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Research Papers

Fatigue Life Prediction for Overlap Friction Stir Linear Welds of Magnesium Alloys

[+] Author and Article Information
Ruijie Wang

Department of Mechanical Engineering,
Kunming University of Science and Technology,
Kunming 650500, China;
Department of Mechanical Engineering,
University of Michigan-Dearborn,
Dearborn, MI 48128

Hong-Tae Kang

Department of Mechanical Engineering,
University of Michigan-Dearborn,
Dearborn, MI 48128

Chonghua (Cindy) Jiang

AET Integration, Inc.,
Troy, MI 48084

1Corresponding author.

Manuscript received December 22, 2014; final manuscript received December 21, 2015; published online March 9, 2016. Assoc. Editor: Blair E. Carlson.

J. Manuf. Sci. Eng 138(6), 061013 (Mar 09, 2016) (7 pages) Paper No: MANU-14-1704; doi: 10.1115/1.4032469 History: Received December 22, 2014; Revised December 21, 2015

This work was undertaken to analyze the stress/strain state at the critical sites in friction stir welded specimens and, further, to assess the fatigue strength of friction stir welded specimens with conventional fatigue life prediction approaches. Elastoplastic and elastic finite-element stress/strain analyses were carried out for friction-stir-linear-welded (FSLW) specimens made of magnesium alloys. The calculated stress/strain at the periphery of the weld nugget was used to evaluate the fatigue life with local life prediction approaches. First, elastoplastic finite-element models were built according to experimental specimen profiles. Fatigue life prediction was conducted with Morrow's modified Manson–Coffin (MC) damage equation and the Smith–Watson–Topper (SWT) damage equation, respectively, for different specimens under different loading cases. Life prediction results showed that both equations can to some extent give reasonable results, especially within a low-cycle fatigue life regime, with the SWT damage equation giving more conservative results. As for high-cycle life, predicted results were much longer and scattered for both methods. Shell element elastic models were then used to calculate the structural stress at the periphery of the weld nuggets. The correlation between structural stress amplitude and experimental life showed the appropriateness of the structural stress fatigue evaluation for friction stir welds. The effect of the notches at the periphery of the faying surface on life prediction was further discussed.

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Figures

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Fig. 1

FSW specimen configuration

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Fig. 2

Specimen weldline cross sections (a) AZ31 to AM60, (b) AZ31 to AZ31, and (c) AM30 to AM60

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Fig. 3

Typical fatigue fracture mode

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Fig. 4

Experimental results in nominal stress amplitude

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Fig. 5

Finite-element meshes at weld cross section

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Fig. 6

Stable cyclic stress–plastic strain curves for three materials

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Fig. 7

Mises stress (in MPa) distribution near the nugget root

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Fig. 8

The strain state near weld root. (a) Maximum Mises stress and three principal stress (MPa) history and (b) three principal strain history.

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Fig. 9

Life prediction results of MC and SWT approaches

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Fig. 10

FEA representation for a welded joint

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Fig. 11

Mises stress distribution obtained from linear elastic FEA

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Fig. 12

Structural stress range versus fatigue life

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Fig. 13

Nominal stress range versus fatigue life (a) and structural stress range versus fatigue life for 6061-T6 specimens (b)

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Fig. 14

Predicted life versus experimental life using structural stress approach for AZ31–AM60 specimens

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