SUMMARY

The transition from a design philosophy based on traditional fatigue and fatigue crack initiation, also known as safe life, to an approach that assumes an initial flaw is present, also known as damage tolerant or retirement for cause, has been driven by the need to maximize safety while minimizing life cycle costs. The use of the damage tolerant design approach in turbine engine hot section components requires accurate understanding and characterization of material performance in order to meet the safety and life cycle cost goals. Numerous isothermal and a limited number of thermomechanical fatigue crack growth (FCG) studies have been conducted and FCG models created. However, there are no models able to fully account for the numerous factors that influence the FCG rate. These factors include the influence of prior loading, oxidation and material evolution, and creep and stress relaxations. The proposed research will work to characterize and empirically model the FCG performance at high temperature of Inconel 718, a polycrystalline Ni-base superalloy, with specific focus on the impact of time spent at elevated temperature on subsequent crack growth. The model will be capable of handling the complicated thermomechanical spectra these alloys typically see with limited simplification and allow designers and program managers to make more informed decisions on the management of their projects. The effort to produce this model will include conducting isothermal and thermomechanical FCG tests, microstructural and fractographical characterization of active crack growth mechanisms, and finite element simulation of the evolution of stresses at the crack tip.