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Research Papers

A Modified Closed Form Energy Based Framework for Fatigue Life Assessment for Aluminum 6061-T6—Damaging Energy Approach

[+] Author and Article Information
M.-H. Herman Shen

Professor
Fellow ASME
Department of Mechanical
and Aerospace Engineering,
The Ohio State University,
201 W. 19th Avenue,
Columbus, OH 43210
e-mail: shen.1@osu.edu

Sajedur R. Akanda

Department of Mechanical
and Aerospace Engineering,
The Ohio State University,
201 W. 19th Avenue,
Columbus, OH 43210
e-mail: akanda.2@buckeyemail.osu.edu

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received August 15, 2014; final manuscript received December 15, 2014; published online January 30, 2015. Assoc. Editor: Tetsuya Ohashi.

J. Eng. Mater. Technol 137(2), 021008 (Apr 01, 2015) (7 pages) Paper No: MATS-14-1166; doi: 10.1115/1.4029532 History: Received August 15, 2014; Revised December 15, 2014; Online January 30, 2015

A previously developed energy based high cycle fatigue (HCF) life assessment framework is modified to predict the low cycle fatigue (LCF) life of aluminum 6061-T6. The fatigue life assessment model of this modified framework is formulated in a closed form expression by incorporating the Ramberg–Osgood constitutive relationship. The modified framework is composed of the following entities: (1) assessment of the average strain energy density and the average plastic strain range developed in aluminum 6061-T6 during a fatigue test conducting at the ideal frequency for optimum energy calculation, and (2) determination of the Ramberg–Osgood cyclic parameters for aluminum 6061-T6 from the average strain energy density and the average plastic strain range. By this framework, the applied stress range is related to the fatigue life by a power law whose parameters are functions of the fatigue toughness and the cyclic parameters. The predicted fatigue lives are found to be in a good agreement with the experimental data.

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Figures

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Fig. 1

A representative hysteresis loop of size of the average plastic strain range Δɛ¯p and the area of the average damaging energy w¯d to cause fatigue failure of a material at stress range Δσ

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Fig. 2

The hysteresis loop in Fig. 1 with the compressive peak point shifted at the origin of the axes

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Fig. 3

Schematic diagram of a flat dogbone specimen for a monotonic tension test

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Fig. 4

Experimental setup of a monotonic tension test to obtain the mechanical properties and the energy for monotonic fracture

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Fig. 5

Schematic diagram of a round dogbone specimen for a fatigue test

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Fig. 6

Experimental setup of a fatigue test with a high resolution extensometer to record the stress–strain hysteresis loop

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Fig. 7

Engineering stress–strain curve of aluminum 6061-T6

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Fig. 8

True stress–strain curve of aluminum 6061-T6

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Fig. 9

Measuring area under the true stress–strain curve of Fig. 8 by image processing. The gray region quantifies the energy required to break aluminum 6061-T6 by a monotonic tension test.

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Fig. 10

(a) Plastic strain range history and (b) plastic energy history of aluminum 6061-T6 developed from a fatigue test of stress amplitude of 83% of yield strength. Test frequency was 0.05 Hz.

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Fig. 11

Hysteresis loops of aluminum 6061-T6. Cycle 2 is in region 1, cycle 8018 is in region 2, and cycle 16318 is in region 3 of Fig. 10.

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Fig. 12

Experimental and predicted S–N curves for aluminum 6061-T6

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