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Research Papers

Measurement and Assessment of Fatigue Life of Spot-Weld Joints

[+] Author and Article Information
Ahmet H. Ertas

Department of Mechanical Engineering, Bogazici University, Bebek, Istanbul 34342, Turkey

Oktem Vardar

 Isik University, Sile, Istanbul 34398, Turkey

Fazil O. Sonmez

Department of Mechanical Engineering, Bogazici University, Bebek, Istanbul 34342, Turkeysonmezfa@boun.edu.tr

Zafer Solim

 Mercedes-Benz Turk A.S., Bahcesehir, Istanbul 34500, Turkey

J. Eng. Mater. Technol 131(1), 011011 (Dec 22, 2008) (11 pages) doi:10.1115/1.3030941 History: Received February 14, 2008; Revised October 11, 2008; Published December 22, 2008

Spot-weld joints are commonly used to fasten together metal sheets. Because fatigue fracture is the most critical failure mode for these joints under fluctuating loads, understanding their fatigue failure behavior and assessment of their fatigue lives are crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded modified tensile-shear specimens made of a low carbon steel. Two different types of resistance spot welding were investigated (manual and automated). Tests were repeated under different load ranges, and the corresponding fatigue lives were determined. The specimens were also examined under an optical microscope. In the numerical part of this study, a finite element analysis was carried out using commercial software, ANSYS , to determine the stress and strain states within the specimens. The material nonlinearity, local plastic deformations around the welds during loading, and the residual stresses and strains developed after unloading as a result of plastic deformations were taken into account. Based on the predicted stress and strain states, fatigue analyses were performed using several models for life assessment. Then, the measured and predicted fatigue lives were compared, and the suitability of the models was discussed. Among the strain-based models, Coffin–Manson and Morrow’s means stress models yielded the best predictions.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Engineering stress-strain curve

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Figure 2

Geometry of the MTS specimens

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Figure 3

Cracks in a tested and deformed automated-type spot weld: (a) larger view and (b) focused on the left region

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Figure 4

An automated-type spot weld in an untested MTS specimen: (a) larger view and (b) focused on the right region

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Figure 5

A manual-type spot weld in an untested MTS specimen: (a) larger view and (b) focused on the right region

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Figure 6

Porosity in a manual spot-welded MTS specimen

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Figure 7

Depiction of the failure modes observed in the spot-welded MTS specimens

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Figure 8

Typical failure mode (Type-2B) for MTS specimens

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Figure 9

The load transfer path for the MTS specimens under loading

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Figure 10

The resulting curve of the regression analysis for both the manual-type and the automated-type spot welds

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Figure 11

Convergence in terms of element size

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Figure 12

Finite element model for the MTS specimen

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Figure 13

A detail of the mesh at and around the right spot weld of the specimen

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Figure 14

Convergence in terms of substeps

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Figure 15

Boundary conditions of the finite element model

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Figure 16

The distribution of the σxx component (in megapascals) of stress on the inner (upper figure) and outer (lower figure) surfaces of the central plate developed due to maximum load (2700 N)

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Figure 17

The distribution of the σxx component (in megapascals) of stress on the inner (upper figure) and outer (lower figure) surfaces of the central plate developed due to minimum load (150 N)

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Figure 18

The distribution of the τxz component (in megapascals) of stress on the inner surface of the central plate developed due to maximum load (2700 N)

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Figure 19

Comparison of the fatigue lives predicted using stress-based approaches and experimental results

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Figure 20

Comparison of the fatigue lives predicted using strain-based approaches and the experimental results

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Figure 21

Comparison of the fatigue lives predicted using Morrow’s mean stress model with different equivalent strain approaches

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Figure 22

Comparison of the fatigue lives predicted for different gap values using Morrow’s mean stress approach and experimental results

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Figure 23

Comparison of the fatigue lives obtained experimentally by Pan and Sheppard (17) and Pan (18) and predicted using Morrow’s mean stress approach

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