臺灣博碩士論文加值系統

當結構經歷了由負載造成的週期性應力，結構會趨向失效，眾所皆知材料並非完美無瑕，裂縫可能由於製造錯誤、冶金不連續性和疲勞而存在。裂縫的存在會影響材料的強度，並導致高應力集中在裂縫尖端附近，使結構失效。本研究探討彈性材料中不同的裂縫長寬比對半橢圓裂縫成長及壽命週期的影響。
平板的裂縫成長分析使用有限元素法，所有的分析模型使用7075-T6鋁合金材料，根據線彈性破壞力學來考慮應力強度因子及能量釋放率。本文考慮常數長寬比及改變長寬比之模型的半橢圓裂縫成長，裂縫深度在每一次的計算步驟中增加相同長度，平面裂縫長度則依據裂縫長寬比。探討彈性材料在常數裂縫成長率下，應力強度因子、能量釋放率及疲勞負載週期的行為。為了證明所用之裂縫成長模型的準確性，將所得之結果與現有文獻結果進行比較。對於常數裂縫長寬比模型，結果表示當裂縫長寬比從0.4增加到1時，裂縫於垂直自由表面位置的應力強度因子呈現降低，自由表面的應力強度因子則會增加，而造成應力強度因子的變化是由於三維效應靠近裂縫尖端。在這裡注意到當裂縫長寬比增加，因為裂縫靈敏度降低，所以負載週期呈現增加。疲勞裂縫成長分析也表示當長寬比從0.4改變到1時，應力強度因子隨著平板厚度增加變得不太敏感。在不同的初始裂縫長寬比下，進行改變長寬比的半橢圓裂縫成長計算。對於初始裂縫長寬比0.4，裂縫成長後最終裂縫長寬比接近0.5。對於初始裂縫長寬比0.6，裂縫成長後最終裂縫長寬比接近0.57。對於初始裂縫長寬比0.8，裂縫成長後最終裂縫長寬比接近0.63。對於初始裂縫長寬比1，裂縫成長後最終裂縫長寬比接近0.68。
薄管的裂縫成長問題使用有限元素法，所有的分析模型使用SA333Gr.6碳鋼，根據線彈性破壞力學來考慮應力強度因子及能量釋放率。在碳鋼管壁上考慮內側軸向、內側圓周向、外側軸向及外側圓周向的半橢圓裂縫。對於不同的裂縫長寬比，探討管路裂縫在裂縫位置及尺寸的影響。
軸向裂縫的應力強度因子三次大於圓周向裂縫的應力強度因子。當軸向裂縫及圓周向裂縫的長寬比增加，疲勞壽命週期趨向增加。管路厚度對疲勞壽命週期有重大的影響。在管路中軸向裂縫的位置較圓周向裂縫危險，內部裂縫不管任意方向都較外部更為危險。

The structures tend to fail when they undergo periodic stresses due to loading. It is also known that material is never flawless, they may exist due to manufacturing errors, metallurgical discontinuity and due to fatigue. The material strength will be compromised due to the presence of cracks and this results in high stress concentration near the tip of the crack leading to failure of the structure. This study deals with the semi-elliptical crack propagation and Life cycles in elastic material for various crack aspect ratio.
The Finite element method is used to analyze the crack growth in a plate. All the analyzed models are made of Al 7075-T6. Stress intensity factor and energy release rate are considered in Linear Elastic Fracture Mechanics (LEFM). Semi-elliptical crack growth for constant aspect ratio and changing aspect ratio models are considered in this report. The crack depth length will have the same increment for every computational step and the surface crack length depends on the crack aspect ratio. The behavior of Stress intensity factor (SIF), Energy release rate and Fatigue load cycles were investigated for elastic material under constant crack growth rate. In order to prove the affirmation of the present crack growth model the obtained results were compared with existed literature results.
For constant crack aspect ratio model, the results show that as the crack aspect ratios increase from 0.4 to 1 the stress intensity factor at deep point appear to decrease and increase at the surface direction and this variation in SIF is due to surface effect near the crack-tip. It is noticed that as the crack aspect ratio increase the number of loading cycles appears to increase because the sensitivity of the crack decreases. The fatigue crack growth analysis also shows that as the aspect ratio changes from 0.4 to 1 the SIF becomes less sensitive to increase in plate thickness. Calculation of semi-elliptical crack growth for changing aspect ratio is carried out for different initial crack aspect ratios. For initial crack aspect ratio 0.4, the accumulated final crack aspect ratio is simulated to be 0.50. For initial crack aspect ratio 0.6, the accumulated final crack aspect ratio is simulated to be 0.57. For initial crack aspect ratio 0.8, the accumulated final crack aspect ratio is simulated to be 0.63. For initial crack aspect ratio 1, the accumulated final crack aspect ratio is simulated to be 0.68.
Finite element method is used to investigate the crack propagation in a thin-pipe. All the analyzed models are made of SA333Gr.6 carbon steel. Stress intensity factor is considered in Linear Elastic Fracture Mechanics (LEFM). Semi-elliptical cracks are considered for Internal Axial (IA), Internal Circumferential (IC), External Axial (EA) and External Circumferential (EC) in the wall of carbon steel pipe. The effect of crack location and size of the crack on the pipe is carried out for different crack aspect ratios.
The stress intensity factor for Axial cracks is 3 times larger than the Circumferential cracks. Fatigue life cycle tends to increase for axial cracks and circumferential cracks as the aspect ratio increase. The pipe thickness will have a significant effect on fatigue life cycles. The position of axial cracks on the pipe is more critical than the circumferential cracks and internal cracks irrespective of crack orientation are more critical.

English Abstract…………………………………………………………i
Chinese Abstract……………………………………………………iv
Acknowledgement ………………………………………………………vi
Table of Contents……………………………………………………………vii
List of Tables…………………………………………………………………ix
List of Figures…………………………………………………………………x
Symbols……………………………………………………………………… xiv
Chapter 1 Introduction………………………………………………………1
Chapter 2 Literature review…………………………………………………4
2.1 Background………………………………………………4
2.2 Theoretical Background………………………………………7
2.2.1 Stress Intensity Factor………………………………7
2.2.2 Energy Release Rate…………………………………………10
2.2.3 J-Integral……………………………………………………11
2.2.4 Fatigue Crack Growth………………………………………13
2.2.5 Elastic-Plastic Fracture Mechanics……………………………14
Chapter 3 Methodology & Verification…………………………………17
3.1 Numerical Procedure for Crack Growth Simulation………17
3.2 Affirmation of Results from Literature………………………20
Chapter 4 Results & Discussion……………………………………………23
4.1 Introduction…………………………………………………23
4.2 FEA results on Rectangular Plate……………………………23
4.3 FEA results on Thin-Pipes……………………………………29
Chapter 5 Conclusion………………………………………………………34
References…………………………………………………………………68
Extended Abstract……………………………………………………………72

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