Implemetation of residual stress to the microstructurally based fracture mechanics at the example of a martensitic steel and application on the fracture-mechanical analysis of the fatigue strength of springs

该项目旨在通过短裂纹的扩展来表征马氏体弹簧钢的疲劳机制。此外，基于材料科学的短裂纹扩展模拟应该通过基于实验观察机制的模型来阐述。因此，为了对残余应力引起的现象进行更具体的研究，例如短裂纹的裂纹闭合，在考虑残余应力的情况下，可以更好地理解HCF循环加载下马氏体钢的金属物理过程。为此目的，将采用材料测试和材料表征的现代实验方法。使用小型材料疲劳测试装置，可以通过共聚焦激光显微镜对材料在单轴循环载荷下的疲劳损伤进行现场分析。一个相同的测试装置将被重新设计为双轴，通过一个扭转力矩的应用到一个试样的循环载荷。残余应力诱导裂纹闭合的影响可以通过干涉法测量位移来测量和量化。通过x射线分析来确定靠近表面的体积内的残余应力状况，即宏观和微观残余应力。此外，为了收集一个全面的疲劳数据库，将使用常规疲劳试验方法对标准样品分别进行拉伸压缩载荷或扭转载荷下的疲劳试验。基于实验确定的材料疲劳机制，将为模拟短裂纹扩展提供一种合理的方法。建立了可以用边界元法求解的短裂纹扩展模型作为基础。为了模拟马氏体钢的短裂纹扩展，对其进行了改进。真实地描述马氏体组织和残余应力状况，模拟空间残余应力分布对短裂纹向试件纵深扩展的影响。由于力学和材料科学专家的密切合作，结果使人们对疲劳的基本机制有了更深入的了解，并通过实验和模拟的共同努力获得了新的见解。项目完成后，在考虑马氏体钢疲劳强度的基础上，改进面向应用的马氏体钢疲劳评价，提高结构内部的材料利用率是可行的。

This project is targeted on a characterization of the mechanisms of fatigue in martensitic spring steels by the propagation of short cracks. Furthermore, a materials science based simulation of the propagation of short cracks should be elaborated by means of a model which is based on the experimentally observed mechanisms. Thus, a better understanding of the metal physical processes in martensitic steels under cyclic loading in the HCF regime is established considering also residual stress in order to conduct more specific investigations of phenomena induced by residual stress, e.g. crack closure of short cracks.Modern experimental methods of materials testing as well as materials characterization will be applied for this purpose. The use of a miniaturized materials fatigue testing device allows for in-situ-analysis of the fatigue damage of the material under uniaxial, cyclic loads by means of confocal laser microscopy. An identical testing device will be redesigned for biaxial, cyclic loads by application of a torsional moment to a specimen. The effect of residual stress induced crack closure can be measured and quantified by an interferometric determination of the displacement. The condition of residual stress, i.e. macro and micro residual stress, within volumes near to the surface will be determined by means of x-ray analysis. Furthermore, for gathering a comprehensive fatigue data base additional fatigue testing will be conducted at standard samples under tensile-compression load or torsional load, respectively, by using conventional fatigue testing methods.Based on the experimentally identified fatigue mechanisms of the considered material a reasonable approach for simulation of short crack propagation will be implemented into a mechanism-oriented model. A model of short crack propagation which can be solved by using a boundary element method serves as a basis. It will be modified for the simulation of short crack propagation in martensitic steels. The martensitic structure and the residual stress condition shall be realistically described and the effect of the spatial residual stress profile on the short crack propagation into the depth of the specimen shall be simulated.Due to the close collaboration of experts in mechanics and materials sciences, respectively, a deeper understanding of the fundamental mechanisms of fatigue is emerging by the results and insight is gained by the combined efforts in experiments and simulation. After finalizing the project, it will be feasible to improve the application-oriented fatigue assessment of martensitic steels and to enhance the materials utilization within structures based on the consideration of its fatigue strength.