly refer to AAR standard,BS standard,and IIW standard.However,the fatigue strength evaluation result of C80E car body is quite different between traditional method and actual application.Based on this,the paper is supported by the science and technofogy research and development project of China railway:"Research on Key Technologies of General Purpose Wagon with 27t Axle Load”.C80E general gondola car is as the research object.A fatigue test is carried out on the samples of the typical welded joints on the car body,and dynamic stress test of car body is carried out too.The fatigue strength evaluation method and the structure optimization design method of the car body are studied,and the structural strength evaluation and structure optimization of the car body are carried out by the method which is proposed in this paper.The nain research contents are as follows:(1)According to the main welded joint form on the general purpose car body,the T-joint samples and lap joint samples were made for fatigue test by using the steel plate of Q450NQR1.Aining at the small number of sanples,the method of fitting the P-S-N curve is proposed,which is the maximum likelihood method based on sample information reconstruction and fatigue life of the equivalence principle.Then the method is used to fit and extrapolate P-S-N Curves for two typical welded joints.Finally,the slope of the P-S-N curve in the medium life zone and the fatigue strength at the fatigue life 2× 106 are obtained.(2)The structural characteristics of general purpose gondola car are analyzed,and combining with the areas of the car body where fatigue cracks appear during application process.Based on this,the key areas that need to be focused on during the finite element simulation are identified.Then,the finite element model of C80E car body is established.The method based on the standard deviation weighted stress smoothing is proposed,because the nodal stress at stiffiiess mutations position is small when using the mean value smoothing method.Then,the method is used to smooth the joint stress on the key weld,and the stress average,maximum value and smoothing value of the key weld are conpared and analyzed.In order to further analyze the influence the local structural asylmetry of the car body on the stress distribution of the cross bearer weld,the correlation between the stress at the interior reinforcement of the cross bearer weld and the position of the side sill is measured by Pearson correlation coefficient and multiple correlation coefficient.(3)The method and process of optimizing car body structure are given systematically by researching on the two aspects of car body structure design strategy and structure optimization method.By analyzing the advantages and disadvantages of the traditional structure design strategy,the car body structure design strategy is optimized based on stress maxima and stress trend of the key areas.Modern intelligent algorithms are deeply studied,and the MPSO-BP optimization method based on orthogonal experimental design is proposed for car body optimization C80E car body is optinized by the two-level optimization methods which are proposed in this paper.Firstly,the position between the main components of car body is optimized to ensure that the overall structure of the car body is reasonable.Secondly,the MPSO-BP method is used to optimize the component size,fUrther reduce the stress concentration,and improve the stress distribution trend.Finally,the results show that a reasonable structural car body is achieved.(4)The fatigue strength evaluation method of car body based on structural stiffness mutation stress gradient is proposed.The method is applied to calculate the fatigue damage of key test points,and compared with the fatigue damage obtained by the traditional method.The results show that the damage which obtained by the mean squared stress smoothing and the stress gradient method based on structural stiffeess mutation is similar to the measured stress spectrum damage.Then Load spectra of Daqin line were calibrated corrected based on damage consistency principle,and the modified load spectrum is used to estimate the key test points' fatigue life of the car body before and after optimization The results show that the fatigue life of the optimized car body meets the design life requirements.(5)The stress intensity factor and fatigue crack growth rate of key areas on the car body were studied by linear elastic fracture mechanics theory.In order to study the distribution law of stress intensity factor on the car body floor,which is in the direction of cross bearer welds.The finite element model of the car body with surface crack is established,and the effect of crack angle and crack depth on the stress intensity factor was studied.Finally,the floor cracks connected with the cross bearer welds were simulated,which is in order to study the effect of different load on the crack propagation rate of the floor.