Leistungen
The increasingly severe and complex mechanical and thermal loading conditions which gas turbine blades are being subjected to, often lead to fatigue cracks initiating in regions where cyclic plasticity highly localises, due to mechanical and thermal cyclic stressing. Moreover, creep damage is observed due to high temperatures dwell times, which also generate oxidation-damaging processes. The complexity of such processes is further underlined by the interaction between cyclic plasticity damage and creep damage. Current and future engine developments rely on thermal barrier coatings (TBC) applied on some strategic places of components to raise turbine inlet temperatures and hence increase thermodynamic efficiency. Substrate damage, oxidation and TBC degradation can however significantly reduce component lifetime.
Two distinct aspects have been considered for damage analysis within the project:
-Fatigue damage of uncoated single crystal.
-Degradation of the thermal barrier coating (TBC) itself.
These two aspects were studied in parallel, as they involve completely different damage mechanisms. The study provides the physical basis to model damage by taking into account the role of the various interactions with the microstructure. Micromechanical and macroscopical models were developed.
During the project, the following main achievements have been realised:
-A basic understanding of the dominant high temperature degradation and damage mechanisms in coated and uncoated monocrystalline Ni-base super-alloy components used in gas turbine applications has been developed.
-Assessment tools for the life prediction of coated and uncoated single-crystal super-alloy components operating under combined creep/fatigue loading conditions have been developed. An integrated micro-meso-macro constitutive framework has been developed. It enables the morphology of the precipitates at the micro-scale, the presence of casting defects at the meso-scale and of a protective TB coating at the macroscopic level to be accounted for. This research provides an understanding of the role played by substrate and coating defects and microstructure on deformation failure modes.
In a second macroscopical approach, lifetime assessment tools have been derived on a phenomenological basis. A simple scheme has been directly introduced in engineering design practice. Such an approach is more limited in its application than mechanisms based framework. However, coupled together with knowledge of the physical mechanisms obtained in the other part of the work, it is useful for industrial applications.
-Some of the life assessment methods have been validated on the basis of structure.
These achievements allow to improve design assessment methodologies which was the main objective of industries.
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