本論文研究不同外長軸/外短軸長度比的6063-T5鋁合金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。本研究所探討的橢方管參數如下：不同外長軸/外短軸長度比有: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)有:0°、30°、60°和90°，而橢方管的壁厚皆為1.5 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：±0.5、±0.55、±0.6、±0.65和±0.7 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 與2.0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，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，當固定控制曲率與彎曲方向，外長軸/外短軸長度比增加時，循環至斷裂的圈數就會減少;而當固定控制曲率與外長軸/外短軸長度比，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，並經由理論分析與實驗結果相互比較後發現，兩者的數據非常接近，這表示本文提出的理論模型能夠合理的描述實驗結果。

This paper investigates the response and failure of 6063-T5 aluminum alloy oval rectangular tubes with different outer long axis/outer short axis length ratios under cyclic bending loads in different bending directions. The responses are presented using moment-curvature and outer minor axis variation (change in outer minor axis length/original outer minor axis length)-curvature relationships, while the failure is presented using the control curvature-number of cycles required to initiate fracture relationship. The parameters of the oval rectangular tubes studied are as follows: different outer long axis/outer short axis length ratios of 1.5, 2.0, 2.5, and 3.0; different bending directions (the angle between the major axis direction of the oval rectangular tube and the moment direction) of 0º, 30º, 60º, and 90º; and a constant wall thickness of 1.5 mm. The experiments all used symmetric control curvature cyclic bending loads, with control curvatures of ±0.5, ±0.55, ±0.6, ±0.65, and ±0.7 m⁻¹.
From the moment-curvature relationship, it was found that for a fixed bending direction, when the outer long axis/outer short axis length ratios are 1.5 and 2.0, the moment-curvature loops stabilize quickly; for the other ratios, the moment-curvature loops stabilize after a few cycles. When the outer long axis/outer short axis length ratio is fixed, larger bending directions result in larger moment-curvature loops. From the outer minor axis variation-curvature relationship, it was observed that for a fixed bending direction, the larger the outer long axis/outer short axis length ratio, the faster the outer minor axis variation increases; for a fixed outer long axis/outer short axis length ratio, larger bending directions result in smaller outer minor axis variations. From the control curvature-number of cycles required to initiate fracture relationship, it was found that when control curvature and bending direction are fixed, increasing the outer long axis/outer short axis length ratio reduces the number of cycles required to initiate fracture; when control curvature and outer long axis/outer short axis length ratio are fixed, increasing the bending direction reduces the number of cycles required to initiate fracture. Based on the experimental relationship between control curvature-number of cycles required to initiate fracture and the outer long axis/outer short axis length ratio with bending direction, theoretical equations were derived to describe the control curvature-number of cycles required to initiate fracture relationship. By comparing the theoretical analysis with the experimental results, it was found that the data are very close, indicating that the theoretical model proposed in this paper can reasonably describe the experimental results.

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