Modelling crack-tip behaviour in a directionally solidified nickel alloy under fatigue-oxidation conditions

A finite-element approach, to study crack-tip behaviour for a directionally solidified nickel-based superalloy subjected to high-temperature fatigue loading in vacuum and air, is presented. In vacuum, crack-tip behaviour was entirely controlled by mechanical deformation, hence, a criterion based on accumulated plastic strain was sufficient to describe damage. In air, effects of oxidation on crack-tip deformation were studied using a diffusion-based approach; further developed to investigate a synergetic interaction between fatigue loading and oxygen penetration. Stress-assisted diffusion and dilatation from oxygen penetration into a crack tip, were considered. A local compressive stress induced by oxygen penetration compensated part of the tensile stresses from mechanical loading. To predict crack growth rate under fatigue-oxidation conditions, a two-parameter crack-growth criterion, based on accumulated plastic strain and oxygen concentration at the crack tip, was developed. Obtained numerical results compared well with experimental data in the literature.