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Research Papers: Gas Turbines: Structures and Dynamics

A Mechanism-Based Approach From Low Cycle Fatigue to Thermomechanical Fatigue Life Prediction

[+] Author and Article Information
Xijia Wu, Zhong Zhang

National Research Council Canada,
Ottawa, ON K1A 0R6, Canada

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received September 28, 2015; final manuscript received October 16, 2015; published online December 4, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(7), 072503 (Dec 04, 2015) (7 pages) Paper No: GTP-15-1462; doi: 10.1115/1.4031908 History: Received September 28, 2015; Revised October 16, 2015

Deformation and damage accumulation occur by fundamental dislocation and diffusion mechanisms. An integrated creep–fatigue theory (ICFT) has been developed, based on the physical strain decomposition rule that recognizes the role of each deformation mechanism, and thus relate damage accumulation to its underlying physical mechanism(s). The ICFT formulates the overall damage accumulation as a holistic damage process consisting of nucleation and propagation of surface/subsurface cracks in coalescence with internally distributed damage/discontinuities. These guiding principles run through both isothermal low cycle fatigue (LCF) and thermomechanical fatigue (TMF) under general conditions. This paper presents a methodology using mechanism-based constitutive equations to describe the cyclic stress–strain curve and the nonlinear damage accumulation equation incorporating (i) rate-independent plasticity-induced fatigue, (ii) intergranular embrittlement (IE), (iii) creep, and (iv) oxidation to predict LCF and TMF lives of ductile cast iron (DCI). The complication of the mechanisms and their interactions in this material provide a good demonstration case for the model, which is in good agreement with the experimental observations.

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Figures

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Fig. 1

Cyclic stress–strain curve for DCI at (a) RT, (b) 400 °C, (c) 600 °C, and (d) 800 °C

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Fig. 2

LCF life versus the total strain amplitude for DCI

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Fig. 3

LCF mechanism map for DCI at strain rate of 0.02/s

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Fig. 4

The hysteresis behavior: (a) and (b) strain accumulation during OP-TMF in the range of 300–800 °C

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Fig. 5

The hysteresis behavior: (a) and (b) strain accumulation during IP-TMF in the range of 300–800 °C

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Fig. 6

The hysteresis behavior: (a) and (b) strain accumulation during constrained TMF with the constraint ratio of 100% in the range of 160–600 °C

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Fig. 7

Comparison of the model predicted life with experiments for DCI

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Fig. 8

OP-TMF specimen failed with ±0.15% mechanical strain in the temperature range of 450–800 °C: (a) the specimen surface and (b) the fracture surface

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Fig. 9

OP-TMF specimen failed with ±0.25% mechanical strain in the temperature range of 450–800 °C: (a) the specimen surface and (b) the fracture surface

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Fig. 10

IP-TMF failed with ±0.3% mechanical strain in the temperature range of 300–800 °C: (a) the specimen surface and (b) the fracture surface

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Fig. 11

Fracture surface of TMF specimen with a constraint ratio of 100% in the temperature range of 160–600 °C

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