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Life Prediction Method of CC and DS Ni Base Superalloys Under High Temperature Biaxial Fatigue Loading

[+] Author Affiliations
Takashi Ogata, Takayuki Sakai

Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa, Japan

Paper No. GT2009-59069, pp. 107-113; 7 pages
doi:10.1115/GT2009-59069
From:
  • ASME Turbo Expo 2009: Power for Land, Sea, and Air
  • Volume 6: Structures and Dynamics, Parts A and B
  • Orlando, Florida, USA, June 8–12, 2009
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-4887-6 | eISBN: 978-0-7918-3849-5
  • Copyright © 2009 by ASME

abstract

Polycrystalline conventional casting (CC) and directionally solidified (DS) Ni base superalloys are widely used as gas turbine blade materials. It was reported that surface of a gas turbine blade is subjected to biaxial tensile-compressive fatigue loading during start-stop operation based on finite stress analysis results. It is necessary to establish life prediction method of these superalloys under biaxial fatigue loading for reliable operation. In this study, the in-plane biaxial fatigue tests with different phase of x and y directional strain cycles were conducted on both a CC and a DS Ni base superalloys (IN738LC and GTD111DS) at high temperatures. The strain ratio, φ was defined as a ratio between x and y directional strains at 1/4 cycle and was varied from 1 to −1. In φ = 1 and −1, cracks propagated in both x and y directions in the CC supealloy. On the other hand, the main cracks of the DS superalloy propagated only in the x direction indicating failure resistance in the solidified direction is weaker than that in the direction normal to the solidified direction. Although biaxial fatigue life of the CC superalloy was correlated with conventional Mises equivalent strain range, that of DS superalloy was not. New biaxial fatigue life criterion, equivalent normal strain range for the DS superalloy was derived from iso-fatigue life curve on a principal strain plane defined in this study. Fatigue life of the DS superalloy was correlated with the equivalent normal strain range. Fatigue life of the DS superalloy under equi-biaxial fatigue loading was significantly reduced by introducing compressive strain hold dwell. Life prediction under equi-biaxial fatigue loading with the compressive strain hold was successfully made by the nonlinear damage accumulation model indicating that the proposed method can apply to life prediction of gas turbine blades under biaxial fatigue loading.

Copyright © 2009 by ASME

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