Experimental study on fatigue performance of resistance spot-welded sheet metals

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Resistance spot welding is used as a reliable joining process in many engineering applications because of its effectiveness, automation capability, and low cost. The spot-welded sheet metals, on the other hand, are prone to mechanical fatigue failure, especially under cyclic loadings. Therefore, understanding and elucidating the fatigue phenomenon of the spot-welded joints are crucial in terms of estimating and preventing undesired failure conditions. In the design phase, there exist a considerable amount of challenges to overcome; one of the most important challenges is to select optimum working conditions. Hence, in this study, the fatigue phenomenon of the spot-welded sheet metals is investigated experimentally, by taking electrode force into consideration. For this purpose, spot-welded modified tensile shear (MTS) test specimens were utilized. A series of fatigue life tests were conducted to examine the influence of electrode force on fatigue life. The results obtained through an optical microscope were presented and interpreted. Experimental data showed that the number of cycles to failure changes depending on the spot-generating schemes in terms of electrode force and welding schedules. Through the investigation of an optical micrograph of partially failed spot-welded MTS specimens for different groups of spot welds created under different electrode force effects, it is seen that the fatigue failure is dominated by the through-thickness cracking. Comparing both crack formation and also fatigue lives of different groups of spot-welded MTS specimens, it is shown that the electrode force and accordingly thermal interaction play an important role in the fatigue strength of the spot-welded specimens.