A model for oxidation-assisted low cycle fatigue of superalloys

Cracks in superalloys subject to an isothermal, sustained hold, compressive strain cycle experience cycle-by-cycle extension. The phenomenon has been attributed to the strains induced by the formation a thermally grown oxide (TGO) on the crack faces with the strain accommodated by alloy creep. Basic features have been elucidated through finite element calculations. The present objective is to develop a straightforward model that explicitly relates the fatigue to the TGO strains and the alloy creep rates. For this purpose, insights are first developed through finite element simulations. The results are rationalized through simple mechanics assessments, leading to analytic expressions for the crack growth rate. Insights are gained using a plasticity model that demonstrates how the TGO growth strains are accommodated and the stress distributed at various stages during the strain cycle. Creep calculations highlight the time-dependent phenomena occurring during the hold interlude. Finally, an expression for the crack growth rate is presented in terms of the TGO growth strain rate, its thickness, the alloy creep strength and the hold time.