论文总字数：23160字

摘 要

工业纯钛TA2是压力容器常用的材料之一，凭借着其密度小、比强度大、耐腐蚀性和断裂韧性强等特点，也广泛运用于航空航天、核电、医疗等领域。钛在服役过程中难免会产生疲劳失效。在实际工程中，承压结构往往承受来自各个方向的载荷工况，为保证钛制设备在使用过程中的安全性，本文通过试验和理论分析的方法研究加载角度和负载荷比对工业纯钛TA2疲劳裂纹扩展的影响。

本文选用改进的CTS试样对工业纯钛I-II复合型疲劳裂纹扩展试验进行研究。从裂纹启裂角、偏离角、扩展角的变化对材料的疲劳寿命、疲劳裂纹扩展速率、疲劳裂纹扩展路径深入分析，并揭示了应力强度因子K_I、K_II在不同载荷状态下对I-II复合型裂纹扩展的作用。最后，采用柔度偏离法测量出裂纹张开力，讨论了负载荷比下的裂纹闭合水平。

研究发现加载角度的增加和负载荷比的减小都加快裂纹扩展的速率，导致疲劳裂纹扩展寿命减少。负载荷比对裂纹路径基本没有影响，而加载角对裂纹扩展路径起着决定性作用，加载角度越大，裂纹扩展路径越陡峭。预制裂纹承受具有一定加载角度的载荷时会立刻发生偏转，产生一个启裂角。通过对MTS、S、G准则进行分析比较发现三种准则预测的启裂角基本与试验吻合，MTS准则的精度最高。随着加载角度的增加裂纹的偏离角也会加大，但偏离角的大小不会超过加载角，说明K_I、K_II相互竞争，加载角越大K_II的作用就会越大，裂纹越来越偏离I型而向型扩展。通过比较不同载荷状态下的闭合系数可知载荷比的增加和加载角度的增加都会导致裂纹闭合水平减小。

关键词：工业纯钛；I-II复合型裂纹；疲劳裂纹扩展；负载荷比

Abstract

Commercial pure titanium TA2 is one of the most commonly used materials for pressure vessels.Fatigue failure of titanium is inevitable in service.Pressurized structures are generally subjected to multi-axis load conditions during actual engineering. In order to ensure the safety of titanium equipment , the effect of loading angle and load ratio on fatigue crack growth of commercial pure titanium TA2 was studied by means of experiment and theoretical analysis.

The mixed mode I-II fatigue crack growth behavior of commercial pure titanium was experimentally studied through improved CTS specimens in this paper. Fatigue crack growth rate and path of the material were deeply analyzed from the changes of crack initiation angle, deflection Angle and growth angle. The effect of stress intensity factor K_I and K_II on mixed-mode fatigue crack growth under different loading states was revealed. At the same time, the crack opening force was measured by the flexibility deviation method, and the crack closure level under the load ratio was discussed.

It is found that the increase of loading angle and decrease of negative load ratio both accelerate the crack growth rate and lead to drop of fatigue crack propagation life. The load ratio has little effect on the crack path, but the loading angle plays a significant role in the crack growth path. The crack growth path becomes steeper as the loading angle increases. When the pre-crack crack is subjected to a load with a certain loading angle, the pre-crack crack will be deflected immediately, resulting in a crack initiation angle. Through the analysis and comparison of MTS, S and G criteria, it can be found that the crack initiation angle obtained by the experiments agree well with the three fracture criteria. Among them, the MTS criterion has the best effect for predicting the mixed mode crack initiation angle. With the increase of the loading angle, the deflection angle of the crack will also increase, but the magnitude of the deflection angle will not exceed the loading angle, the crack deviates more and more from mode I crack to mode II crack. By comparing the crack closure coefficients under different loading states, it can be seen that the decrease of the negative ratio and the decrease of the loading angle will lead to the level decrease of crack closure.

KEYWORDS: Commercial pure titanium;mixed mode I-II crack;Fatigue crack growth; Negative load ratio

目 录

摘要 I

Abstract II

第1章 前言 1

1.1课题研究的背景及意义 1

1.2国内外疲劳裂纹扩展研究现状及分析 1

1.2.1疲劳裂纹扩展机理 1

1.2.2复合型疲劳裂纹断裂准则 3

1.2.3裂纹闭合研究现状 4

1.2.4 I-II复合型疲劳裂纹扩展试验研究 6

1.3本论文研究的主要内容 7

第2章I-II复合型疲劳裂纹扩展试验 8

2.1 试验方案 8

2.2试件的材料与尺寸 8

2.3试验设备及试验步骤 10

2.3.1 试验参数 10

2.3.2 试验设备及步骤 10

2.4试验数据处理 12

2.5 本章小结 14

第3章 I-II复合型疲劳裂纹扩展试验分析 15

3.1 加载角度对疲劳裂纹扩展的影响 15

3.1.1 加载角对疲劳裂纹扩展速率的影响 15

3.1.2 加载角对疲劳裂纹扩展路径的影响 17

3.2负载荷比对裂纹扩展的影响 22

3.2.1负载荷比对裂纹扩展速率的影响 22

3.2.2负载荷下裂纹闭合水平 24

3.3本章小结 26

第4章 总结与展望 27

4.1总结 27

4.2 展望 27

参考文献 29

致谢 32

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