Study On Fatigue Reliability Of Heavy Haul Freight Car Couplings Based On Measured Load Spectrum

With the increase of traction weight and operating times, longitudinal load is increased considerably and its occurrence numbers lead to aggravate the service condition of heavy haul freight car couplings, so that the service life of couplings is reduced, occurrence probability of appearing crack and fracture is increased, which affect the operating security.Secondly, couplings life is affected by uncertain factors of load characteristics, the casting defects, material mechanical properties and so on, which have randomness, if the deterministic method is analyzed, the results aren't conformed with the actual situation. At the same time, at present our country is lack of industry standards, which is suitable for fatigue test on heavy haul freight car couplings, to evaluate the anti-fatigue ability. Therefore, the research on fatigue reliability of couplings based on service conditions and manufacture level is carried out to assess the service life, develop the reasonable maintenance period and realize the cost control of total life, at the same time, the research on fatigue reliability help provide reference for couplings safe standards system under the new operational conditions.The dynamic load of couplings is the direct cause of fatigue failure and service life decrease, which directly affects the fatigue reliability of couplings. Therefore, load characteristics and load spectrum are got according to the coupler load-time history of China's coal transportation lines. Fatigue reliability is studied respectively on the basis of load spectrum, and load spectrum of fatigue test is compiled. The main contents of the thesis are as follows:1. Characteristics of coupler load are analyzed and the load spectrum is complied.According to the structural characteristics of couplings, the coupler load-time history is measured on coal transportation train by making the high precision measuring load coupler. According to the locomotive manipulation record, load characteristics are analyzed on the different operating conditions. The maximum tensile load and the maximum compressive load of 10,000 t and 20,000 t were obtained by using the kernel density estimation method. According to the coupler load characteristics, the method of compiling load spectrum considering the influence of average load is studied by using the rain flow counting method, and the optimal number model suitable for packet of coupler load spectrum is established based on the theory of Miner linear cumulative damage. Finally, the tensile load spectrum and compressive load spectrum for 10,000 km were compiled, and comparative analysis of load spectrum is carried out under different operating conditions, different vehicle location and different hauling weight.2. Fatigue strength reliability of couplings is analyzed based on censoring test specimen. To accurately estimate the coupler fatigue life on weak areas, which are coupler pulling lugs, knuckle pulling lugs and pulling surface, the fatigue test is carried out under load spectrum of 20,000t, and the results of appearing crack and no the crack are as the statistical samples to estimate life based on the censoring specimen using the maximum likelihood method. The finite element models of coupler and knuckle are established, and the relationship of load and stress on the fatigue weak zone is determined using the quasi-static method. According to the Miner linear cumulative damage theory, the relationships of equivalent stress and service mileage are established,so that the fatigue strength and P-S-N curve are got on the fatigue weak zone. y-P-S-N curve of grade E steel is modified by the defect coefficient, and the defect coefficients for fatigue design of couplings are obtained under different reliability.3. The fatigue life reliability and reliability sensitivity of couplings are analyzed by Monte Carlo method based on latin hypercube sampling. Fatigue life reliability model of couplings is established based on the Miner linear cumulative damage theory to determine the random variable parameters that affect the fatigue life of couplings.Couplings fatigue life reliability of 10,000 t and 20,000 t couplings with the service mileage are analyzed. The reliability sensitivity with parameters mean and standard deviation are carried out to determine the influence on the change scope of fatigue life reliability, so that the improved methods and measures are proposed to use to increase the fatigue life reliability.4. Fatigue fracture reliability is analyzed by establishing the fatigue fracture model.The finite element models with 3D crack of coupler and knuckle were established by using the sub-model and the three types stress intensity factors were analyzed by mutual integration method so as to determine the crack propagation form and bearing load. On the basis of above results, the couplings fatigue fracture reliability model suitable for 10,000 t and 20,000 t heavy haul freight car was established. The fatigue fracture reliability with the service mileage and the fatigue fracture reliability sensitivity with parameters mean and standard deviation are analyzed. Finally, the influences of random variable parameters and their change on the fatigue fracture reliability are determined.5. Load spectrum of fatigue test is complied for 10,000 t heavy haul freight car according to the load-time history. Considering the great influence of locomotive operation on coupler load's discreteness, the load spectrum is extrapolated by quantile fractile used the method of rain flow counting and non-parametric statistical, and the amplification factor of load spectrum under different quantile fractile is obtained. In order to improve the efficiency of the test, coupler load is decomposed into the trend load and the small cyclic load. According to the equivalent principle of fatigue damage,the damage caused by the small amplitude load is superimposed on the trend load to increase the trend load to simplify load occurrence number. Finally, fatigue test load spectrum and basic load block suitable for 10,000 t heavy haul freight car couplings are compiled. According to the relationship of the amplification factor of load spectrum and quantile fractile, the modified method of test spectrum is proposed.