Alain Köster

A physical-base model for life prediction of single crystal turbine blades under creep-fatigue loading and thermal transient conditions

Abstract The damage estimation model developed herein can be used to predict service life under creep-fatigue loading in both isothermal (LCF) and variable temperature (TMF) conditions. This model is based upon a careful identification of basic physical mechanisms taking part in the damage process of a unit microstructural element. The damage is, in fact, considered to be the growth of micro cracks originating from initial material defects. The distribution of these defects has been determined through microscopic observations and their propagation due to fatigue and creep loading is simultaneously calculated in this model. A large near surface pore may develop into a surface crack and its propagation may be accelerated by the weakening of material through oxidation embrittlement. This model has been successful in predicting creep-fatigue life of CMSX4 test specimens at 950°C, and that of tubular specimens under (450–950°C) TMF conditions. It has also been applied to a thermal fatigue (TF), wedge-shaped structural element undergoing thermal transient loading; early crack growth rate has been satisfactorily predicted by this model.

Three-Dimensional Damage Evolution Measurement in EB-PVD TBCs Using Synchrotron Laminography

This study provides the first results of three dimensional microstructural evolution analysis of a typical EB-PVD thermal barrier coating subjected to thermal cycling. Results have been obtained using the technique of synchrotron-radiation computed laminography (SRCL), enabling the scanning of samples that are thin but extended in two dimensions. A correlation with quantitative measurements of phase transformation from β-NiAl to γ’-Ni3Al was achieved from 3D images and 2D cross-section analysis. Morphological features of phase transformation and damaged areas within the bond-coat layer obtained by SRCL are discussed.

Thermal-mechanical Fatigue and the Modelling of Materials Behaviour Under Thermal Transients

Thermal-mechanical fatigue is addressed using the following methodology: volume element tests are used to check constitutive models as well as to investigate synergy effects and damage models. Structure tests as in thermal shock are used to validate models. This methodology is applied to two gas turbine materials: a wrought polycrystalline alloy, Superwaspaloy, for moderate temperature use, and aluminized single crystal AM1 superalloy for blades. The capabilities of viscoplastic constitutive models with internal variables are illustrated. A damage model is shown which describes the synergy between oxidation, creep and fatigue. Detrimental effects of aluminide coating for specific thermal mechanical loading paths are tentatively rationalized.