| Summary: | 碩士 === 國立中正大學 === 機械工程所 === 97 === Effects of tool size on fracture and fatigue behaviors of friction stir spot welds (FSSW) in cross-tensile specimens of 6061-T6 aluminum sheets were investigated based on experimental observations. Welds made by three tools with different sizes, T1, T2 and T3, at different rotational speed, dwelling time and indentation depth were tested under quasi-static opending conditions. The nugget pullout, interfacial and mixed type failure modes can be observed. The experimental results indicate that the failure strengths and failure modes of “good” welds made under different processing conditions show significant dependence on the tool size. Then, welds made by the three tools at the optimum processing parameters of T3 tool were tested under cyclic opening conditions. Under cyclic loading conditions, the fatigue lives and failure modes of the welds strongly depend on the tool size and applied load amplitude. For T1 tool, the mixed mode failure mode can be found. For T2 tool, when the welds are subjected to high and low cycle fatigue loading conditions, the upper-sheet and lower-sheet nugget pullout failure modes can be found, respectively. For T3 tool, except the two failure modes mentioned above, another transient failure mode can be found between them. Finally, optical and scanning electron micrographs of the welds made by the three tools before and after failure were examined and micro indentation tests of these welds were conducted.
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