The safe and efficient operation of modern heavy duty gas turbines requires a reliable prediction of fatigue behavior of turbine components. Fatigue damage is located in areas where cyclic stress and strain amplitudes are highest. Thus, geometrical notches associated with stress/strain concentrations and stress/strain gradients appear to be the most important sites for fatigue crack initiation. The paper addresses a nonlocal concept for cyclic life prediction of notched components. Contrary to various local approaches in the field, the proposed method explicitly accounts for stress and strain gradients associated with notches arising from grooves, cooling holes, fillets, and other design features with stress raising effect. As a result, empirical analytical expressions for considering either strain or stress gradients for cyclic life prediction are obtained. The method has been developed from cyclic test data on smooth and notched specimens made of a ferritic 1.5CrNiMo rotor steel. The analytical formulations obtained have then been applied to test data on the nickel base superalloy MAR-M247 CC showing a good agreement between prediction and measurement. Moreover, the proposed nonlocal lifing concept has been validated by component tests on turbine blade firtrees. The predicted number of cycles to failure correlates well with the experimental results showing the applicability of the proposed method to complex engineering designs.
The performance of heavy duty gas turbines is closely related to the material capability of the components of the 1st 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.
In the paper the well-known strain-and stress-based approach to cyclic fatigue assessment of isotropic materials is extended towards anisotropic structures. The methodology is considered in detail for transverse isotropy, as it occurs in directionally solidi"ed materials. Some remarks on cubic anisotropy for singlecrystal applications are given as well. Using a modi"ed HILL approach to describe the anisotropic failure surface, the HILL parameters, which are a function of the actual loading, are expressed in terms of the parameters of the failure laws for independent material axes. Finally, the procedure is veri"ed by experiments in additional directions showing that the experimental results "t well the results of computational anisotropic failure assessment.
SUMMARYA p-version of the finite element method and an hp-extension is applied to a geometrically non-linear model problem. Starting from a standard formulation in finite elasticity, some implementation details are outlined. The robustness and efficiency of this method, as already well known for linear problems, is demonstrated. In two numerical examples high accuracy and exponential rate of convergence in energy are shown. The method presented can be used effectively in problems where standard low-order plane strain elements fail to give accurate results.
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