Experimental and numerical investigation of the overload effect on fatigue behaviour of spot-welded steel sheets

Spot-weld joints are commonly used to fasten together metal sheets in automotive industry. The car frame used in Renault vehicles is a representative example of the usefulness of this method. Most of the spot-welds experience fatigue damaging occurrence due to rough roads or driving conditions which apply periodical overloads to the vehicle. Understanding their fatigue behaviour is crucial from the viewpoint of failure prevention in design. In this study, a series of experiments was conducted to study the fatigue failure of spot-welded tensile-shear specimens made of a deep-drawing steel (XES) and High strength low alloy steels (HE360D and XE360D). Two different types of fatigue tests were performed, the first one with a constant-amplitude sinusoidal loading (loading ratio equal to 0.1) and the second one with one incidental overload cycle introduced per 100 cycles. The experimental results show a favourable effect of overloads for HSLA steel specimens, whereas the effect is the opposite for XES steel specimens. A finite element analysis was carried out using the open-source Salome-Meca platform to determine the stress states within the specimens around the weld spot and explains both failure modes observed on the specimens at high and low loads.