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

本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建立在实验观察机制基础上的模型来阐述。因此，为了更具体地研究由残余应力引起的现象，例如短裂纹的闭合，可以更好地理解马氏体钢在HCF区循环加载下的金属物理过程。为此，将应用现代材料测试和材料表征的实验方法。利用微型材料疲劳试验装置，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原位分析。通过对试件施加扭矩，将重新设计用于双向循环载荷的相同测试装置。残余应力引起的裂纹闭合的影响可以通过对位移的干涉测定来测量和量化。表面附近体积内的残余应力，即宏观和微观残余应力的状态将通过X射线分析来确定。此外，为了收集全面的疲劳数据库，还将使用常规的疲劳试验方法，在标准试件上分别进行拉压载荷和扭转载荷下的附加疲劳试验。基于实验确定的所考虑材料的疲劳机理，将在面向机理的模型中实现合理的短裂纹扩展模拟方法。以可用边界元方法求解的短裂纹扩展模型为基础。并对其进行了改进，以模拟马氏体钢中的短裂纹扩展。应真实地描述马氏体组织和残余应力条件，并模拟空间残余应力分布对短裂纹向试件深度扩展的影响。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的理解，并通过实验和模拟的共同努力获得了洞察力。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估，提高结构内部材料利用率将是可行的。

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.