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Biaxial Thermo Mechanical Fatigue Life Propety of a Ni Base DS Super Alloy

[+] Author Affiliations
Takashi Ogata, Masato Yamamoto

Central Research Institute of Electric Power Industry, Komae, Tokyo, Japan

Paper No. GT2006-90758, pp. 979-984; 6 pages
doi:10.1115/GT2006-90758
From:
  • ASME Turbo Expo 2006: Power for Land, Sea, and Air
  • Volume 4: Cycle Innovations; Electric Power; Industrial and Cogeneration; Manufacturing Materials and Metallurgy
  • Barcelona, Spain, May 8–11, 2006
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 0-7918-4239-8
  • Copyright © 2006 by ASME

abstract

Development of life assessment methods of high temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated. Thermo-Mechanical-Fatigue (TMF) loading during start-steady state-stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between 450C and 870C on a Ni base directional solidified (DS) supper alloy. Strain ratio, φ was defined as shear strain range, Δγ, to normal strain range, Δε, and φ varied from 0 to infinite. The “Blade waveform” which simulated surface temperature and strain loading condition of the blade, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under biaxial TMF loading showed strain ratio dependency giving shorter life with increasing φ. Considering biaxial stress effect on failure life, an equivalent shear strain range was derived based on Γ -plane theory, and biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade waveform. It was concluded that creep damage gradually accumulated during cycles resulting in TMF life reduction. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, failure life could be predicted within factor of 1.5 on observed life. It was found that fatigue life of CoCrAlY coated material reduced 1/2 to 1/3 from that of without coated material. From observation of longitudinal section of the coated specimens, many cracks started from coating surface and penetrated into the substrate. It was concluded that CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

Copyright © 2006 by ASME
Topics: Alloys , Fatigue life

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