Fatigue crack growth under non-proportional mixed mode loading considering mode coupling effects

在其使用寿命期间，机器和部件会承受复杂的操作载荷，这些载荷通常以非比例部件为特征。为了计算裂纹结构的剩余寿命，必须考虑由模式I、II和III叠加产生的空间混合模式载荷。对于平面内比例载荷，现有的裂纹扩展概念往往能够描述疲劳裂纹扩展。相比之下，只有几个，不普遍有效的方法，包括所有三个裂纹模式的概念，用于描述疲劳裂纹扩展。此外，在疲劳裂纹扩展过程中的非比例加载会导致模态混合度的时间和局部变化，从而也会发生主导断裂行为的变化。拟议的研究项目的目的是在金属材料非比例空间混合模式载荷下，在线弹性断裂力学框架内发展可靠的剩余寿命预测概念。调查集中在疲劳裂纹扩展过程中的裂纹路径和裂纹几何形状的实验测定，伴随着对有效机制的理解，作为相应的裂纹扩展概念的发展的重要基础。在此基础上，应获得有关裂纹扩展方向以及疲劳裂纹扩展过程中裂纹扩展速率的基本知识。在伺服液压拉伸/扭转试验机上使用夹紧单边缺口试样（SEN（TC）-试样）进行预期研究，以实现所有三种裂纹模式的目标耦合。为了识别疲劳裂纹扩展的有效机制，产生不同的载荷路径。通过使用各种实验技术暴露断裂表面，可以获得裂纹的三维几何形状以进行测量。数字图像相关（DIC）是用于自动评估裂纹扩展和裂纹加载试样表面上。实验研究是伴随着线弹性有限元模拟的应力强度因子（SIF）的测定实验测得的空间裂纹的几何形状的基础上。裂纹载荷由J积分数值确定，而应力强度因子解由相互作用积分（M积分）分离。

During their service life, machines and components are exposed to complex operating loads, which are often characterized by non-proportional parts. For the calculation of the remaining lifetime of cracked structures, spatial mixed-mode loading resulting from a superposition of mode I, II and III must therefore be taken into account. For in-plane, proportional loads, existing crack propagation concepts tend to be able to describe fatigue crack propagation. In contrast, there are only a few, not universally valid approaches for the inclusion of all three crack modes in a concept for the description of fatigue crack propagation. In addition, non-proportional loading during fatigue crack growth leads to a temporal and local change in the mode-mixity, so that a change in the dominant fracture behaviour can also occur. The aim of the proposed research project is the development of concepts within the framework of linear-elastic fracture mechanics for a reliable remaining lifetime prediction under non-proportional spatial mixed-mode loading in metallic materials. The investigations focus on the experimental determination of crack paths and crack geometries during the fatigue crack growth, accompanied by an understanding of the effective mechanisms, as an essential basis for the development of corresponding crack propagation concepts. Based on this, basic knowledge on the crack propagation direction as well as on the crack growth rate during fatigue crack growth shall be gained.The intended investigations are carried out using clamped single-edge notched specimens (SEN(TC)-specimens) on a servohydraulic tension/torsion testing machine for achieving a targeted coupling of all three crack modes. In order to identify the effective mechanisms of fatigue crack growth, different load paths are generated. The three-dimensional geometries of the cracks are made accessible for measuring by exposing the fracture surfaces using various experimental techniques. The digital image correlation (DIC) is used for the automated evaluation of crack growth and crack loading on the specimen surfaces. The experimental investigations are accompanied by linear-elastic finite element simulations for the determination of stress intensity factors (SIF) based on the experimentally measured spatial crack geometries. The crack loading is numerically determined by the J-Integral, while the SIF solutions are separated by means of the interaction integral (M-integral).