Abstract: Based on the method of calculating the equivalent constant amplitude stress of the variable amplitude stress cycle and the von_Mises theory, the fatigue life analysis method of the structure subjected to variable amplitude cycles and under multi-axial stress state is proposed. The weld coordinate system is established according to geometric features, the stress components of nodes are calculated in this system. The node's stress spectrum is obtained according to the road environment percent occurrence spectrum and its equivalent constant amplitude stress is also calculated. According to the joint fatigue resistance, the components of degree of utilization and comprehensive degree of utilization are calculated to evaluate the structural fatigue strength under a specified life. The evaluation of the fatigue strength of typical welds on gondola carbody shows that the fatigue strength of the studied welds is mainly affected by the normal stress component, and the shear stress has little effect on the structural fatigue strength. The comparison of the characteristics of node's degree of utilization under different analysis methods have shown that, when the observation points have significant multi-axial stress characteristics, there are large differences between the results obtained by the AAR standard and the multi-axial stress method. On the basis of the multi-axial stress method, the node's degree of utilization is 18.5% less than the AAR method. The use of multi-axial stress method to evaluate the fatigue strength is beneficial to the lightweight design of the carbody structure.

Key words: fatigue life, multiaxial fatigue criteria, variable amplitude load spectrum, wagon carbody, weld coordinate