超載負荷及裂縫閉合對鈦合金電子束銲件疲勞裂縫成長之影響

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顧繼珩(Ji-Heng Ku) 查詢紙本館藏

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機械工程學系

論文名稱

超載負荷及裂縫閉合對鈦合金電子束銲件疲勞裂縫成長之影響

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摘要(中)

本研究以Ti-6Al-4V鈦合金為實驗材料，分析超載負荷及裂縫閉合對疲勞裂縫成長之影響，並量測可用以準確評估疲勞裂縫成長壽命之材料參數。藉掃瞄式電子顯微鏡之觀察以瞭解材料的疲勞破壞機構。研究結果顯示，超載負荷造成裂縫成長速率下降，需要多次的週期數才能穿越塑性區並回到原始裂縫成長速率。超載負荷比愈大，裂縫成長延遲現象也愈明顯。在施加超載負荷的期間，裂縫前緣會產生新月形的損傷區域。Ti-6Al-4V鈦合金在Wheeler模式中的延遲形狀參數為3.3。在裂縫閉合方面，應力比或應力強度因子範圍愈力，則裂縫閉合應力愈高。在低應力比時，Ti-6Al-4V的裂縫閉合係數約為0.83，遠高於鋼鐵材料的0.5。亦即Ti-6Al-4V受到裂縫閉合的影響較小，若採用鋼鐵材料的裂縫閉合經驗參數值進行裂縫成長評估，會嚴重高估其疲勞裂縫成長壽命。本研究進一步得到裂縫閉合係數與應力比之關係式。

摘要(英)

In this study, Ti-6Al-4V titanium alloy was used as experimental material to analyze the effects of overload load and crack closure on fatigue crack growth. The fatigue failure mechanism of materials was studied by scanning electron microscope. The results show that overload load reduces fatigue crack growth rate. It takes more load cycles to go through the plastic zone and return to the original crack growth rate. The larger the overload load ratio, the more obvious the crack growth retardation phenomenon. During the loading period, crescent-shaped areas of damage were produced at the crack front. The delay shape parameter of Ti-6Al-4V titanium alloy in Wheeler model is 3.3. In terms of crack closure, the greater the stress ratio or the stress intensity factor, the higher the crack closure stress. At low stress ratio, the crack closure coefficient of Ti-6Al-4V is about 0.83, which is much higher than 0.5 of steels. That is to say, Ti-6Al-4V is less affected by crack closure. If fatigue crack growth evaluation is carried out with the empirical crack closure coefficient of steels, the fatigue crack growth life will be apparently overestimated. Furthermore, the relationship between crack closure coefficient of Ti-6Al-4V and stress ratio was obtained.

關鍵字(中)

★ 鈦合金
★ 電子束銲接
★ 超載負荷
★ 疲勞裂縫成長
★ 裂縫閉合

關鍵字(英)

★ Titanium Alloy
★ Ti-6Al-4V
★ Electron Beam Welding
★ Overload
★ Fatigue Crack Growth
★ Crack Closure

論文目次

中文摘要 i
ABSTRACT ii
誌謝 iii
目錄 iv
圖目錄 vii
表目錄 x
符號說明 xi
第一章、前言 1
1.1 研究背景 1
1.1.1 電子束銲接原理及其特性 3
1.1.2 結構件疲勞設計之重要性及方法 4
1.2 研究動機與目的 8
1.3 文獻回顧 9
1.3.1 鈦合金介紹 9
1.3.2 鈦合金銲接之文獻回顧 10
第二章、理論說明 11
2.1 疲勞裂縫成長理論 11
2.1.1 線彈性破壞力學 (Linear Elastic Fracture Mechanics, LEFM) 11
2.1.2 Griffith脆性破裂理論 14
2.1.3 疲勞裂縫成長曲線 15
2.2 疲勞裂縫成長實驗規範 19
2.3 超載負荷 21
2.3.1 超載負荷理論 22
2.3.2 超載負荷機制 22
2.4疲勞裂縫閉合 23
2.4.1 裂縫閉合理論 23
2.4.2 裂縫閉合機制 24
2.5 預測修正模式 26
2.5.1 Wheeler 模式 26
2.5.2 Willenborg 模式 27
第三章、研究方法與步驟 30
3.1 研究規劃及流程 30
3.2鈦合金材料 30
3.3銲接方法 31
3.4試片加工 32
3.5疲勞裂縫長度量測方法 33
3.6 機械性質測試 35
3.6.1 拉伸性質測試 35
3.6.2 超載負荷疲勞裂縫成長實驗 36
3.6.3 超載負荷量測 37
3.6.4 裂縫閉合實驗 38
3.6.5 裂縫閉合量測 39
3.7破斷面觀察 41
第四章、結果與討論 43
4.1 拉伸性質 43
4.2 疲勞裂縫成長速率 44
4.3 超載負荷之塑性區尺寸分析 47
4.4 超載負荷對疲勞裂縫成長之影響 50
4.5 疲勞裂縫閉合 56
4.6 破斷面分析 67
第五章、結論 75
第六章、未來研究方向 76
參考文獻 77

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指導教授

黃俊仁(Jiun-Ren Hwang)

審核日期

2019-7-29

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