本研究旨在探討不同初始外半徑之 SUS304 不鏽鋼直角扇形管，在對稱循環彎曲負載下之外半徑變化、臨界外半徑變化及破壞行為。試件之初始外半徑R0/t分別為 20、30、40 與 50 ，管壁厚度均為 1 mm，並於六種對稱控制曲率±0.50、±0.55、±0.60、±0.65、±0.70及±0.75 m-1下進行循環彎曲試驗，所有試驗均持續至試件完全斷裂為止。
依據試件循環彎曲過程中的外觀變化，可將整體破壞歷程區分為三個階段。第一階段為試驗開始至試件首次出現局部塌陷；第二階段為局部塌陷後至首次觀察到裂紋或孔洞；第三階段則為裂紋形成後至試件完全斷裂。各階段之外半徑變化結果顯示，第一階段的外半徑變化上升較快，第二階段仍持續增加但變化較為緩慢且穩定，進入第三階段後，外半徑變化強烈快速上升，而既有裂紋則持續擴展並最終造成試件斷裂。
實驗結果顯示，外半徑相對變化量會隨循環圈數增加而逐漸累積。在初始外半徑固定的條件下，控制曲率愈大，第一階段與第二階段所累積之外半徑相對變化量愈高。此外，本研究將第二階段結束前最後一個穩定外半徑變化點定義為臨界外半徑變化量。結果顯示，在相同初始外半徑下，臨界外半徑變化量整體上會隨控制曲率增加而提高；在相同控制曲率下，初始外半徑較大之試件通常具有較高的臨界外半徑相對變化量。
最後，本研究利用四種初始外半徑試件於六種控制曲率下之第一階段與第二階段端點，建立外半徑變化量之經驗關係式，另以對數形式建立臨界外半徑變化量與控制曲率之經驗關係。比較結果顯示，所建立之經驗關係式可合理描述本研究試驗範圍內，直角扇形管於第一階段與第二階段之外半徑變化累積趨勢，以及進入第三階段前之臨界變形特性。

This study investigates the changes in outer radius, critical changes in outer radius, and failure behavior of SUS304 stainless-steel right-angle sector tubes with different initial outer radii subjected to symmetric cyclic bending. The initial outer-radius-to-thickness ratios, (R0/t), of the specimens were 20, 30, 40, and 50, while the wall thickness was fixed at 1 mm. Cyclic bending tests were conducted under six symmetric controlled curvatures of ±0.50, ±0.55, ±0.60, ±0.65, ±0.70,and ±0.75 m-1. All tests were continued until the specimens fractured completely.
Based on the changes in the external appearance of the specimens during cyclic bending, the overall failure process was divided into three stages. Stage I extended from the beginning of the test to the first occurrence of local collapse. Stage II extended from local collapse to the first observation of a crack or hole. Stage III extended from crack initiation to complete fracture of the specimen. The results showed that the change in outer radius increased relatively rapidly during Stage I. During Stage II, the change continued to increase but at a slower and more stable rate. After entering Stage III, the change in outer radius increased sharply and rapidly, while the existing cracks continued to propagate and ultimately caused complete fracture of the specimen.
The experimental results indicated that the relative change in outer radius accumulated progressively with an increasing number of bending cycles. For specimens with the same initial outer radius, a higher controlled curvature resulted in a greater accumulated relative change in outer radius during Stages I and II. In addition, the last stable outer-radius change measured before the end of Stage II was defined as the critical change in outer radius. For specimens with the same initial outer radius, the critical relative change in outer radius generally increased with increasing controlled curvature. Under the same controlled curvature, specimens with a larger initial outer radius generally exhibited a greater critical relative change in outer radius.
Finally, empirical relationships for the change in outer radius were established using the endpoints of Stages I and II obtained from specimens with four initial outer radii under six controlled curvatures. A logarithmic empirical relationship between the critical change in outer radius and the controlled curvature was also developed. The comparison results demonstrated that the proposed empirical relationships reasonably describe the accumulation trends of the change in outer radius during Stages I and II, as well as the critical deformation characteristics of right-angle sector tubes before entering Stage III, within the experimental range investigated in this study.

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