PSI - Issue 5

Jidong Kang et al. / Procedia Structural Integrity 5 (2017) 1425–1432 Jidong Kang/ Structural Integrity Procedia 00 (2017) 000 – 000

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automotive body structures. One of the advantages of using the structural stress model is that it does not consider the material property and hence resistance spot welds between dissimilar and similar materials can be plotted together to obtain a single master fatigue curve. The present model by Rupp provides a set of mathematical formulae to calculate critical stresses based on observed fracture mode (a) sheet fracture and (b) interfacial nugget fracture. Since the dominant fracture mode in this present study is sheet fracture, only the appropriate formulae set are presented. Figure 8(a) is a schematic representation of the structural stress concept based on a circular plate model for sheet metals.The equivalent stresses for sheet fracture is calculated using a superposition of formulae for a plate subjected to central loading: σ ss = σ max (F x )cosθ - σ max (F y )sinθ + σ max (F z )- σ max (M x )sinθ - σ max (M y )cosθ (1) where max (F x ) = F x /π dt , max (F y ) = F y /π dt , max (F z ) = κ(1.774 F z / t 2 ) for F z > 0, max (M x ) = κ((1.872 M x /( dt 2 )) max (M y ) = κ ((1.872 M y /( dt 2 )). And d is the diameter of the weld nugget, t is the thickness of the sheet that fractured /cracked, κ= 0.6√ t , M x , M y , F x , F y , are bending moments and maximum applied forces in the sheet. The forces and moments in Eq. (1) are calculated using a simple free body diagram. The nugget diameter for AA6022-T4 to AA6022-T4 and AA6022-T4 to IF steel were measured for each test specimen. Using Eq. (1) and substituting relevant geometrical data, structural stresses were calculated for each of the test specimens from the two stacks. The fatigue test results were then replotted using the maximum stresses calculated and the result is presented in Fig. 8(b).

Fig. 7. (a) Concept of Rupp’s structural stress based on circular plate mod el for sheet metals [17] and, (b) fatigue test results of resistance spot welded AA6022-T4 to AA6022-T4 and AA6022-T4 to IF steel joints plotted using maximum structural stress. From Fig. 8(b), it is observed that the fatigue life from the test results versus the calculated structural stress gives a reasonable correlation. The fatigue life of the RSWs falls close to a single master curve, irrespective of stack configuration and weld nugget diameter. These results suggest that the nugget diameter, which corresponds to tensile strength, dominates the fatigue life in the two stacks of RSWs presented in this study. Overall, the difference in fatigue life attributed to nugget diameter is well captured. The lap-shear tensile and load controlled fatigue behavior of the similar AA6022-T4 to AA6022-T4 and dissimilar AA6022-T4 to IF steel spot welds were studied. Based on the results, the following conclusions can be drawn: 1. It is feasible to weld AA6022-T4 to IF steel effectively by using the GM proprietary MRD electrode. An average lap-shear strength of 3900 N was achieved as compared to an average lap-shear strength of 3100 N in AA6022-T4 to AA6022-T4 RSWs welded using MRD electrodes. 4) Conclusions