本文運用彎管試驗機針對不同圓孔方向及不同圓孔直徑的6061-T6鋁合金圓孔管進行純彎曲鬆弛負載的實驗，藉以探討相關的彎曲力矩、橢圓化與時間的相互關係。所謂圓孔管的純彎曲鬆弛負載是將圓孔管施加一穩定的純彎曲力矩，使圓孔管達到想要控制的曲率後，維持該曲率一段時間。實驗的圓孔管共五種不同的圓孔方向分別為0°、45°、90°、135°與180°，而每個圓孔方向分別各有五種不同的直徑大小分別為2、4、6、8與10 mm，對於每一組的圓孔管，本研究將以四種不同的控制曲率進行純彎曲鬆弛負載的實驗。實驗結果顯示，圓孔管一旦進入純彎曲鬆弛負載時，彎曲力矩會隨時間急遽的降低，之後彎矩隨時間的變化量明顯的降低且逐漸減少，而橢圓化程度則隨時間些許的增加後，便逐漸趨近於一個穩定的量。由於橢圓化程度呈現一個穩定的量，所以圓孔管不會發生破裂失效。最後，本文提出相關的理論公式來描述鬆弛彎矩與時間的關係，並與實驗結果相互比較後發現，理論公式可以合理的描述實驗結果。

This paper presents an experiment and analysis to examine the response of round-hole 6061-T6 aluminum alloy tubes with different hole directions of 0°, 45°, 90°, 135° and 180°, and different hole diameters of 2, 4, 6, 8 and 10 mm subjected to pure bending relaxation. Pure bending relaxation involves bending the tube to the required curvature and maintaining the curvature for a period of time. Based on the pure bending relaxation experimental results, the bending moment decreases rapidly with time, where subsequently, amount of bending moment change over time drops significantly and gradually decreases. However, the ovalization increases slightly over time and gradually becomes a stable value. In addition, higher holding curvature or hole diameter results in higher relaxation moment and ovalization. Due to the stable ovalization value, the round-hole tube does not break. Finally, the formulas proposed by Lee et al. (2014) were modified to simulate the relaxation moment-time for pure bending relaxation. A comparison of the simulation results with the experimental results indicated that a theoretical analysis can reasonably be used to describe the experimental results.

[1]P. K. Shaw and S. Kyriakides, “Inelastic Analysis of Thin-Walled Tubes under Cyclic Bending,” International Journal of Solids and Structures, Vol. 21, No. 11, pp. 1073-1100, 1985.
[2]S. Kyriakides and P. K. Shaw, “Inelastic Buckling of Tubes under Cyclic Bending,” ASME Journal of Pressure Vessel Technology, Vol. 109, No. 2, pp. 169-178, 1987.
[3]E. Corona and S. Kyriakides, “An Experimental Investigation of the Degradation and Buckling of Circular Tubes under Cyclic Bending and External Pressure,” Thin-Walled Structures, Vol. 12, No. 3, pp. 229-263, 1991.
[4]S. Kyriakides and G. T. Ju, “Bifurcation and Localization Instabilities in Cylindrical Shells under Bending-I. Experiments,” International Journal of Solids and Structures, Vol. 29, No. 9, pp. 1117-1142, 1992.
[5]D. Kujawski and E. Kremp1, “The Rate (Time)-Dependent Behavior of Ti-7A1-2Cb-1Ta Titanium Alloy at Room Temperature under Quasistatic Monotonic and Cyclic Loading,” Journal of Applied Mechanics, Vol. 48, No. 1, pp. 55-63, 1981.
[6]L. Guo, S. Yang and H. Jiao, “Behavior of Thin-Walled Circular Hollow Section Tubes Subjected to Bending,” Thin-Walled Structures, Vol. 73, pp. 281-289, 2013.
[7]J. Lara-Bocanegra, A. Majano-Majano, F. Arriaga and M. Guaita, “Long-Term Bending Stress Relaxation in Timber Laths for the Structural Design of Lattice Shells,” Construction and Building Materials, Vol. 193, pp. 565-575, 2018.
[8]W. F. Pan, T. R. Wang and C. M. Hsu, “A Curvature-Ovalization Measurement Apparatus for Circular Tubes under Cyclic Bending,” Experiment Mechanics, Vol. 38, No. 2, pp. 99-102, 1998.
[9]W. F. Pan and C. H. Fan, “An Experimental Study on the Effect of Curvature-Rate at Preloading Stage on Subsequent Creep or Relaxation of Thin-Walled Tubes under Pure Bending,” JSME International Journal Series A, Vol. 41, No. 4, pp. 525-531, 1998.
[10]K. L. Lee and W. F. Pan, “Pure Bending Creep of SUS304 Stainless Steel Tubes,” Steel and Composite Structures, Vol. 2, No. 6, pp. 461-474, 2002.
[11]K. L. Lee, C. M. Hsu and W. F. Pan, “Response of Sharp-Notched Circular Tubes under Bending Creep and Relaxation,” Mechanical Engineering Journal, Vol. 1, No. 2, pp. 1-14, 2014.
[12]C. C. Chung, K. L. Lee and W. F. Pan, “Collapse of Sharp-Notched 6061-T6 Aluminum Alloy Tubes under Cyclic Bending,” International Journal of Structural Stability and Dynamics, Vol. 16, No. 7, 2016.
[13]K. L. Lee, K. H. Chang and W. F. Pan, “Failure Life Estimation of Sharp-Notched Circular Tubes with Different Notch Depths under Cyclic Bending,” Structural Engineering and Mechanics, Vol. 60, No. 3, pp. 365-386, 2016.
[14]K. L. Lee, L. C. Chin and W. F. Pan, “Elastoplastic Response and Failure of Round-Hole Tubes under Cyclic Bending,” Informatica, Vol. 31, No. 4, pp. 23-41, 2020.
[15]翁慕理，”純彎曲鬆弛負載下圓孔管之行為”，工程科學系，國立成功大學，2020。
[16]ASTM E328-13, Standard Test Methods for Stress Relaxation Tests for Materials and Structures.