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Materials and Fabrication

Failure Mechanism of Laser Welds in Lap-Shear Specimens of a High Strength Low Alloy Steel

[+] Author and Article Information
Jwo Pan

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

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNALOF PRESSURE VESSEL TECHNOLOGY. Manuscript received March 17, 2011; final manuscript received March 16, 2012; published online October 18, 2012. Assoc. Editor: Xian-Kui Zhu.

J. Pressure Vessel Technol 134(6), 061402 (Oct 18, 2012) (5 pages) doi:10.1115/1.4006560 History: Received March 17, 2011; Revised March 16, 2012

In this study, the failure mechanism of laser welds in lap-shear specimens of a high strength low alloy (HSLA) steel under quasi-static loading conditions is examined based on the experimental and computational results. Optical micrographs of the welds in the specimens before tests were examined to understand the microstructure near the weld. A micrographic analysis of the failed welds in lap-shear specimens indicates a ductile necking/shear failure mechanism near the heat affected zone. Micro-hardness tests were conducted to provide an assessment of the mechanical properties of the joint area which has varying microstructure due to the welding process. A finite element analysis was also carried out to identify the effects of the weld geometry and different mechanical properties of the weld and heat affected zones on the failure mechanism. The results of the finite element analysis show that the geometry of the weld protrusion and the higher effective stress–plastic strain curves of the heat affected and weld zones result in the necking/shear failure of the load carrying sheet. The deformed shape of the finite element model near the weld matches well with that near a failed weld. A finite element analysis based on the Gurson yield function with consideration of void nucleation and growth was also carried out. The results of the finite element analysis indicate that the location of the material elements with the maximum void volume fraction matches well with that of the initiation of ductile fracture as observed in the experiments.

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References

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Figures

Grahic Jump Location
Fig. 1

(a) A top view and (b) a bottom view of a laser welded lap-shear specimen and (c) a schematic of a lap-shear specimen

Grahic Jump Location
Fig. 2

A micrograph of the cross section of a laser weld in a lap-shear specimen

Grahic Jump Location
Fig. 3

(a) A micrograph of the cross section of a weld after the microhardness indentations and (b) the distributions of the hardness values across the weld

Grahic Jump Location
Fig. 4

Load–displacement curves from three quasi-static tests

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Fig. 5

Micrographs of the cross sections near the welds from (a) a specimen just prior to failure and (b) a failed specimen

Grahic Jump Location
Fig. 6

A schematic of the six zone finite element model with different material sections near the weld

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Fig. 7

Equivalent plastic strain distributions near a weld from the finite element analysis based on the six zone model at a displacement of (a) 0.1 mm and (b) 2.0 mm

Grahic Jump Location
Fig. 8

(a) The distribution of the void volume fraction of the finite element analysis at the displacement of 1.2 mm and (b) an SEM picture of the upper portion of the fracture surface of the lower left loading carrying sheet

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