Under service conditions, turbine blades experience a superposition of complex thermal and mechanical loading especially during start and shut-down of the engine. Very high transient temperature gradients occur and result in combination with the alternating mechanical loads in significant lifetime reductions. Under certain conditions, the presence of diffusion coatings can lead to brittle cracking of the coating system, and thus, to drastical loss of fatigue life, due to the brittleness of the intermetallic coating system at low temperatures.
A physically-based lifetime model for the aluminide coated single crystalline nickel-base superalloy PWA 1484 under thermo-mechanical loading was developed. An essential element is the brittle cracking of the coating system. Therefore, an approach for modeling of the deformation behavior by Wilhelm et al. Was used and superimposed by a modified crack propagation model based on the calculation of the cyclic crack-tip opening displacement (CTOD) as introduced by Schweizer et al.
The simulation of the fatigue crack growth behavior was validated by in-situ crack propagation measurements using the beachmark technique in the thermomechanical fatigue tests. The comparison of the simulated lifetimes to the experimental results provides remarkable accuracy of the physically-based lifetime model. It should be noted that all material parameters implemented in the model were evaluated from monotonic isothermal tensile and creep tests as well as from isothermal low-cycle fatigue (LCF) experiments.
Anahtar Kelimeler: Diffusion coating - Nickel-based superalloy - Fatigue crack growth measurement - Lifetime modelling -
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