表面纳米化金属材料的疲劳失效是涉及材料微观结构、力学性能等多重因素的损伤演化过程。大量研究表明，表面晶粒的纳米化往往能抑制裂纹萌生从而较大程度地提高疲劳寿命。然而，目前针对表面纳米化材料的表面疲劳微裂纹扩展门槛值的影响机理研究还较为缺乏，一定程度上制约了材料表面纳米化疲劳研究的发展。本项目将从微观组织结构及其力学性能的角度深入地探索不同表面纳米化程度对表面疲劳微裂纹扩展门槛值的作用机制。首先，基于表面微裂纹的扩展传播特性研究微裂纹尺寸与扩展门槛值之间的关联机制，从而确定适用于表面纳米化材料的微裂纹大范围屈服条件；其次，从细观尺度分析表面晶粒尺寸、纳米层厚度和残余应力分布等关键参数对抗疲劳微裂纹扩展性能的影响机理；最后，基于上述微观机理分析提出一种适用于表面纳米化材料疲劳微裂纹扩展门槛值的预测方法。本项目的完成将有助于深化金属材料表面纳米化的抗疲劳机理研究，同时为其抗疲劳设计提供设计参考。

The fatigue failure in surface nanocrystallization of metallic material is a complex damage evolution process involving influential parameters, such as material microstructures and mechanical properties. It has already been demonstrated that nanocrystal in surface can contribute to suppressing fatigue crack initiation thereby improving fatigue life. Nevertheless, the current study on the fatigue small crack threshold mechanism in surface nanocrystallization of metals is scarce, which hinders the further development of surface nanocrystallization fatigue research to some extent. From the viewpoint of the material microstructure and mechanical properties, this project attempts to systematically explore the fatigue small crack threshold mechanism in metals influenced by the surface nanocrystallization. Firstly, based on the characteristics of surface small crack propagation, we try to investigate the correlative mechanism between small crack length and small crack threshold so that the large scale yielding condition in metals of surface nanocrystallization can be redefined. Secondly, the influence of several important factors such as grain size in surface, nanolayer thickness and residual stress distribution, on small crack propagation resistance will be analyzed. Lastly, one new prediction method for fatigue small crack threshold of surface nanocrystallization metals is attempted to propose based on above mechanism analysis. This project will be beneficial to further understanding for fatigue mechanism of surface nanocrystallization metals. Meanwhile, one kinds of design reference in practical engineering for anti-fatigue design of surface nanocrystallization metals will be provided.