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Research Papers: JOINING

Failure Mode and Fatigue Behavior of Dissimilar Friction Stir Spot Welds in Lap-Shear Specimens of Transformation-Induced Plasticity Steel and Hot-Stamped Boron Steel Sheets

[+] Author and Article Information
S. H. Hong, S.-J. Sung

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109

J. Pan

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: jwo@umich.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received December 1, 2014; final manuscript received July 25, 2015; published online September 4, 2015. Assoc. Editor: Blair E. Carlson.

J. Manuf. Sci. Eng 137(5), 051023 (Sep 04, 2015) (8 pages) Paper No: MANU-14-1649; doi: 10.1115/1.4031235 History: Received December 01, 2014; Revised July 25, 2015

Failure mode and fatigue behavior of dissimilar friction stir spot welds in lap-shear specimens of transformation-induced plasticity steel (TRIP780) and hot-stamped boron steel (HSBS) sheets are examined in this paper. Optical micrographs of the dissimilar TRIP780/HSBS friction stir spot welds made by a concave silicon nitride tool before and after testing are obtained and examined. These micrographs indicate that subject to quasi-static and cyclic loading conditions, the TRIP780/HSBS welds fail from cracks growing through the TRIP780 sheets where the tool was plunged into and the thickness was reduced. The bending moments and the transverse shear force near the welds are derived with consideration of the load offset, the weld gap, and the bend distance for calculation of analytical global stress intensity factor solutions for the welds in lap-shear specimens. A fatigue model of kinked crack growth is used to estimate fatigue lives based on the local stress intensity factor solutions for kinked cracks. The estimated fatigue lives with consideration of the weld gap and the bend distance are in agreement with the fatigue test results under low-cycle loading conditions and lower than the fatigue test results under high-cycle loading conditions. The estimated fatigue lives suggest that the weld gap and the bend distance can significantly affect fatigue lives of the friction stir spot welds in lap-shear specimens under cyclic loading conditions.

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References

Figures

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Fig. 1

(a) A silicon nitride tool tip, (b) a top view of a lap-shear specimen, and (c) a schematic of a lap-shear specimen with a spot weld

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Fig. 2

(a) A micrograph of a weld and (b) a close-up micrograph near the left crack tip

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Fig. 3

Fatigue test results and estimated fatigue lives for TRIP780/HSBS welds

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Fig. 4

(a) The cross section of a failed weld and (b) the failed weld on the HSBS sheet under quasi-static loading conditions

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Fig. 5

(a) The cross section of a failed weld and (b) the failed weld on the HSBS sheet under low-cycle fatigue loading conditions

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Fig. 6

(a) The cross section of a partially failed weld, (b) the cross section of a failed weld, and (c) the failed weld on the HSBS sheet under high-cycle loading conditions

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Fig. 7

(a) A schematic of a friction stir spot weld with the load offset s, the weld gap g, and the bend distance e in a lap-shear specimen under the force F, the bending moments MA and MB, and the transverse shear force V at the grips; (b) a free-body diagram for the lower sheet from the edge of the left grip to the center of the friction stir spot weld; (c) a free-body diagram for the friction stir spot weld with the force F, the transverse shear force V, and the bending moments MC ; and (d) a free-body diagram for the friction stir spot weld with the force F1, the transverse shear force V, and the bending moments Ml and Mu.

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Fig. 8

Schematic plots of the cross sections near dissimilar TRIP780/HSBS spot welds under (a) quasi-static/low-cycle and (b) high-cycle loading conditions

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Fig. 9

A schematic of a friction stir spot weld in a lap-shear specimen under clamped grip loading conditions

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