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.
Copper has many applications especially in rotor parts subjected to fatigue loading. From them, a typical copper busbar is chosen to mainly investigate the low-cycle fatigue (LCF) behaviors, fracture morphologies, dislocation patterns and crack initiation mechanism. It is found that with total strain amplitude increasing, cyclic softening appears in all the samples; the fatigue crack initiation sites transform mainly from slip bands to grain boundaries; the dislocation spacing of the post-fatigue samples decreases; by comparison, the Basquin-Manson-Coffin and the simplified hysteresis energy relations are relatively simple and accurate methods to predict fatigue life. Finally, the LCF properties of the coppers with different grain sizes prepared by different technologies are compared and optimized.
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