SUMMARY

Creep and creep-fatigue crack growth in directionally-solidified Ni-base superalloys does not adhere to existing time-dependent fracture mechanics concepts. Many of the underlying assumptions do not apply to these types of materials. The result is unpredictable, transient crack growth rates which thwarts single parameter fracture mechanics-based approaches to service life estimation. This poses a significant challenge to designers who use these materials for hot-section blades in industrial gas turbine engines. A closer investigation of creep and creep-fatigue crack growth behavior in these materials is warranted.

The goal of this work is to understand the contributing factors to transient crack growth rate behavior observed in directionally-solidified Ni-base superalloys. Failure analyses and material characterization must be performed on specimen from creep and creep-fatigue crack growth experiments. This will reveal the nature of crack growth and the length- and time-scales pertinent to the fracture process zones. Finite element analyses of experimental conditions will elucidate the evolution of crack tip stress fields for growing cracks in these materials given the nature of the fracture process zones. The results will validate the applicability of single parameter fracture mechanics-based approaches to this class of materials. Additional creep and creep-fatigue crack growth testing must be performed so that a variety of transient crack growth conditions can be studied. This data will be highly valuable to broader scientific community as creep and creep-fatigue crack growth rate data for directionally-solidified Ni-base superalloys is currently limited.