本研究主要針對SUS304不鏽鋼方形管在不同彎曲方向下，探討對稱控制曲率的循環彎曲負載作用下呈現的力學響應與破壞特性。其中方形管不同外邊長的尺寸分別為:20、30、40及50 mm，以及循環彎曲負載所使用的對稱控制曲率分別為:±0.5、±0.55、±0.6、±0.65、±0.7和±0.75 m-1，而不同的彎曲方向分別為0°、22.5°和45°，所有方形管的壁厚皆為1 mm。
根據實驗結果發現，彎矩-曲率的關係顯示，所有方形管在循環彎曲過程最終會形成穩定的彈塑性迴圈。在固定彎曲方向的條件下，隨著外邊長增加，彎矩值亦隨之上升；相反地，當外邊長固定時，彎曲方向越大，所需的彎矩值也越高。此外，從外邊長變化-曲率的關係可觀察到，無論任何彎曲方向或外邊長尺寸，隨著循環次數增加，此關係均呈現出對稱、棘齒狀增長的趨勢，且外邊長或彎曲方向越大時，外邊長變化幅度亦隨之增加。
在控制曲率與循環至斷裂圈數的關係方面，實驗發現，在相同的控制曲率，外邊長較大時，循環至損壞圈數就越少。進一步將上述關係繪製於雙對數座標圖中，可發現不同外邊長的方形管各自對應形成四條線性關係。最後，本研究建立了一套理論模型，以描述不同外邊長的SUS304不鏽鋼方形管在不同彎曲方向下，於對稱控制曲率循環彎曲負載條件下的控制曲率與循環至斷裂圈數的關係。理論分析與實驗數據比較後發現，兩者相當契合，證實該理論可有效描述實驗結果。

This study focuses on investigating the mechanical response and failure characteristics of SUS304 stainless steel square tubes under cyclic bending with symmetric curvature control in different bending directions. The square tubes have outer side lengths of 20, 30, 40, and 50 mm, respectively. The symmetric curvature values used for cyclic bending are ±0.5, ±0.55, ±0.6, ±0.65, ±0.7, and ±0.75 m⁻¹. The bending directions examined are 0°, 22.5°, and 45°, and all square tubes have a wall thickness of 1 mm.
According to the experimental results, the moment-curvature relationship indicates that all square tubes eventually form stable elastoplastic hysteresis loops during the cyclic bending process. Under a fixed bending direction, the bending moment increases with the outer side length. Conversely, when the outer side length is fixed, a larger bending direction requires a higher bending moment. Furthermore, from the relationship between outer side length variation and curvature, it is observed that regardless of the bending direction or outer side length, this relationship exhibits a symmetric, sawtooth-like growth trend as the number of cycles increases. Additionally, the magnitude of side length variation increases with either a larger outer side length or a greater bending direction.
Regarding the relationship between controlled curvature and the number of cycles to failure, experimental results show that under the same controlled curvature, square tubes with larger outer side lengths require fewer cycles to reach failure. When this relationship is plotted on a log-log scale, four distinct linear trends emerge, each corresponding to a different outer side length. Finally, this study establishes a theoretical model to describe the relationship between controlled curvature and the number of cycles to failure for SUS304 stainless steel square tubes with different outer side lengths and bending directions under symmetric cyclic bending conditions. A comparison between the theoretical predictions and experimental data shows good agreement, confirming that the model effectively captures the experimental behavior.

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