本論文主要是探討不同外邊長的6063-T5鋁合金方形管在不同彎曲方向對稱控制曲率循環彎曲負載下的響應與失效，其中以彎矩-曲率關係與外邊長變化-曲率關係來呈現響應，而以控制曲率-循環至斷裂圈數關係來呈現失效。本研究不同的外邊長有：20、30、40與50 mm，而不同的彎曲方向有：0º、22.5º和45º，至於方形管的壁厚皆1 mm。
從實驗彎矩-曲率關係中發現，所有方形管皆有循環硬化的現象發生，但最後都會形成一個穩定的彈塑性迴圈。當固定彎曲方向時，在相同的控制曲率情況下，隨著外邊長尺寸的增加，其彎矩值會相對的增加；而當固定外邊長尺寸時，彎曲方向越大彎矩值就越大。從外邊長變化-曲率關係中發現，不論是固定彎曲方向或外邊長尺寸時，隨著循環圈數的增加，該關係都會呈現對稱、棘齒與增加的趨勢。且外邊長尺寸或彎曲方向越大時，外邊長變化就越大。在控制曲率-循環至斷裂圈數關係中發現，當外邊長尺寸越大時，其循環至斷裂的圈數就越小；當彎曲方向越大時，循環至斷裂的圈數就越大。此外，控制曲率-循環至斷裂圈數關係在雙對數座標關係圖中可以發現，在固定彎曲方向時，四種外邊長會對應出四條直線。最後，本文統整出理論方程式來描述不同外邊長的6063-T5鋁合金方形管在不同彎曲方向對稱曲率控制循環彎曲負載下的控制曲率-循環至斷裂圈數關係，並且將理論分析與實驗結果比較後發現，兩者數據非常的接近，表示理論能夠合理的描述實驗結果。

The main focus of this paper is to investigate the response and failure of 6063-T5 aluminum alloy square tubes with different outer side length under symmetric curvature-controlled cyclic bending loads in various bending directions. The response is presented through the moment-curvature relationship and the outer side length variation-curvature relationship, while the failure is represented by the relationship between controlled curvature and the number of cycles required to initiate fracture. The different outer side lengths studied are 20, 30, 40, and 50 mm, with bending directions of 0º, 22.5º, and 45º. The wall thickness of the square tubes is 1 mm.
From the experimental moment-curvature relationships, it was found that all square tubes exhibit cyclic hardening, eventually forming a stable elastic-plastic loop. When the bending direction is fixed, the moment value increases with the outer side length under the same controlled curvature condition. When the outer side length is fixed, the moment value increases as the bending direction increases. The outer side length variation-curvature relationship shows that, regardless of whether the bending direction or outer side length is fixed, the relationship becomes symmetrical, ratcheting, and increases with the number of cycles. Furthermore, the larger the outer side length or bending direction, the greater the outer side length variation. In the controlled curvature-number of cycles required to initiate fracture relationship, it was found that as the outer side length increases, the number of cycles required to initiate fracture decreases. Conversely, as the bending direction increases, the number of cycles required to initiate fracture also increases. Additionally, in the double logarithmic plot of the controlled curvature-number of cycles required to initiate fracture relationship, four straight lines correspond to the four outer side lengths for a fixed bending direction. Finally, the study derives theoretical equations to describe the relationship between controlled curvature and the number of cycles required to initiate fracture for 6063-T5 aluminum alloy square tubes with different outer side lengths under symmetric curvature-controlled cyclic bending loads in different bending directions. A comparison of theoretical analysis and experimental results shows that the data are very close, indicating that the theory can reasonably describe the experimental results.

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