Prediction of time-dependent crack growth with retardation effects in nickel base alloys

Accurate crack growth prediction methods are playing an increasing role in the design and evaluation of rotating gas turbine engine components. Fracture mechanics methods used to predict the cyclic lives (without time-dependent effects) are well established. As mission times and temperature increase, nickel base superalloys experience time-dependent crack growth where the crack growth response is a function of time under load (hold time) as well as overpeaks that occur prior to hold times. Linear elastic fracture mechanics methods have been developed that accurately predict the acceleration associated with hold times at elevated temperatures using a linear superposition of cyclic and static crack growth rates. The beneficial effects of retardation induced by overpeaks can be predicted using a modified Willenborg retardation model. Results of subcomponent validation tests for a variety of conditions and materials used to validate these methods are reported. Applicability of these model to predict complex missions and combinations of complex missions is also discussed.