本論文研究不同外長軸/外短軸長度比的SUS 304不鏽鋼橢方管在不同彎曲方向循環彎曲負載下的響應與損壞，其中以彎矩-曲率和外短軸變化(外短軸變化量/原始外短軸長度)-曲率來呈現響應，而以控制曲率-循環彎曲至斷裂圈數來呈現損壞。本研究探討的橢方管參數如下：不同外長軸/外短軸長度比:1.5、2.0、2.5和3.0，不同彎曲方向（橢方管長軸方向與彎矩方向的夾角）:0°、30°、60°和90°，而橢方管的壁厚皆為0.7 mm。實驗皆採用對稱控制曲率循環彎曲負載，而控制的曲率有：±0.6、±0.65、±0.7、±0.75和±0.8 m⁻¹。
從彎矩-曲率的關係中可以發現，若固定彎曲方向，外長軸/外短軸長度比 = 1.5時，彎矩-曲率的迴圈會很快的穩定，至於其它的比值則需要一些循環圈數後，彎矩-曲率的迴圈才會穩定;若固定外長軸/外短軸長度比時，越大的彎曲方向，會呈現出越大的彎矩-曲率迴圈。從外短軸變化-曲率的關係中可以發現，若固定彎曲方向，而外長軸/外短軸長度比越大時，外短軸變化增加就越快;若固定外長軸/外短軸長度比時，越大的彎曲方向(除了90°外)，會呈現出越小的外短軸變化。從控制曲率-循環彎曲至斷裂圈數的關係中可以發現，當外長軸/外短軸長度比增加時，循環至斷裂的圈數就會增加，而當彎曲方向變大時，循環至斷裂的圈數則會減少。本文根據實驗控制曲率-循環彎曲至斷裂圈數和外長軸/外短軸長度比與彎曲方向之間的關係，推導出理論方程式來描述控制曲率-循環彎曲至斷裂圈數的關係，在與實驗數據比對之後發現，理論和實驗結果相當吻合。

This paper studies the response and failure of SUS 304 stainless steel oval rectangular tubes with different outer major/minor axis length ratios under cyclic bending loads in different bending directions. The response is presented in terms of moment-curvature and short axis variation (variation in the outer minor axis length/original outer minor axis length)-curvature, while the failure is presented in terms of controlled curvature-number of cycles required to initiate fracture. The parameters of the oval rectangular tubes stud-ied are as follows: different outer major/minor axis length ratios: 1.5, 2.0, 2.5, and 3.0; different bending directions (angle between the outer major axis of the oval rectangular tube and the bending moment direction): 0°, 30°, 60°, and 90°; and the wall thickness of the oval rectangular tubes is 0.7 mm. The experiments all adopt symmetrical controlled curvature cyclic bending loads, and the controlled curvatures are: ±0.6, ±0.65, ±0.7, ±0.75, and ±0.8 m⁻¹.
From the moment-curvature relationship, it can be observed that when the bending direc-tion is fixed, for the outer major/minor axis length ratio of 1.5, the moment-curvature loop stabilizes quickly. For other ratios, the moment-curvature loop stabilizes after some cycles. When the outer major/minor axis length ratio is fixed, a larger bending direction results in a larger moment-curvature loop. From the short axis variation-curvature rela-tionship, it can be observed that when the bending direction is fixed, a larger outer ma-jor/minor axis length ratio leads to a faster increase in the short axis variation. When the outer major/minor axis length ratio is fixed, a larger bending direction (except for 90°) results in a smaller short axis variation. From the controlled curvature-number of cycles required to initiate fracture relationship, it can be observed that as the outer major/minor axis length ratio increases, the number of cycles required to initiate fracture increases. Conversely, as the bending direction increases, the number of cycles required to initiate fracture decreases. Based on the experimental controlled curvature-number of cycles re-quired to initiate fracture relationship, this paper derives a theoretical equation to de-scribe the aforementioned relationship. Comparison with experimental data shows that the theoretical and experimental results are in good agreement.

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