Effects of various strength defects of spot weld on the connection strength under both static and cyclic loading conditions: empirical and numerical investigation

During the spot welding process, it is always possible to be made defects, and the most important reasons for defects can be due to incorrect settings of current, force, and time. In other words, at different time intervals depending on the working conditions of the welding machine (i.e., both manual and robotic machines), such as the erosion of the electrode head, it is necessary to optimize the welding parameters and apply the essential changes in the next settings. The presence of various defects reduces the quality and strength of spot welds and eventually structures, which is one of the main concerns in automotive, aerospace, and marine industries. Therefore, in the present research, the authors have tried to investigate the effects of two important strength defects, including undersized and stick, on the spot-welded connection strength under static and cyclic loading. To this end, different samples of three-sheet spot-welded joint with defects of undersized and stick and free-defect were prepared based on the results of previous studies and using available relationships for the nugget diameter in terms of welding parameters, i.e., force, quadrilateral times (squeeze, up slope, welding time, and hold), and current of spot welding process. Next, metallographic, tensile, and axial fatigue tests were performed to obtain the nugget diameter, static strength of the connection, and fatigue behavior of the connection in different classifications of the samples, respectively. Furthermore, numerical analysis including the simulation of the welding process and the simulation of tensile and fatigue tests was performed in the finite element software. Numerical and experimental results have an acceptable correlation. Finally, both methods show that the nugget diameter is an influencing parameter on the tensile strength and fatigue life of spot welds, which reduces the fatigue life by 60% when the nugget diameter is reduced by 44%. Moreover, the stick defect also decreases the fatigue life of the spot-welded connection, while its effect is different depending on the geometry of welding core, including penetration height and nugget diameter.