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Research Papers: Gas Turbines: Oil and Gas Applications

A Cyclic Life Prediction Approach for Directionally Solidified Nickel Superalloys

[+] Author and Article Information
Roland Mücke, Piyawan Woratat

 ALSTOM, Brown Boveri Strasse 7, CH-5401 Baden, Switzerland

J. Eng. Gas Turbines Power 132(5), 052401 (Mar 04, 2010) (7 pages) doi:10.1115/1.3205027 History: Received April 08, 2009; Revised April 15, 2009; Published March 04, 2010; Online March 04, 2010

The performance of heavy duty gas turbines is closely related to the material capability of the components of the first turbine stage. In modern gas turbines single crystal (SX) and directionally solidified (DS) nickel superalloys are applied, which, compared with their conventionally cast version, hold a higher cyclic life and a significantly improved creep rupture strength. SX and DS nickel superalloys feature a significant directional dependence of the material properties. To fully exploit the material capability, the anisotropy needs to be accounted for in both the constitutive and 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 toward 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, material symmetry, and anisotropic fatigue strength of the material. In this 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 the measured and predicted fatigue lives of the test specimens. Finally, the proposed method is applied to the cyclic life prediction of a gas turbine blade.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Sketches of the microstructure of a conventionally cast and a directionally solidified material

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Figure 2

Anisotropic Young’s modulus of the DS materials

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Figure 3

Example for anisotropy of cyclic strength data of the DS material at elevated temperature

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Figure 4

Definition of the crystallographic orientation of uniaxial specimens

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Figure 5

Comparison of predicted and experimental uniaxial test results for the strain approach

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Figure 6

Cycles to failure (failure surface) for constant strain in logarithmic scale

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Figure 7

Comparison of predicted and experimental uniaxial test results for the stress approach

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Figure 8

Cycles to failure (failure surface) for constant stress in logarithmic scale

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Figure 9

Details on notch specimen testing

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Figure 10

Axial stress range and Hill stress

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Figure 11

Hill equivalent mechical strain and predicted number of cycles to failure

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Figure 12

Predicted first and stabilized tenth load cycles

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Figure 13

Lifetime prediction analysis

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Figure 14

Example of application to turbine components

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