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

在其使用寿命中，机器和部件面临着复杂的运行载荷，这些载荷通常以非比例部件为特征。因此，在计算含裂纹结构的剩余寿命时，必须考虑由I、II和III型叠加产生的空间混合模式载荷。对于面内比例载荷，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，在描述疲劳裂纹扩展的概念中，只有少数几种不是普遍有效的方法来包含所有三种裂纹模式。此外，疲劳裂纹扩展过程中的非比例加载导致了模式混合度的暂时和局部变化，因此也可能发生主导断裂行为的变化。提出的研究项目的目的是在线弹性断裂力学框架内发展概念，以便在金属材料的非比例空间混合模式加载下进行可靠的剩余寿命预测。这些研究集中在疲劳裂纹扩展过程中裂纹路径和裂纹几何形状的实验确定，并伴随着对有效机制的理解，作为发展相应的裂纹扩展概念的重要基础。在此基础上，获得关于疲劳裂纹扩展过程中裂纹扩展方向和裂纹扩展速率的基本知识。在伺服液压拉/扭试验机上使用夹持式单边缺口试件(SEN(TC)试件)进行预期的研究，以实现所有三种裂纹模式的定向耦合。为了识别疲劳裂纹扩展的有效机制，产生了不同的加载路径。通过使用各种实验技术暴露断裂面，可以获得裂缝的三维几何形状，以便进行测量。利用数字图像相关法(DIC)对试件表面的裂纹扩展和裂纹载荷进行了自动评价。在实验研究的同时，还进行了线弹性有限元模拟，根据实验测量的空间裂纹几何形状确定应力强度因子(SIF)。裂纹载荷由J积分确定，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).