基于小时间尺度原位试验的疲劳小裂纹扩展机理研究和寿命预测

Fatigue life prediction of metallic structural material is a relatively old subject in aeronautical field. A fatigue life prediction method has been mainly established by describing the process of long crack propagation and fracture based on Liner Elastic Fracture Mechanics (LEFM). People make good efforts to extend this approach to the smaller crack size. The small crack is of great significance, because in the most structural materials the flaws always exist, which can work as the initial small crack, and its propagation usually consumes the major fatigue life. However, due to the small crack effects, the LEFM theory cannot be directly used to analyze the small crack growth. Furthermore, the intrinsic quantitative relationship between the micro-mechanisms and the instantaneous crack growth is still not clear. It is difficult to establish a physical model of the small crack growth. So, the fatigue life evaluation involving the small crack and variable amplitude loading is a big challenge. In this project, the state-of-the-art experimental techniques (In-Situ SEM/Optical Microscopy Fatigue Crack Growth Testing) are performed to investigate fatigue small crack growth mechanisms and behaviors at a small time scale within one load cycle. Then, the small crack growth physical model can be built up based on the analysis of the experimental observations. Combining the previous progress of long crack growth model, the whole fatigue life prediction model can be derived. Finally, the study on fatigue life distribution prediction is conducted, considering the uncertainties of initial flaws, material properties, and loading spectra.

在航空领域中，金属结构材料的疲劳寿命预测问题由来已久，人们已基本建立了利用线弹性断裂力学理论描述长裂纹扩展直至断裂过程的寿命预测方法，并始终致力于把这种方法向更小的裂纹尺寸延伸。小裂纹的问题尤为重要，因为结构材料内不可避免的含有可视为小裂纹的初始缺陷，而小裂纹扩展消耗绝大部分的疲劳寿命。然而，由于“小裂纹效应”的影响，线弹性断裂力学分析无法直接应用于小裂纹的扩展，而且其微观机理以及与裂纹增量之间的定量关系尚不明确，物理模型难以建立，变幅载荷下小裂纹扩展寿命评估面临挑战。本项目采用先进的小时间尺度原位试验技术，开展变幅载荷下扫描电镜和光学显微镜疲劳试验，揭示金属疲劳小裂纹在一个载荷周期内瞬时的行为和机理。分析试验结果，建立小裂纹扩展物理模型，结合申请人在前期研究中建立的长裂纹扩展建模，推导疲劳全寿命预测模型。最后，考虑材料参数、初始缺陷以及载荷的不确定性，进行疲劳寿命预测方法研究。

项目针对小裂纹效应现象，以基于断裂力学的金属结构疲劳寿命预测方法为工程需求背景，采用试验、数值、理论分析结合的方法对小裂纹扩展行为进行深入的研究。从小时间尺度揭示裂纹在恒幅/变幅载荷下的裂纹扩展规律，进而建立统一的长短裂纹扩展模型，形成疲劳寿命/概率疲劳寿命预测方法。. 项目建立了一套小时间尺度原位疲劳试验方法，使用SEM和光学显微镜，可分别实现一个加载周期内的裂尖形貌和裂尖塑性区实时测量。试验观察到了过载前的裂纹闭合现象及过载后的瞬时加速和延迟迟滞现象。获得了过载前后裂尖物理量的变化，揭示了过载后的瞬时加速和延迟迟滞现象的机理；使用数值仿真模型，分析了疲劳裂尖塑性区演化过程和残余应力对裂纹扩展行为的影响；在以上机理分析的基础上，考虑小裂纹阶段裂纹闭合的演化和微观结构差异性，建立了以CTOD为驱动因子的长短裂纹统一的扩展模型，并采用多种材料不同载荷的试验数据进行了验证，取得了令人满意的结果。最后，基于基于Bootstrap和裂纹扩展模型，建立了考虑材料、初始缺陷和载荷不确定性的概率疲劳寿命评估模型。. 项目完成了整个研究内容，达到了研究目的，在变幅载荷下裂纹扩展机理研究和小裂纹扩展建模方面，取得较为突出的进展，形成的方法可用于损伤容限分析和全寿命评估。