本研究先以有限元素分析軟體ANSYS Workbench 15.0來分析不同深度局部切痕6061-T6鋁合金管承受對稱曲度循環彎曲負載下的應力幅度，接著，再根據王建峻[24]、李致頤[25]、吳政昌[26]實驗所獲得疲勞損壞圈數，提出適當疲勞理論並求得相關的疲勞參數。本文所考慮的6061-T6鋁合金管尺寸為:圓形凹痕有三種分別為:外徑= 30mm厚度 = 0.5mm、外徑= 31mm厚度 = 1mm與外徑 = 33mm厚度= 2mm，而圓形凹痕的深度為: 0、0.3、0.6、0.9及1.2 mm。尖銳凹痕為:外徑 = 33mm厚度= 2mm，而尖銳凹痕的深度為: 0、0.3、0.6、0.9及1.2 mm。
由有限元素ANSYS分析結果顯示，當凹痕深度相同時，控制曲度越大時，應力值也就越大。此外，當考慮控制曲度相同時，隨著凹痕深度越大，應力值也就越大。雖然所考慮彎曲負載的曲度為對稱的情況，但循環彎曲的最大應力值和最小應力值有差異，因此分析時必須考慮平均應力的影響。最後，根據有限元素ANSYS所求得的應力幅度，並導入Walker[20]所提出的形式，求得完全反覆的應力幅度後，再與上述實驗所得的循環至損壞圈數繪製在雙對數座標中。接著，利用最小平方法近似出一條直線，則相關的疲勞理論可簡易的建立，且所對應的參數即可透過直線的斜率與截距求得。
關鍵字:有限元素分析、局部凹痕圓管、完全反覆應力

Fatigue Life Prediction of Local Round-dented and Sharp-
dented Circular Tubes under Cyclic Bending
KE, BO-QUAN
PAN,WEN-FUNG
Department of Engineering Science
NationalChengKungUniversity

Summary
In this study, the finite element software ANSYS Workbench 15.0 is used to analyze stress amplitudesfor dented 6061-T6 aluminum tubes with different dent depths under cyclic bending. Next, according to experimental cycles of fatigue from Wang [24], Li [25] and Wu [26], a proper fatigue theory is proposed and related material parameters are determined. The dimensions of 6061-T6 aluminum tubes includethree different round dents of outer diameter =30 mm, thickness =0.5 mm, outer diameter =31mm, thickness =1mm and outer diameter =33 mm, thickness =2 mm. The depths of the rounddent are 0,0.3,0.6,0.9 and 1.2 mm. Sharpdentsarewith outer diameter = 33 mm and thickness =2 mm. The depths of the sharp dent are 0,0.3,0.6,0.9 and 1.2 mm.
According to the analysis from ANSYS, higher controlled curvatures lead to larger stress amplitudes for a constant dent depth. In addition, higher dent depths cause larger stress amplitudes for a constant controlled curvature. Although the controlled curvature is symmetrical under cyclic bending, the amounts of the maximum and minimum stresses are different. Therefore, the fatigue analysis should consider the mean stress effect.
Finally, according to the stress amplitudesobtained from the finite element ANSYS and importing the form proposed by Walker [20], the complete reverse stress amplitudes are determined. Then, the values and experimental number of cycles to produce failure are plotted on the log-log scale. Next, the least-square method is employed to approximate a straight line. Thus, the related fatigue theory can easily to set up and the parameters can be determined from the slope and intercept of the straight line.

Keywords: Finite Element ANSYS Analysis, Local-dented 6061-T6 Aluminum Alloy Tubes, Cyclic Bending,Stress Amplitudes, Fatigue Model.

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