本研究針對不同外長軸/外短軸長度比SUS304不鏽鋼橢方管在不同對稱控制曲率之循環彎曲負載下的力學響應和破壞模式進行了深入的探討，其中橢方管有四種外長軸/外短軸長度比分別為:1.5、2.0、2.5、和3.0，而循環彎曲負載則使用五種對稱的控制曲率值分別為: ±0.6、±0.65、±0.7、±0.75與±0.8 m-1。
由實驗過程發現，SUS304不鏽鋼橢方管從開始彎曲至完全斷裂可分成三個階段:起始階段、第二階段與第三階段，而第二階段與第三階段幾乎包含了總循環圈數，又以第三階段佔總循環圈數的70%。由彎矩-控制曲率關係顯示，在不同對稱控制曲率循環彎曲負載下該關係皆會呈現出一個穩定的彈塑性迴圈，而當外長軸/外短軸長度比為1.5，該關係在第一圈即呈現穩定的迴圈，當外長軸/外短軸長度比為2.0、2.5與3.0時，該關係要經過一些循環圈數後才會呈現穩定的迴圈。此外，隨著外長軸/外短軸長度比增加時，彎矩的極值會些許的減少。由外短軸變化-控制曲率關係顯示，在四種不同的外長軸/外短軸長度比，該關係皆呈現了逐步增加的棘齒狀態，且在剛開始的圈數，外短軸變化增加的很快，之後外短軸變化增加變的比較緩慢。當外長軸/外短軸長度比為1.5與3.0時，該關係呈現對稱的狀態，而當外長軸/外短軸長度比為2.0與2.5時，該關係呈現不對稱的狀態。且當外長軸/外短軸長度比越大時，外短軸變化就會越大。至於由控制曲率-循環至斷裂圈數關係顯示，在固定控制曲率的情況下，循環至斷裂圈數會隨著外長軸/外短軸長度比的增加而增加。若以雙對數座標系呈現控制曲率-循環至斷裂圈數關係，則四種外長軸/外短軸長度比會對應出四條不同斜率之直線。最後，本研究將Shaw和Kyriakides於1987年所提出的描述光滑圓管承受循環彎曲負載下的控制曲率-循環至挫曲圈數關係加以修改，以適用於描述SUS304不鏽鋼橢方管的控制曲率-循環至斷裂圈數的關係。最終理論與實驗結果的對比顯示高度一致性，驗證了本研究所提出的經驗公式的可行性。

This study conducted an in-depth investigation into the mechanical responses and failure modes of SUS304 stainless steel oval rectangular tubes with different outer long axis/outer short axis length ratios under cyclic bending loads with varying symmetric control curvatures. The oval rectangular tubes had four different outer long axis/outer short axis length ratios: 1.5, 2.0, 2.5, and 3.0. The cyclic bending loads were applied using five symmetric control curvature values of ±0.6, ±0.65, ±0.7, ±0.75, and ±0.8 m⁻¹.
From the experimental process, it was found that the bending of SUS304 stainless steel oval rectangular tubes from the initial bending to complete fracture can be divided into three stages: the initial stage, the second stage, and the third stage. The second and third stages nearly encompass the total number of cycles, with the third stage accounting for 70% of the total cycle count. The moment-curvature relationship shows that under cyclic bending loads with different symmetric control curvatures, this relationship exhibits a stable elastoplastic loop. When the outer long axis/outer short axis length ratio is 1.5, the relationship forms a stable loop in the first cycle. However, when the outer long axis/outer short axis length ratios are 2.0, 2.5, and 3.0, the relationship requires several cycles before stabilizing into a loop. Additionally, as the outer long axis/outer short axis length ratio increases, the peak moment slightly decreases. The relationship between outer short axis variation and control curvature shows a progressively increasing ratcheting behavior across the four different outer long axis/outer short axis length ratios. At the beginning of the cycles, the outer short axis variation increases rapidly, and then the rate of increase slows down. When the outer long axis/outer short axis length ratios are 1.5 and 3.0, the relationship exhibits a symmetric pattern, whereas, for the ratios of 2.0 and 2.5, the relationship is asymmetric. Additionally, as the outer long axis/outer short axis length ratio increases, the outer short axis variation also becomes larger. As for the relationship between control curvature and the number of cycles required to ignite fracture, it shows that under a fixed control curvature, the number of cycles required to ignite fracture increases with the increase in the outer long axis/outer short axis length ratio. When this relationship is presented on a double logarithmic coordinate system, the four different outer long axis/outer short axis length ratios correspond to four straight lines with different slopes.
Finally, this study modified the relationship between control curvature and number of cycles required to ignite buckling, as proposed by Shaw and Kyriakides in 1987 for smooth circular tubes under cyclic bending loads, to describe the relationship between control curvature and the number of cycles required to ignite fracture for SUS304 stainless steel oval rectangular tubes. The comparison between the theoretical results and the experimental data showed a high degree of consistency, validating the feasibility of the empirical formula proposed in this study.

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