本研究針對不同外長軸/外短軸長度比（1.5、2.0、2.5、3.0）之6063-T5鋁合金橢圓方管，進行對稱控制曲率下的循環彎曲實驗，以探討其外短軸的變化與臨界外短軸變化之趨勢。根據外短軸變化與循環圈數的關係曲線，大致可將其變化行為區分為三個階段：
初始階段: 外短軸的變化迅速增大;第二階段: 外短軸的變化速率趨緩，呈穩定增長;第三階段：外短軸變化趨於穩定，不再顯著增加，最終導致試件破壞。從實驗結果可觀察到，控制曲率愈大時，其對應之臨界外短軸變化亦隨之增大；此外，外長軸與外短軸的長度比越大時，臨界外短軸變化亦呈增大趨勢。
本研究進一步導入Lee等人針對SUS304不鏽鋼圓管所提出之橢圓化經驗模型，並修正其模型以擬合本研究中不同外長軸/短軸比之6063-T5鋁合金橢圓方管試件的行為。透過最小平方法進行非線性回歸分析，獲得不同長短軸比下的擬合參數，以描述其在循環彎曲負載下，第一與第二階段中短軸變化與循環圈數的關係。
此外，研究亦發現臨界外短軸變化與控制曲率之對數呈現線性關係，進而提出相對應之預測方程式，用以描述不同幾何參數下的6063-T5鋁合金橢圓方管在循環彎曲下之臨界變化行為。最後，理論預測與實驗結果對比顯示高度一致性，驗證了本研究所提出經驗模型與參數設定之可行性與準確性。

This study investigates the mechanical behavior of 6063-T5 aluminum alloy oval square tubes with four different outer major-to-minor axis ratios (1.5, 2.0, 2.5, and 3.0) subjected to cyclic bending under symmetric curvature control. The variation of the outer minor axis and the critical condition leading to structural instability were systematically examined. Experimental observations revealed that the evolution of outer minor axis variation can be categorized into three distinct stages: (1) a rapid increase during the initial stage, (2) a gradual and stable growth during the second stage, and (3) a saturation stage where further increase was negligible before final failure. Results showed that higher controlled curvature values corresponded to greater critical outer minor axis variation, while larger axis ratios also led to increased critical variation.
Furthermore, a modified version of the empirical ovalization model originally proposed by Lee et al. for SUS304 stainless steel circular tubes was applied. Nonlinear regression using the least squares method provided fitting parameters describing the relationship between minor axis variation and the number of cycles during the first and second stages. In addition, a logarithmic linear correlation between the critical minor axis variation and the controlled curvature was established, leading to a predictive formula capable of estimating the critical deformation behavior under varying geometric parameters. The high consistency between theoretical predictions and experimental data validates the reliability and accuracy of the proposed empirical model and its parameterization.

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