材料损伤诊断的三维表面形貌研究

The demands for fatigue control techniques have increased in the design of aerospace structures, and these improvements require a deep scientific understanding of the fatigue processes. The fatigue initiation stage including crack nucleation and micro-crack propagation constitutes 90% of the fatigue life for most components, life prediction based on fracture mechanics may give unreliable values for this stage, therefore, developing a quantitative and accurate understanding of crack initiation processes should be considered as one of the most important tasks in fatigue study. One major challenge of fatigue study during fatigue initiation stage is that no physical measurement has been identified to characterize plastic deformation. To facilitate fatigue study, we propose to use the plastic deformation induced surface roughness as a diagnosis tool to assess material damage at crack initiation stage. In order to obtain a direct expression with nano-meter precision of specimen surface in different loading stages and fatigue cycles, in-situ three dimensional surface profiling of compact specimens under cyclic loading are designed, and scanning whitelight interferometry technique will be exploited for three dimentional surface profiling on specimen. In consideration of the mutiscale of material failure and damage evolution with cyclic loading, multiscale simulation model which can be updated with the material physical state will be developed. Through developing a system identification algorithm, the system parameters will be updated and the model will be optimized by correlating simulated and measured surface profile. Finally, integrating stage research results, a valid physical method based on the characteristics of three dimentional surface profiling will be proposed for material damage assessment at early fatigue life, and an adaptively adjustable multiscale simulation model that is capable of realistically describing the evolution of the material physical state will be developed. Adaptively adjusting the multi-scale simulation model will provide a mechanism-based mathematical model of the material damage state, enabling reliable prediction of material damage development. . Knowledge gained from this project will provide valuable insights on the interactions between microstructures, plastic deformation and material damage development. The understandings resulting from this project will enable fundamental advances in technologies with direct impacts on fatigue control of aerospace structures.

航空航天结构设计中，对疲劳控制技术的要求日益提高，所有这些技术都需要对疲劳过程的科学理解。而疲劳初始阶段（裂纹形核和微裂纹扩展）占近90%的疲劳寿命，因此定量认识裂纹萌生过程中材料的损伤发展应视为疲劳研究中最重要的任务之一。本研究提出用塑性变形引发的表面粗糙度作为诊断工具，评价主裂纹出现之前的材料损伤。为了获得不同加载阶段和疲劳周期试件表面纳米精度的直接表征，拟设计紧凑拉伸微型实验装置，集成白光干涉扫描显微镜，进行试件三维表面轮廓的现场原位测试；考虑到材料断裂的多尺度和真实损伤状态的演化，拟建立一种可随材料损伤状态动态调节的多尺度仿真模型，并开发系统识别算法，通过对比仿真和实测的表面轮廓，更新系统参数，优化仿真模型；最后，将集成阶段性研究成果，综合出一套能根据材料物理状态演化自适应调节的多尺度仿真模型及实现算法，提出裂纹萌生阶段的材料损伤评价方法，为进一步用于航空结构疲劳控制提供理论参考。

本项目首先通过多种材料不同应力状态的破坏试验，研究了断裂过程中塑性变形对损伤机理的影响规律，给出了材料损伤破坏机理与断裂过程中发生的塑性变形量之间的关系。认识到了材料受力变形、损伤至断裂的过程中，塑性变形的主要作用为：一是改变构件几何形状，降低应力集中，微裂纹出现后，塑性变形量直接决定着损伤机理；二是位错滑移使材料强化。达强化极限后，微裂纹一旦出现，材料不具备继续发生塑性变形的能力，易发生低宏观应力下的断裂。基于塑性变形和弹性变形在机理和尺度上的不同，提出了将塑性变形和弹性变形对结构的效应区分考虑的思想，并用于损伤研究中。疲劳初始阶段的寿命主要取决于循环载荷作用下塑性变形的不断消耗及材料的强化，而位错滑移是导致塑性变形的主要因素，材料经历的塑性变形与滑移带的发展演化直接相关，表现为表面粗糙度的变化。在此基础上，通过对纯铜和镍合金轴向疲劳试件不同加载工况的疲劳试验，采用引起塑性变形的细观因素滑移带的发展演化和塑性变形引发的表面粗糙度作为诊断工具，完成了不同疲劳周期试件表面形貌的表征，对加载至不同循环周期的试样表面滑移带和表面粗糙度的变化规律进行了系统的分析，建立了表面粗糙度和表面滑移带与循环周期的联系。提出了能描述疲劳初始阶段材料损伤特征和能反映疲劳寿命早期材料错综复杂演化特征的实际测试方法，建立了对疲劳初始阶段的定量认识。通过对比研究不同疲劳周期试件表面滑移带和表面粗糙度的变化，发现了疲劳初始阶段表面粗糙度与表面滑移带演化特征之间的关系，早期的表面粗糙度增大主要表现为滑移带的形成和滑移带高度的增加，后期粗糙度增大主要与邻近晶粒的面外位移相关。通过对复杂环境下材料损伤特性的试验研究，建立了统计损伤本构模型，描述了材料细观结构的损伤演化特征。研究结果为工程结构损伤分析和检测提供了理论依据。