本研究主要針對不同外長軸/外短軸長度比SUS304不鏽鋼橢圓管承受循環彎曲負載下的行為做研究。主要關注的行為有:外短軸變化-循環圈數的關係與臨界外短軸變化-控制曲率的關係，其中外短軸變化定義為外短軸長度變化量/初始外短軸長度，而本研究使用了四種不同外長軸/外短軸長度比分別為：1.5、2.0、2.5與3.0。
依據實驗結果，本研究所觀察到的外短軸變化-循環圈數的關係可分為四個階段: 初始、第二、第三與第四階段，其中初始階段與第二階段的外短軸變化呈現穩定成長，從第二階段結束後進入第三階段，橢圓管開始出現裂痕，外短軸變化量會急速上升，而在第三階段結束後進入第四階段，外短軸變化量趨於平緩，但很快的橢圓管就完全斷裂，依據此現象，本研究將聚焦於前兩階段的探討。
由實驗外短軸變化-循環圈數的關係可得知，在固定控制曲率下，橢圓管的外長軸/外短軸長度比越大時，其外短軸變化成長就越快，到達斷裂的循環圈數就越少；而在相同的外長軸/外短軸長度比下，控制曲率越大時，則外短軸變化成長就越快，到達斷裂的循環圈數就越少。至於在實驗臨界外短軸變化-控制曲率的關係可得知，當外徑長/短軸比越大，臨界外短軸變化就越大。最後，本文提出可描述外短軸變化-循環圈數關係與臨界外短軸變化-控制曲率的相關理論公式，並將理論數據與實驗數據比對後發現，本研究所提出的理論公式能夠合理描述實驗結果。

This study focuses on the behavior of SUS304 stainless steel elliptical tubes with different outer long axis/outer short axis length ratios under cyclic bending loads.The main behaviors of interest are the relationship between the outer short axis variation and the number of cycles, and the relationship between the critical outer short axis variation and the controlled curvature. The outer short axis variation is defined as the change in the outer short axis length divided by the original outer short axis length. This study uses four different outer long axis/outer short axis length ratios: 1.5, 2.0, 2.5, and 3.0.
Based on the experimental results, the relationship between the outer short axis variation and the number of cycles can be divided into four stages: initial, second, third, and fourth stages. In the initial and second stages, the outer short axis variation exhibits stable growth. After the second stage ends, the elliptical tube enters the third stage, where cracks begin to appear, and the outer short axis variation increases rapidly. After the third stage ends, the outer short axis iii variation levels off in the fourth stage, but the elliptical tube soon completely fractures. Based on this phenomenon, this study focuses on the first two stages.
From the experimental relationship between the outer short axis variation and the number of cycles, it can be observed that at a fixed controlled curvature, the larger the outer long axis/outer short axis length ratio of the elliptical tube, the faster the outer short axis variation grows, and the fewer cycles it takes to reach fracture. At the same outer long axis/outer short axis length ratio, the larger the controlled curvature, the faster the outer short axis variation grows,and the fewer cycles it takes to reach fracture. Regarding the experimental relationship between the critical outer short axis variation and the controlled curvature, it can be observed that the larger the outer diameter long/short axis ratio, the larger the critical outer short axis variation.
Finally, this paper proposes theoretical formulas that can describe the relationship between the outer short axis variation and the number of cycles, as well as the critical outer short axis variation and the controlled curvature. By comparing the theoretical data with the experimental data, it is found that the theoretical formulas proposed in this study can reasonably describe the experimental results.

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