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

Experimental Validation of Fretting Fatigue Strength and Fretting Wear Rate at Contact Surface of Turbine-Blade-Shroud Cover

[+] Author and Article Information
Takeshi Kudo

e-mail: takeshi.kudo.fn@hitachi.com

Hideo Yoda

e-mail: hideo.yoda.xq@hitachi.com
Power Systems Hitachi Works, Hitachi Ltd.,
1-1, Saiwai-cho 3-chome,
Hitachi-shi, Ibaraki-ken 317-8511, Japan

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received October 18, 2012; final manuscript received September 6, 2013; published online December 10, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(4), 042101 (Dec 10, 2013) (7 pages) Paper No: GTP-12-1410; doi: 10.1115/1.4025833 History: Received October 18, 2012; Revised September 06, 2013

In continuously coupled blade structures, fretting fatigue and wear have to be considered as supposed failure modes at the contact surface of the shroud cover, which is subject to steady contact pressure from centrifugal force and the vibratory load of the blade. We did unique fretting tests that modeled the structure of the shroud cover, where the vibratory load is only carried by the contact friction force, i.e., a type of friction. What was investigated in this study are fretting fatigue strength, wear rate, and friction characteristics, such as friction coefficient and slip-range of 12%-Cr steel blade material. The friction-type tests showed that fretting fatigue strength decreases with the contact pressure and a critical normal contact force exists under which fretting fatigue failure does not occur at any vibratory load. This differs from knowledge obtained through pad-type load carry tests that fretting fatigue strength decreases with the increase of contact pressure and that it almost saturates under a certain contact pressure. Our detailed observation in the friction-type tests clarified that this mechanism was the low contact pressure narrowing the contact area and a resulting high stress concentration at a local area. The fretting wear rate was explained by the dissipated energy rate per cycle obtained from the measured hysteresis loop between the relative slip range and the tangential contact force. It was found that the fretting wear rate is smaller than the wear rate obtained by one-way sliding tests, and the former is much smaller than the latter as the dissipated energy decreases. Finally, to prevent fretting fatigue and wear, we propose an evaluation design chart of the contact surface of the shroud cover based on our friction-type fretting tests.

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Fig. 1

Structure of continuously coupled blade

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Fig. 2

Comparison of fretting fatigue dependence on contact pressure between load carrying and friction tests

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Fig. 3

Experimental setup

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Fig. 4

Shape of test pieces

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Fig. 5

Definition of nominal stress

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Fig. 6

Schematic view of relation between tangential contact force and relative displacement

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Fig. 7

Variation of tangential contact force coefficient, slip range in type P, (a) p = 3.4 × pc_P, Δa = 79 μm, (b) p = 0.85 × pc_P, Δa = 66 μm

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Fig. 8

Relation between tangential contact force coefficient and slip-range in type P

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Fig. 9

Fretting fatigue test results for three kinds of test pieces, (a)–(c) relation between normalized normal force and Δa, (d)–(e) normalized stress amplitude against normalized contact pressure, and (g)–(i) relation between normalized contact pressure and slip-range, where open marks mean nonfracture data under 2 × 107 cycles and solid marks mean fracture data

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Fig. 10

Comparison of critical contact force Fnc obtained by experiments and calculation

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Fig. 11

Distance from crack to geometric contact edge obtained by tests in type P

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Fig. 12

FE analysis model in type P

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Fig. 13

Definition of reference stress

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Fig. 14

Stress distribution from contact edge along evaluation path

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Fig. 15

Relation between noncontact length da and reference stress concentration when da = db

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Fig. 16

Schematic illustration of one-way sliding wear test. (Thrust cylinder type.)

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Fig. 17

Measured friction coefficient obtained from one-way sliding wear tests. (p = 1.2 MPa, ω = 2 rpm)

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Fig. 18

Wear volume obtained by one-way sliding wear tests

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Fig. 19

Comparison of fretting wear volume and accumulated consumption energy

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Fig. 20

Comparison of fretting wear volume and consumption energy per cycle

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Fig. 21

Schematic view of design diagram of shroud cover




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