Tensile-Shear Forces and Fracture Modes in Single and Multiple Weld Specimens in Dual-Phase Steels

[+] Author and Article Information
Manuel Marya

 Colorado School of Mines, Golden, CO 80401

Kathy Wang

 GM Global Performance Integration, Warren, MI 48090

Louis G. Hector, Xiaohong Gayden

 GM R&D Center, Warren, MI 48090

J. Manuf. Sci. Eng 128(1), 287-298 (Aug 30, 2005) (12 pages) doi:10.1115/1.2137751 History: Received April 21, 2005; Revised August 30, 2005

In this article, weld fracture criteria based upon low strain rate (i.e., ε̇103102s1) tensile-shear tests of spot welds in dual-phase (DP) steels DP600, DP780, and DP980 are developed. Three empirical equations are inferred from least-squares root-fitting analyses of tensile-shear testing data. Building upon existing results in the literature, the first equation relates the tensile-shear force to the weld diameter. The second and third equations relate, respectively, a critical weld diameter and a critical tensile-shear force for interfacial fracture to the sheet thickness and hardness extrema in the heat-affected zone. These idealized equations can serve as the basis for further development of fracture criteria resembling material flow laws that account for higher strain rates and more complicated deformation paths. The effect of spot-weld placement in specific patterns or arrays on deformation and fracture behavior was also investigated to explore underlying effects from deformation field interactions between adjacent spot welds.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

The various types of weld fracture in resistance spot welds. (a) to (d) show button-pullout fracture, while (e) and (f) show interfacial fracture in two views. (a) Button-pullout (side view)—case of an extremely strong weld. (b) Button-pullout (top view)—case of an extremely strong weld. (c) Button-pullout (side view)—case of a strong weld. (d) Button-pullout (top view)—case of a strong weld. (e) Interfacial fracture (side view)—case of a weld weaker than in the case of button-pullout fracture. (f) Interfacial fracture (top view)—case of a weld weaker than in the case of a button-pullout fracture.

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Figure 2

Tensile-shear specimen dimensions

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Figure 3

Representative spot-weld configurations considered in this study

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Figure 4

Force-displacement curve for a 4mm DP600 spot weld that resisted interfacial fracture during tensile-shear testing divided into six sections; ΔL=6mm. #1: initial coupon slippage; #2: elastic deformation; #3; elastic deformation of the bulk, small plastic deformation in the weld; #4: maximum force (TSF) achieved; #5: stress localization in the weld; #6: force decreases gradually with the displacement which is characteristic of button-pullout fracture

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Figure 5

Set of images showing the progression of weld button rotation during tensile-shear testing (corresponds to #6 in Fig. 4)

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Figure 6

Force-displacement curves for spot welds of various diameters in 1.0mm thick DP600

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Figure 7

Variation of the tensile-shear force (TSF) with the square of the weld diameter, d2, for DP600, DP780, and DP980 dual-phase steels (sheet thickness range: 1.0–2.2mm)

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Figure 8

Variation of tensile-shear force with the product of initial sheet thickness (t) and weld diameter (d) for DP600, DP780, and DP980 (sheet thickness range: 1.0–2.2mm)

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Figure 9

Tensile-shear force variation with weld diameter, d, for DP600, DP780, and DP980 (sheet thickness range: 1.0–2.2mm)

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Figure 10

Variation of total coupon displacement, ΔL, immediately following weld fracture with tensile-shear force for the welds in Figs.  67

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Figure 11

Variation of minimum weld diameters to prevent weld interfacial fracture during tensile-shear testing with initial sheet thickness, t, for various dual-phase steels

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Figure 12

Variation of TSF with weld diameter, d, for DP600, DP780, and DP980

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Figure 13

Vickers microindentation hardness variation across DP600, DP780, and DP980 welds shown in the top row of figures. Indentations were made in 1.8mm thick dual-phase steels using identical welding parameters.

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Figure 14

Process map showing fracture mode development during tensile-shear testing in terms of hardness ratios and initial sheet thickness, t. Welds associated with points falling above the dotted line failed by interfacial fracture, while those falling below the dashed line failed by button-pullout.

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Figure 15

Variation of TSF with total number of welds at selected weld diameters and configurations (shown in Fig. 3)

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Figure 16

Images showing various coupons where interfacial fracture was prevented. (a) and (b) show button-pullout fractures with two welds with a 4.0mm diameter; (c) shows a coupon with three welds (each with a 3.0mm diameter) where fracture occurred in the base material.

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Figure 17

Tensile-shear force measured in coupons with three miniature welds. Locations and orientation of the welds are depicted in Figs.  3333.

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Figure 18

Comparison of total number of weld interfacial fractures and weld button-pullout fractures in DP600, DP780, and DP980 dual-phase steels, with total number and percentage of wrong predictions, as determined by comparing results from Eqs. 5,8 with experimental observations




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