Failure criteria for the prediction of compound journal bearing fatigue - Extension of the methodology considering microstructural effects, Project stage II

本项目的重点是描述白金属合金在滑动轴承应用中的疲劳行为与微观结构的关系。在项目的第一阶段，通过热弹性流体力学(TEHD)和有限元(FE)模拟计算了复合滑动轴承的局部应力状态。通过失效假设将多轴应力状态与部件疲劳强度联系起来。所实施的失效假设需要通过实验确定的疲劳强度参数来评估所研究的白色金属合金。由于白色金属具有由韧性固溶体基质和金属间化合物相组成的异质组织，导致析出硬化，因此该组织对材料的动态强度有重要影响。这种微观结构的影响已经在材料和部件的疲劳试验中被检测到。因此，只有在确定材料参数以计算疲劳极限的试件的微观结构与待评估部件中的微结构相同的情况下，才有可能进行适当的寿命预测。因此，微观结构对宏观强度的影响将在第二阶段进行研究和描述。在细观力学领域，代表性体积元(RVE)方法是一种公认的将微观特征与宏观材料性能联系起来的工具。为了计算RVE，将白色金属的微观图像转换为网格有限元模型。对于各阶段的边界条件和具体的材料模型，RVE暴露在操作载荷下。这是针对不同的RVE进行的，这些RVE在相分数、尺寸和分布等关键参数上有所不同。通过均化步骤，可以从各个相的材料性质得到关于有效材料性质的宏观响应。通过考虑细观模拟和均匀化研究的结果，宏观有限元模型可以根据局部有效材料行为进行调整。这样，应力状态就可以作为有效微结构的函数来计算。此外，通过综合的数值算例研究，可以确定影响宏观静、动强度的关键参数。这是通过将RVE中的应力和应变集中与外部载荷相关联来实现的，这使得能够确定复合材料滑动轴承的损坏标准。通过实验材料和部件试验对数值计算结果进行了验证。最后，通过考虑有效材料参数作为微观结构的函数，使用各个步骤的结果来增强在第一个项目阶段开发的滑动轴承寿命预测的方法。

Focus of this project is the description of the fatigue behaviour of white metal alloys in plain bearing applications depending on the microstructure.In the first stage of the project, the local stress state in a compound journal bearing has been calculated by thermo-elastohydrodynamic (TEHD) and finite element (FE) simulations.The multiaxial stress state has been related to the component fatigue strength by means of failure hypotheses. The implemented failure hypotheses require fatigue strength parameters, which have been determined by experiments, for the evaluation of the investigated white metal alloys.As white metals have a heterogeneous microstructure, composed by a ductile solid solution matrix with intermetallic phases causing precipitation hardening, the microstructure has significant influence on the dynamic strength of the material. This influence of the microstructure has been detected in both material and component fatigue tests. Hence, an adequate lifetime prognosis is only possible, if the microstructure of the specimens, from which material parameters are determined to calculate the fatigue limit, is identical to the microstructure in the component to be evaluated. Therefore, the influence of the microstructure on the macroscopic strength will be examined and described in the second project stage.In the field of micromechanics the method of representative volume elements (RVE) is an established and accepted tool for linking microscopic features with macroscopic material properties. In order to calculate RVE, micrographs of white metal are converted into meshed FE models. Regarding the boundary conditions and specific material models of the phases, the RVE is exposed to the operational loads. This is done for different RVE that vary in key parameters such as phase fraction, size and distribution. By the step of homogenization the macroscopic response regarding effective material properties can be derived from material properties of the individual phases. By considering the findings of the mesoscopic simulations and homogenization studies the macroscopic FE model can be adjusted to the local effective material behavior. In this way, the stress state can be calculated as a function of the effective microstructure. Furthermore, key parameters affecting the macroscopic static and dynamic strength can be identified by comprehensive numerical case studies. This is done by correlating stress and strain concentrations in the RVE with external loads, which enables the determination of a damage criterion for composite plain bearings. The validation of the numerical results is done by experimental material and component tests. Ultimately, the results of the individual steps are used to enhance the methodology of lifetime prediction of journal bearings developed in the first project stage by considering effective material parameters as a function of the microstructure.