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

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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.