本研究主要針對不同外邊長之鍍鋅鋼方形管在循環彎曲負載下之行為進行實驗研究。本研究選用之方形管截面包含四種不同外邊長，分別為：20、30、40 和 50" mm" 。實驗係以彎管實驗機進行循環彎曲負載的測試，配合油壓伺服控制系統、橢圓化量測器及電腦監控系統，測量並分析鍍鋅鋼方形管在循環彎曲載荷下的彎矩-曲率、外邊長變化（外邊長變化量/原始外邊長）-曲率，以及控制曲率-循環至斷裂圈數之關係。
實驗結果顯示，彎矩-曲率關係呈現出遲滯迴圈的型態，在循環加載過程中表現出循環硬化的現象，隨著循環次數增加，峰值彎矩呈現微幅上升或維持穩定。在外邊長變化-曲率關係方面，呈現出對稱、棘齒與隨循環圈數增加的趨勢，且外邊長越大，外邊長變化的累積速率就越快。至於控制曲率-循環至斷裂圈數關係則呈現，在相同的無因次控制曲率下，外邊長較大時，循環至損壞的圈數就越少。若將上述的關係繪製於雙對數坐標下，則四種不同的外邊長對應出四條直線。最後，本研究針對控制曲率-循環至斷裂圈數關係引入外邊長/壁厚比作為修正因子進行理論分析，在與實驗結果比較後，顯示修正後的理論模型能有效描述不同外邊長鍍鋅鋼方形管的循環至斷裂圈數，實驗結果與理論分析具有良好的一致性。

This study conducted experimental research on the behavior of galvanized steel square tubes with different outer side lengths under cyclic bending loads. The square tube cross-sections selected for this study included four different outer side lengths: 20, 30, 40, and 50 mm. The experiments were performed using a tube bending machine integrated with a hydraulic servo control system, an ovalization measurement device, and a computer monitoring system to measure and analyze the relationships among moment-curvature, outer side length variation (change in outer side length/original outer side length)-curvature, and controlled curvature-number of cycles to fracture for the galvanized steel square tubes under cyclic bending loads.
The experimental results showed that the moment-curvature relationship exhibited hysteresis loops, demonstrating cyclic hardening during the cyclic loading process. As the number of cycles increased, the peak bending moment showed a slight increase or remained stable. Regarding the outer side length variation-curvature relationship, a symmetric, ratcheting, and increasing trend was observed with the increase of cycles; moreover, the larger the outer side length, the faster the accumulation rate of the outer side length variation. As for the relationship between the controlled curvature and the number of cycles to fracture, the results indicated that under the same dimensionless controlled curvature, a larger outer side length resulted in fewer cycles to failure. When these relationships were plotted on a double logarithmic coordinate system, four straight lines corresponding to the four different outer side lengths were observed. Finally, this study conducted a theoretical analysis of the controlled curvature-number of cycles to fracture relationship by introducing the outer side length-to-wall thickness ratio as a correction factor. After comparing with the experimental results, it was shown that the modified theoretical model could effectively describe the number of cycles to fracture for galvanized steel square tubes with different outer side lengths, and the experimental results were in good agreement with the theoretical analysis.

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