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Research Papers: Gas Turbines: Structures and Dynamics

Development of a Probabilistic Methodology for Predicting Hot Corrosion Fatigue Crack Growth Life of Gas Turbine Engine Disks

[+] Author and Article Information
Kwai S. Chan

Fellow ASME

Jonathan P. Moody

Southwest Research Institute,
San Antonio, TX 78238

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 15, 2013; final manuscript received August 20, 2013; published online November 1, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 022505 (Nov 01, 2013) (8 pages) Paper No: GTP-13-1306; doi: 10.1115/1.4025555 History: Received August 15, 2013; Revised August 20, 2013

Advanced Ni-based gas turbine disks are expected to operate at higher service temperatures in aggressive environments for longer time durations. Exposures of Ni-based alloys to alkaline-metal salts and sulfur compounds at elevated temperatures can lead to hot corrosion fatigue crack growth in engine disks. Type II hot corrosion involves the formation and growth of corrosion pits in Ni-based alloys at a temperature range of 650 °C to 750 °C. Once formed, these corrosion pits can serve as stress concentration sites where fatigue cracks can initiate and propagate to failure under subsequent cyclic loading. In this paper, a probabilistic methodology is developed for predicting the corrosion fatigue crack growth life of gas turbine engine disks made from a powder-metallurgy Ni-based superalloy (ME3). The key features of the approach include: (1) a pit growth model that describes the depth and width of corrosion pits as a function of exposure time, (2) a cycle-dependent crack growth model for treating fatigue, and (3) a time-dependent crack growth model for treating corrosion. This set of deterministic models is implemented into a probabilistic life-prediction code called DARWIN. Application of this approach is demonstrated for predicting corrosion fatigue crack growth life in a gas turbine disk based on the ME3 properties from the literature. The results of this study are used to assess the conditions that control the transition of a corrosion pit to a fatigue crack and to identify the pertinent material parameters influencing corrosion fatigue life and disk reliability.

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Figures

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Fig. 1

Schematics of time-dependent pit and crack growth in Ni-based superalloys during corrosion fatigue at elevated temperatures. Modified from Wang et al. [14].

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Fig. 2

Pit depth or width as a function of stress based on the LCF specimens of ME3. The low stress ground (LSG) specimens contained little or no residual stresses, while the shot-peened specimens contained residual surfaces which were fully characterized by Gabb et al. [5].

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Fig. 3

Computed pit aspect ratio 2 d/w and normalized driving force ΔKKth compared to the experimental data as a function of the pit width: (a) LSG specimens without residual stresses, and (b) SP specimens with residual stresses

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Fig. 4

Comparison of the computed pit aspect ratio 2 d/w and the normalized driving force K/Kth compared to the experimental data as a function of the pit width

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Fig. 5

Pit aspect ratio 2 d/w and computed kt as a function of the pit width

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Fig. 6

Comparison of the pit growth rates of the depth d· and width w· against the fatigue crack growth rate da/dt = f da/dN, as a function of the pit width. Pit growth dominates at ΔK < Kth, while fatigue crack growth dominates at ΔKKth.

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Fig. 7

A comparison of the damage mechanisms for Type II hot corrosion fatigue and pure fatigue in a Kitagawa diagram: (a) Type II hot corrosion fatigue, and (b) pure fatigue. Experimental data are from Gabb et al. [5].

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Fig. 8

Comparison of the measured and computed da/dN response of ME3 at several temperatures

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Fig. 9

Comparison of the computed and measured da/dt response of ME3 at several temperatures

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Fig. 10

Fatigue crack growth rate da/dN as a function of frequency for ME3 at ΔK = 16.5 MPa (m1/2) and R = 0.5

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Fig. 11

A schematic illustration of the dependence of the population of critical pit size on hot corrosion time and how it affects the risk of corrosion fatigue crack growth in an engine disk

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Fig. 12

Fracture model superimposed upon a generic gas turbine engine

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Fig. 13

DARWIN pit and crack growth simulation of Type II hot corrosion fatigue crack growth in a generic ME3 disk

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Fig. 14

DARWIN risk simulation of Type II hot corrosion fatigue crack growth in a generic ME3 disk

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