Resistance spot welding (RSW) of multiple sheets (more than two) and combining multiple materials (i.e. different steel grades) are increasingly realized with the growing demand of safety and lightweight vehicle structures. However, the mechanical strength and rupture of such new generation of RSW under multi-axial loadings is not yet well studied. To the knowledge of the authors, most of published works for combined loadings concern two-sheet spot-welded assemblies (Langrand and Markiewicz, 2010). When three-sheet assemblies are of considered only few works relate the study of the mechanical strength under quasi-static loadings using conventional tensile–shear specimens (Tavasolizadeh et al. 2011; Li et al. 2015).In this work it is proposed to study the mechanical strength and the rupture of multi-sheet, multi-steel grade spot-welded assemblies under pure and mixed modes I/II loading conditions. An advanced experimental device has been developed, based on the Arcan test principle (Chtourou et al. 2017). Thus, the loading modes I/II are combined and well controlled, with a reduced contribution of the plates' strength surrounding the weld nugget in the macroscopic response. The studied specimen consists of a three-sheet assembly of various thicknesses and involving two steel grades: (P1) is 1.6 mm thick and made of an ultra-high steel 22MnB5, (P2) is0.65 mm thick and made of mild steel DX54D and (P3) is 2 mm thick and made of 22MnB5.The spot welding process was performed by our industrial partner ‘Renault' according to his know-how. Different weld nugget sizes were realized according to internal quality rules: standard size nugget, oversize nugget, and under size nugget.

The experimental study allows to characterize the mechanical strength and to understand failure mechanisms by varying the loading angle α. Three principal failure modes are therefore identified: Pull out failure (pure tensile tests), interfacial failure (pure shear tests) and mixed failure (combined loading tests). The study of the specimen with different spot welds size shows that the peak load and the energy dissipated increase when the size of the spot welds increases. This influence is much more remarkable when the loading angle increases. The investigation of the failure modes can explain the influence of the loading angle to the mechanical response of the spot welded assembly.

References:

Langrand, B., Markiewicz, E., 2010. Strain-rate dependence in spot welds: Non-linear behaviour and failure in pure and combined modes I/II. International Journal of Impact Engineering, 37, 792-805.

Tavasolizadeh, A., Marashi, S. P. H., Pouranvari, M., 2011. Mechanical performance of three thickness resistance spot welded low carbon steel. Materials Science and Technology, 27, 219-224.

Li, Y., , Yan, F., Luo, Z., Chao, Y. J., Ao, S., Cui, X., 2015. Weld Growth Mechanisms and Failure Behavior of Three-Sheet Resistance Spot Welds Made of 5052 Aluminum Alloy. Journal of Materials Engineering and Performance, 24, 2546-2555.

Chtourou R., Leconte N., Chaari F., Haugou G., Markiewicz E., Zouari B. (2017). Macro-modeling of the strength and failure of multi-layer multi-steel grade spot welds: connector formulation, assembly model and identification procedure, Thin-Walled Structures, DOI: 10.1016/j.tws.2017.01.023.