Strength of spot welding

Because spot welding has generally replaced riveting in many applications, it is reasonable to compare these two processes for strength. Often, riveted joints are designed on the basis d = 1.2 t, where 'd' is the rivet diameter and 't' is the sheet thickness. In a riveted joint, the holes for the plate weakens the plate. With a spot welded joint, the weld is integral with the plate so that the only weakening occurring is that due to the softening effect of the welding heat. Higher efficiencies can therefore be achieved in welding. Since, weld diameter is so closely related to electrode tip diameter, the later maybe quoted for spot welding and the electrode diameter d is made equal to t. d₂ = (0.1 + 2t) is another empirical formula used in the case of plates which are not very thick or thin. It is important to recognise when considering the strength of a spot weld that in a sheet metal the weld is rarely, if ever, stressed solely in shear because of distortion which take place around the weld under load. Under these conditions, the ductility of the parent metal at the periphery of the metal can have a dominating influence. A useful indication of weld ductility is obtained by taking the ratio of cross-section strength (f_t) and shear strength(f_s). This ductility ratio f_t/f_s approaches 1 for maximum ductility and approaches '0' where extreme brittleness is present.