| Summary: | 碩士 === 國立中山大學 === 材料科學研究所 === 94 === The dislocation structure evolution of polycrystalline copper at constant strain amplitude during low cycle fatigue develops loop patches, vein structure, persistent slip bands, dislocation walls, dislocation cells, and cells with misorientation dislocation step-by-step by increasing fatigue cycles. However, the dislocation structure evolution will change in low cycle fatigue under reduced loading amplitude.
The polycrystalline copper of 99.99 at% purity and 60µm in grain size was used in the low cycle fatigue test. First, the test is controlled at Δε/2= ±0.4%, ±0.2%, and ±0.1% strain amplitude until the specimens crack. And control the fatigue test after 2500 cycles at ±0.4% strain amplitude. Then we can observe the dislocation structure of above specimens by electron microscope.
After 2500 cycles at ±0.4% strain amplitude, change the strain amplitude from ±0.4% to ±0.2%. We chose the steps of low cycle fatigue test under reduced loading amplitude at 1000, 10000, and 30000 cycles. By the same token, change the strain amplitude from ±0.4% to ±0.2%. We chose the steps of low cycle fatigue test under reduced loading amplitude at 1000, and 50000 cycles. Then observe the dislocation structure of above specimens by electron microscope, and we can know the dislocation morphology of evolution process under reduced loading amplitude.
After 2500 cycles at ±0.4% strain amplitude, change the strain amplitude from ±0.4% to ±0.2% and from ±0.4% to ±0.1%. After 1000 cycles, the dislocation wall can be observed at grain boundary. After 10000 cycles under changed loading amplitude from ±0.4% to ±0.2%, we can observe that the dislocation cells are broken and evolve loop patches. And after 50000 cycles under changed loading amplitude from ±0.4% to ±0.1%, large area of dislocation walls and some loop patches can be observed.
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