Study On Load Spectrum Test And Fatigue Strenth Evaluation Of Railway Freight Car Body

With the development of speed increase and heavy haul technology of railway freight transportation, the vehicle structure fatigue reliability problems have become increasingly prominent. Freight car structure fatigue strength problem is a key factor restricting the promotion of freight car equipment technology, especially car body as bearing part of the goods transport. Fatigue crack damage prematurely occurs at significantly lower than the design life, which will lead to high maintenance cost and seriously endangers safety of freight car. Therefore, it is necessary to comprehensively understand China’s railway freight service load environment and build the most comprehensive freight car body load spectra and fatigue assessment methods, which can make fatigue strength design and assessment on freight car scientifical and reasonable. It can also provide theory support for the reasonable arrangement of the maintenance cycle.At present, AAR standard load spectra are mainly adopted for fatigue strength assessment of freight car body, and the combination of static finite element simulation method, with fatigue damage or equivalent stress assessing fatigue strength of car body. However, a large number of fatigue damage appearing on railway freight car body during the service period in China shows that AAR standard load spectra may not suitable for China’s line conditions and freight car type, and the static finite element simulation method has been unable to meet the requirements of fatigue design and assessment. Therefore, in this paper, under the support of the railway ministry science and technology research and development project:"Railway Wagon C70Type Gondola Car General Line Load Spectrum Test and Application Research", a70-ton general gondola car was as the research object, load spectrum measurement test of car body was carried on the domestic typical general line. Effective car body load spectra were compiled. Fatigue strength assessment method of the car body dynamic load response was studied, and the car body line simulation test bench fatigue assessment method was explored. The main research work are as follows:(1) Small sample specimens were made according to the practical application of parent metal and welded joint of car body. S-N curves of plate and welding joint were obtained through the indoor load test. The mathematical expressions of S-N curves were fitted, which were appropriately extended. Fatigue performance parameters for welded structure design of American AAR standard and European BS EN standard were also analyzed.(2) Load and response signal data of C70E type gondola car body were measured on three typical general railway line. The effective signal time history was obtained by processing the signal data. The signal time history was converted to load time history by using load identification arithmetic method. Load spectra and stress spectra of car body were compiled using the rain-flow count method. It is found that load spectra obeys three parameters Weibull distribution using the method of probability and statistics. The maximum load amplitude was inferred which would arise during the service period of car body. Load spectrum characteristics of three typical lines were compared from two aspects of the maximum value and cumulative frequency per kilometer. The measured maximum spectra with the AAR standard load maximum spectra were compared in the form of a frequency per kilometer.(3) Through the establishment of the whole car body finite element model, static finite element calculation under different load was carried to get the big stress positions on the car body and load-stress transfer relationship. Three dimensional finite element rigid and soft combination model of car body and coal was built. The corrected D-P rule was put forward to describe the constitutive model of granular coal. Surface-surface contact was set between car body and granular coal. Car body static pressure was simulated and results were compared with results of traditional calculation. Three dimensional distribution of granular coal on the body end and side wall displacement, stress and side pressure response were comprehensively analyzed.(4) The modal analysis of car body was carried out to extract the all orders of modal frequencies and mode shapes. Based on this, the structure transient dynamic finite element method was adopted to numerically simulate the dynamic stress response of fatigue key points of car body under dynamic sink-float load and longitudinal load. Dynamic load-stress transfer relationship and big stress positions on car body were calculated under dynamic loading. Comparing the static load-stress transfer coefficient and dynamic load-stress transfer coefficient, it is found that the certain difference exists between the two under different load types, and dynamic load response is larger. On rigid and soft contact combination model of car body and granular coal, structure dynamic strength of car body was simulated under the longitudinal impact by using the direct integral method. The body’s dynamic response was got. The problem of dynamic pressure on side wall considering granular coal mobility in impact process was solved.(5) Based on the measured stress spectra, according to AAR standard evaluation method, BS EN standard evaluation method and the measured S-N curve nominal stress evaluation method, combined with the Miner fatigue cumulative damage rule, the stress spectrum damage of fatigue key points on car body was respectively calculated. According to the measured load spectrum, load spectra damage on car body was calculated by respectively using static load transfer relation and the dynamic load transfer relations. Two calculation results were analyzed in comparison. Load spectra were calibrated corrected by using multipoint optimization algorithm based on damage consistency principle for making it cover real fatigue damage of all the key points, on the basis of which the fatigue life of car body is evaluated. The result shows that car body meets the design life.(6) The fatigue strength assessment method of the indoor car body bench simulation test of car body was studied based on the line the measured load time history. In order to obtain bench test load spectra, the test load spectrum compiling method using stress measuring points as reference point for synchronously concentrating load time history was put forward. According to the damage equivalent principle, the correctness and effectiveness of concentrated load spectrum were verified.