041.pdf

Welding Journal | January 2014

1+patch+1, and 1+++1+1-mm configurations, which have the same thermal and notch effects, highlighting that a mechanical effect is also present. To better analyze this mechanical effect, the CTSs were normalized with respect to their weld and sheet dimensions. A parameter αwas developed equaling the CTS divided by the product of the weld diameter and the thickness of the thinnest sheet used in the stackup. Although the parameter is not perfect, this normalization turns out to be the most robust for a wide variety of cases, and has already been used in a study by Dancette et al. (Ref. 6). After the α parameter was calculated for each weld, the values for each configuration were averaged resulting in a value for each stackup. It should be noted that the units N/mm2 are used for αand not MPa. This choice was made as in most cases αis not a stress; however, α is close to the shear stress in case of full button pullout. Instead, this parameter is meant as only an equivalent stress to be used to compare the various joint configurations. The average α obtained for each configuration is given in Table 4. First, theseαvalues confirm the qualitative analysis for three-sheet configurations. From Table 4, theαvalues of the 1+1+0 joint are slightly higher than the 1+1 configuration. As well, both values for these joints are lower than those for the 1+2, 1+patch+1, and the 1+++1+1 joints. This is all in agreement with Figs. 6 and 7. This again confirms the strength of the mechanical effect when compared to when only the thermal and notch effects are present. The α values of the two-sheet stackups were plotted in Fig. 9 as a function of the sheet thickness ratio. Even if some scatter can be seen for the similar configurations (thickness ratio of 1), there is a clear increase (“positive deviation”) in α value for dissimilar configuration, which WELDING JOURNAL 41 is correlated to thickness ratio. To better understand the influence of thickness ratio, a detailed mechanical analysis was found in literature (Ref. 15). In this work, the authors developed an analytical theory of elastic loading of spot welds. The stress intensity factors at the notch around the spot weld are derived as a function of the material elastic properties, the sheet thicknesses and the “nominal stress” (i.e., global loading) applied to the spot weld. Of course, in the present study the loading during cross-tension testing was not fully elastic. As this assumption may be reached locally around the notch, the elastic analysis from Ref. 15 was considered here as a means of understanding the mechanical effect on positive deviation. For the cross-tension case (opening mode), the nominal stress, proportional to the cross-tension global load, is called σbu ++, and the relevant stress intensity factors able to explain the spot weld failure are KI (stress intensity factor in opening mode I) or Kres (a resulting stress intensity factor taking into account the mode I and mode II stress intensity factors), which turn out to be close to each other since the contribution of mode Table 3 — Welding Parameters Minimum Sheet Thickness Electrode acc. ISO 5821 Welding Force (kN) Welding Time (ms) Holding Time (ms) in the Assembly (mm) (type-shank diameter-tip curvature radius-tip diameter) 1 G0 - 16 - 40 - 6 3.5 260 260 1.25 G0 - 16 - 40 - 6 4 320 320 2 G0 - 20 - 50 - 8 5 720 (four 180 ms 400 pulses separated by 40 ms cold times) Fig. 5 — Cross-tension strength for DP980 1.25+1.25, 1.25+2, and 2+2 configurations. Fig. 6 — Cross-tension strength for DP980 1+1, 1+1+0, and 1+patch+1 configurations. Fig. 7 — Cross-tension strength for DP980 1+1, 1+2, 1+1+1, and 1+++1+1 configurations.


Welding Journal | January 2014
To see the actual publication please follow the link above