Research Papers: Gas Turbines: Structures and Dynamics

Strain Rate and Loading Waveform Effects on an Energy-Based Fatigue Life Prediction for AL6061-T6

[+] Author and Article Information
Todd Letcher

e-mail: etcher.7@osu.edu

M.-H. H. Shen

e-mail: shen.1@osu.edu
Mechanical and Aerospace
Engineering Department,
Building 148, 201 W. 19th Ave.,
The Ohio State University,
Columbus, OH 43210

Charles Cross

Turbine Engine Fatigue Facility,
Air Force Research Laboratory,
Wright-Patterson AFB, OH 45433

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received May 29, 2013; final manuscript received August 28, 2013; published online November 1, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(2), 022502 (Nov 01, 2013) (6 pages) Paper No: GTP-13-1150; doi: 10.1115/1.4025497 History: Received May 29, 2013; Revised August 28, 2013

The energy-based lifing method is based on the theory that the cumulative energy in all hysteresis loops of a specimens' lifetime is equal to the energy in a monotonic tension test. Based on this theory, fatigue life can be calculated by dividing monotonic strain energy by a hysteresis energy model, which is a function of stress amplitude. Recent studies have focused on developing this method for a sine wave loading pattern—a variable strain rate. In order to remove the effects of a variable strain rate throughout the fatigue cycle, a constant strain rate triangle wave loading pattern was tested. The testing was conducted at various frequencies to evaluate the effects of multiple constant strain rates. Hysteresis loops created with sine wave loading and triangle loading were compared. The effects of variable and constant strain rate loading patterns on hysteresis loops throughout a specimens' fatigue life are examined.

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Fig. 1

Hysteresis loop in generalized coordinates

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Fig. 2

Specimen dimensions

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Fig. 3

Fatigue test setup using an MTS load frame

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Fig. 4

Sinusoidal and triangular loading waveforms

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Fig. 5

Hysteresis loop—sine wave, 241.3 MPa, 2 Hz

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Fig. 6

Hysteresis loop—triangle wave, 241.3 MPa, 2 Hz

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Fig. 7

Different specimen for stress level and waveform (refer to Table 1)

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Fig. 8

Same specimen for each stress level (refer to Table 2)

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Fig. 9

Cumulative energy over specimen lifetime

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Fig. 10

Fatigue life predictions based on two loading waveforms on a single specimen



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