本文使用有限元素分析軟體ANSYS Workbench 16.0進行不同外徑/壁厚比對圓形凹痕圓管在循環彎曲負載下行為影響之理論研究，所分析的圓管材料為6061-T6鋁合金，不同的外徑/壁厚比為：16.5、25.2、31、48.4及60；不同的凹痕深度為：0、0.3、0.6、0.9及1.2 mm，所探討的物理量為：彎矩、橢圓化、曲度之間的相互關係。對於外徑/壁厚比較小的圓管，凹痕深度對彎矩與曲度關係曲線的影響不顯著；而外徑/壁厚比較大且凹痕深度較深的圓管，在控制曲度極值處所對應的彎矩有顯著的不同。此外，由於凹痕兩側上方凸起會相互接觸，所以在第一次最小控制曲度對應的彎矩絕對值較大，之後會逐漸減小並趨於穩定。對於不同外徑/壁厚比的圓管，在無凹痕深度時的橢圓化-曲度關係圖為左右對稱的棘齒狀曲線，而隨著凹痕深度增加而漸呈左高右低的趨勢。在相同凹痕深度下，外徑/壁厚比愈大，在相同控制曲度對應的彎矩愈小而橢圓化愈大；在相同外徑/壁厚比下，凹痕深度愈深，在相同控制曲度對應的橢圓化愈大。此外，由彎矩-曲度關係圖發現，不同外徑/壁厚比的圓管會在不同凹痕深度區間，發生明顯的變化，因此，本文提出理論方程式來合理描述臨界凹痕深度/壁厚比與外徑/壁厚比的關係。

In this thesis, the finite element software ANSYS Workbench 16.0 is used to study the influence of diameter-to-thickness ratios on the behavior of round-dented circular tubes subjected to cyclic bending. The material of the tubes is 6061-T6 aluminum alloy. The diameter-to-thickness ratios are 16.5, 25.2, 31, 48.4 and 60; the round-dented depths are 0, 0.3, 0.6, 0.9 and 1.2 mm. The investigating physical quantities are the relationship among moment, ovalization and curvature. It can be found that the dent depth has no significant influence on moment-curvature relationship for tubes with a smaller diameter-to-thickness ratio. However, the values of the moment at the maximum or minimum control curvature are different for tubes with a higher diameter-to-thickness ratio and deeper dent depth. In addition, due to contact of the dent’s sides, the absolute value of the moment at the minimum control curvature gradually decreases and becomes a stable value. The ovalization-curvature relationship exhibits a left-and-right symmetrical and ratcheting way for tubes without any dent. However, the relationship becomes left-high and right-low trend along with the increase of the dent depth. For a fixed dent depth and controlled curvature, larger diameter-to-thickness ratios lead to small amounts of moment and larger amounts of ovalization; for a fixed diameter-to-thickness ratio and controlled curvature, deeper dent depths lead to larger amounts of ovalization. Moreover, it can be observed from the moment-curvature relationships that the configuration changes at different range of the dent depth for tubes with different diameter-to-thickness ratios. Therefore, a theoretical formulation is proposed in this study to describe the relationship between the critical dent depth/wall thickness ratio and diameter-to-thickness ratio.

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