Franck Gallerneau

Fatigue Life Prediction of Single Crystals for Turbine Blade Applications

This paper describes a fatigue life prediction model accounting for high temperature applications of metallic materials. The formulation of the model is for general anisotropy and multiaxiality of loading. This phenomenological model distinguishes between an initiation phase and a propagation phase, and takes into account oxidation and creep effects on fatigue life. An application of the model is given for a coated single crystal superalloy for turbine blades. Model predictions are in good agreement with a large set of experimental data for mechanical loading including thermomechanical fatigue tests. A new experimental device, designed to produce a thermal gradient in the thickness of thin walled specimens, is also presented. This device has been used with polycrystalline and monocrystalline superalloys. Corresponding life predictions, performed by using recent anisotropic models, are presented for the single crystal superalloy.

Dissolution of fine γ' precipitates of MC2 Ni-based single-crystal superalloy in creep-fatigue regime

The dissolution kinetics of an ultra-fine γ’ precipitation occurring in the γ matrix between the standard secondary precipitates of MC2 Ni-based single crystal superalloy was investigated. Creep-fatigue experiments at 1050°C including an overheating at 1200°C were performed on <111> oriented specimens to study the effects of fine γ’ particles on the plastic deformation. During these experiments, a decrease of the plastic deformation rate was observed just after the temperature peak. This hardening effect disappears once the fine γ’ precipitates had been dissolved. A mean time for this hyperfine precipitation dissolution could then be highlighted. Based on both simple binary diffusion and complex diffusion analysis, the mean time for the dissolution of the fine γ’ precipitates is analyzed and compared to the experimental ones. It is shown that considering only a simple binary diffusion is not sufficient and it should be considered a more complex diffusive analysis involving additional interplays.

A Strain Rate Sensitive Formulation to Account for the Effect of \gamma ^{\prime } Rafting on the High Temperature Mechanical Properties of Ni-Based Single Crystal Superalloys

The **Polystar** model was recently developed to fulfill the effects of possible fast microstructure evolutions occurring upon high temperature non-isothermal loadings. New internal variables were introduced in a crystal plasticity framework to take into account microstructure evolutions such as \(\gamma ^{\prime }\) dissolution/precipitation and dislocation recovery processes, and their effects on the creep behavior and creep life. Nevertheless, this model does not take into account one of the main microstructural evolutions occurring specifically at high temperature, the \(\gamma ^{\prime }\) directional coarsening. Fedelich and Tinga have already proposed models respectively based on a modification of the kinematic hardening and on the level of the von Mises stress. Nevertheless, if the Fedelich’s model is implicitly strain rate sensitive, improvements have to be performed for strain controlled tests under fast conditions for which such a model may overestimates the \(\gamma \) channel width evolutions. A new formulation has been proposed to explicitly account for such a strain rate sensitivity and was successfully implemented in the **Polystar** model. The effect of \(\gamma ^{\prime }\) rafting on the mechanical behavior is well reproduced for both cyclic and monotonic tension tests.

Simulation numérique de la propagation de fissure dans les superalliages monocristallins: Simulation numérique de la propagation de fissure dans les monocristaux

L’objet de cette étude est de développer une méthodologie de simulation numérique de propagation de fissure par fatigue, en appliquant une approche couplée. On s’intéresse plus particulièrement à l’utilisation d’éléments d’interface cohésifs, avec une loi de comportement spécifique, afin de simuler, sans discontinuité, et avec un caractère prédictif l’avancée d’une fissure. Nous présentons, tout d’abord, le modèle d’endommagement utilise que nous validons par la suite grâce à une étude de convergence de la solution avec la taille de maille. Nous appliquons le modèle développé au cas d’une éprouvette préfissurée en superalliages monocristallin. Enfin, nous définissons la méthode d’identification des paramètres du modèle de zone cohésive sur la base des essais de propagation de fissure conduits en régime de fatigue pure.