A MULTISCALE MICROMECHANICS APPROACH TO DESCRIBE ENVIRONMENTAL EFFECTS ON SURFACE CRACK INITIATION UNDER CYCLIC LOADING

摘要

In this work, a multiscale mechanistic approach is employed to study the effects of casting-related porosities and environment on the initiation of surface cracks in single crystal superalloys under predominantly cyclic loading conditions. At the scale of the porosity (i.e. mesoscale), a micromechanics-based probabilistic formulation is relied upon to describe the initiation and growth of fatigue cracks from spherical defects. The effects of loading at the scale of the component (i.e. macroscale) and of the interaction of the void with a free surface on the mesoscopic stress variations within a loading cycle are quantified from detailed finite element analyses of a representative material volume element. The effect of a reduction in the volume fraction of the γ' precipitate phase due to surface oxidation on the time-dependent notch stresses is analysed numerically using a crystallographic formulation for the single crystal which depends explicitly on the precipitate volume fraction at the microscale. The framework is then used to investigate the fatigue behaviour of a typical notched tensile bar.