SUMMARY

Turbine engine blades are subjected to extreme conditions characterized by significant and simultaneous excursions in both stress and temperature. These conditions promote thermo-mechanical fatigue (TMF) crack growth which can significantly reduce component design life beyond that which would be predicted from isothermal/constant load amplitude results. A thorough understanding of the thermo-mechanical fatigue crack behavior in single crystal superalloys is crucial to accurately predict blade life.

The proposed research will be conducted on PWA1484, a single crystal superalloy used by Pratt & Whitney for turbine blades. Initially an isothermal constant amplitude fatigue crack growth rate database will be developed, filling a void that currently exists in the literature. Through additional experimental testing, fractography, and analysis the effects of temperature interactions, load interactions, oxidation and secondary crystallographic orientation on the fatigue crack growth rate and the underlying mechanisms responsible can be determined. This information will then be used to build crack growth prediction models. Modeling will be developed using a two pronged approach, with one route adding temperature interaction and updated load interaction functionality to crack closure based FASTRAN. A crack trajectory modeling approach will use blade primary and secondary orientations to determine whether crack propagation will occur on crystallographic planes or normal to the applied load while taking into account anisotropic single crystal plasticity. Finally, since the material being studied is highly anisotropic, both in an elastic and plastic sense, the applicability of conventional fracture mechanics approaches is open to question and the possibility of building on previous work to develop a “local approach” to defining a crack growth forcing function will be investigated. Model verification will be performed through the prediction of simple spectrum tests. These models can then be used to create inspection intervals that when coupled with advanced nondestructive inspection techniques ensure proper retirement of superalloy engine blades.