Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

Engineering Approach for Low-Cycle Fatigue Assessment of Porous Alloys

[+] Author and Article Information
Piotr Bednarz

Alstom Power,
Baden, Switzerland
e-mail: piotr.bednarz@power.alstom.com

Jarosław Szwedowicz

Alstom Power,
Baden, Switzerland
e-mail: jaroslaw.szwedowicz@power.alstom.com

1Corresponding author.

Contributed by the Manufacturing Materials and Metallurgy Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 29, 2015; final manuscript received August 3, 2015; published online October 21, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(4), 042101 (Oct 21, 2015) (8 pages) Paper No: GTP-15-1380; doi: 10.1115/1.4031374 History: Received July 29, 2015; Revised August 03, 2015

Most components used in gas and steam turbines are metallic parts produced by either casting or forging processes. Although process control works to eliminate defects, there can be variation in microporosity from component to component. Previously, this microporosity was only able to be detected destructively using metallography. Using computer tomography (CT), one can find voids in the range of a few tenths of a millimeter and know the location of the voids with high precision. This allows one to map the defects present in each component onto the stress and temperature fields for that component. However, there is not yet universal agreement upon a consistent method to evaluate the effect of these small porosities on a components lifetime. Having a robust analysis tool to understand the impact of microporosity would decrease development costs, decrease the time to bring a product to market, and increase the likelihood of failure-free operation. This paper presents an approach using equivalent low-cycle fatigue (LCF) material properties which avoids the need to explicitly model the morphology of the microstructure in the region of the microporosity. The homogenization methodology calculates new LCF curves depending on porosity ratios in material. This approach uses Morrows correlation factor of LCF cycles to crack initiation regarding energy amount dissipated in stable cycling (shakedown) and ultimate strain energy under monotonic loading. The paper generalizes Morrows postulate and formulates the hypothesis that energy stored and dissipated in the material under shakedown conditions corresponds directly to the number of LCF cycles to crack initiation. The paper demonstrates that the reduction of LCF life based on the porosity ratio agrees well with the experimental results. These results also show that the methodology is very sensitive to the void orientation and loading direction.

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Fig. 6

Linear relationship of the cyclic strain to the cyclic energy

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Fig. 5

Example of Coffin–Manson lifetime curve for INCONEL718 [1]

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Fig. 4

Specimen responses under monotonic loading for material with different porosity ratios computed with the conventional (solid curves) and elastoviscoplastic Chaboche damage model [1] (dotted lines)

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Fig. 3

Illustration of the Martin and Feltner and Morrow hypotheses for monotonic (a) and cyclic (b) loading, illustrating the physical meaning of Eqs. (1) and (2)

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Fig. 2

Aspect ratios r of a void illustrated for the aspect ratio r varying from 0.1 to 1.0, where r = 1.0 corresponds to a sphere with ra=rb=rc

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Fig. 1

Illustrations of various small voids and microporosities after (a) casting manufacturing, (b) welding, and (c) braze repair of gas turbine components

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Fig. 11

IN738LC LCF (strain-life) tests with porosities (FVV No. 696) at a temperature of 700 °C (left-hand side picture) and 850 °C (right-hand side picture)

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Fig. 7

Scaling factors for porous material

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Fig. 15

Generated populations of IN738LC LCF lifetime curves for all void parameters with respect to a measured porosity of 9.6% at 700 °C

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Fig. 16

Generated population LCF lifetime curves and the experimental data of IN738LC measured at 850 °C

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Fig. 8

Kinematic hardening responses for different porosity consideration

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Fig. 9

Equivalent response calculated using Massings hypothesis and cyclic loading in Digimat-MF

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Fig. 10

Example of Massings hypothesis application to cyclic loading

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Fig. 12

Calculated ε−N (strain-life) sensitivity in terms of porosity amount, aspect ratio r, and relative orientation to the loading direction, where green, orange, and red boxes correspond with in-phase (θ=0 deg), arbitrary (0 deg<θ<90 deg), and out-of-phase (θ=90 deg) orientation, respectively

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Fig. 13

Strain-life of porous IN738LC in terms of porosity amount for the in-phase orientation angle θ=0 deg related to the reference (nonporous) alloy given in the material database

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Fig. 14

Impact of shear deformations on LCF (strain-life) crack initiation of porous IN738LC in terms of porosity orientation and aspect ratio



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