A Cyclic Life Prediction Approach for Directionally Solidified Nickel Superalloys

The performance of heavy duty gas turbines is closely related to the material capability of the components of the 1^st turbine stages. In modern gas turbines single crystal (SX) and directionally solidified (DS) nickel superalloys are applied which, compared to their conventionally cast (CC) version, hold a higher cyclic life and a significantly improved creep rupture strength. SX and DS nickel superalloys feature a significant directionally dependence of material properties. To fully exploit the material capability, the anisotropy needs to be accounted for in both, the constitutive and the lifing model. In this context, the paper addresses a cyclic life prediction procedure for DS materials with transverse isotropic material symmetry. Thereby, the well-known local approaches to fatigue life prediction of isotropic materials under uniaxial loading are extended towards materials with transverse isotropic properties under multiaxial load conditions. As part of the proposed methodology, a Hill type function is utilized for describing the anisotropic failure behavior. The coefficients of the Hill surface are determined from the actual multiaxial loading, the material symmetry and the anisotropic fatigue strength of the material. In the paper we first characterize the anisotropy of DS superalloys. We then present the general mathematical framework of the proposed lifing procedure. Later we discuss a validation of the cyclic life model by comparing measured and predicted fatigue lives of test specimens. Finally, the proposed method is applied to the cyclic life prediction of a gas turbine blade.