Fatigue failure is one of major problems of the structural components under cyclic loading; however the fatigue test is a time-consuming and high-cost process. Therefore, it is better to establish the relationship between the fatigue property and static mechanical properties of materials. The present studies mainly focus on the relationship between fatigue strength and tensile strength of 316L austenitic stainless steel prepared with four different technologies. The rolling results in severe plastic deformation; dislocation multiplication occurs inside the grains, and the elongated grains are parallel to the rolling direction. With the tensile strength increasing, the optimum fatigue strength reaches at an appropriate tensile strength. The fatigue strength is controlled by the tensile strength and the ability of work hardening, and based on this analysis a new method is established to estimate the fatigue strength. The competitive mechanism between fatigue crack initiation and propagation is also proposed to account for the different shapes of S-N curves.
The tensile properties, high-cycle fatigue properties, fracture surface morphologies, corresponding damage mechanisms, and dislocation patterns of two steels with trace silicon, 550TG and SD320, are investigated. It is found that the SD320 has a higher tensile strength than 550TG, but lower plasticity. In general, some deep cracks appear along the direction of rolling in all the tensile specimens and the fatigue limit of SD320 is higher. In particular, the 550TG shows a continuously decreasing S-N characteristic without fatigue limit at the higher cycle region, which can be explained by their differences of dislocation morphologies. Furthermore, the tensile and fatigue damage mechanisms are deeply analyzed and discussed.
For the steels with trace silicon, the tensile properties, low-cycle fatigue properties, fatigue fracture morphologies, dislocation evolutions, and life predictions of 550TG and SD320, are investigated. It is found that cyclic hardening appears in 550TG and opposite tendency in SD320, respectively; the dislocation density decreases after fatigue experiment for the two materials. The sub-grain boundaries in 550TG and dynamic recovery in SD320 has been discussed. The dislocation planar and wavy slip in 550TG and only planar slip in SD320 can be found. For life prediction, Basquin & Manson-Coffin and hysteretic energy relationships are more appropriate. This study will be useful for the improvement of fatigue property.
Due to the higher reliability needs of the large moving component motor-generator rotor, the assessment of the service life has drawn more and more attention. After finite element analysis of the rotor, the simulation part which can represent the magnetic pole with the most dangerous position of the rotor was designed to investigate the S–N curves. Compared with the conventional specimen, considering the main influencing factors of fatigue life for simulation part, the comprehensive factor was proposed to establish the fatigue life relationship between magnetic pole material and simulation part. It was found that the calculation method of fatigue notch factor based on the notch sensitivity factor is relatively simple and practical, and there is no significant effect of surface roughness on high and low cycle fatigues for low roughness ( R a is about 1 µm), and the dimension factor changes linearly with the scale factor. Based on those results, a fatigue life prediction method was proposed and validated, and the predicted results were in good agreement with the experimental data. This study will provide a reasonable reference to determine the fatigue life prediction of large moving components.
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