Fracture and fatigue failure of spot welds under general combined loading conditions

Failure of spot welds under static, impact and cyclic combined loading conditions is investigated to develop accurate spot welds models for engineering applications. Accurate spot weld models can be implemented into finite element codes to conduct component or full vehicle simulations to understand the durability and separation of spot welded components under complex combined loading conditions. In this study, experiments were first conducted to investigate the failure loads and failure mechanisms of mild steel spot welds under static combined opening and shear loading conditions. Lower bound limit load analyses have been conducted to characterize the failure loads of spot welds under combined loading conditions. Based on the lower bound limit load solutions, general failure criteria are proposed to characterize the failure loads of spot welds under combined three forces and three moments. The general failure criteria can be used to characterize the failure loads of spot welds under complex combined loading conditions with consideration of the effects of sheet thickness and nugget size. Experimental results for spot welds under different combined loads are shown to be characterized well by the general failure criteria.; Experiments were also conducted to investigate the failure loads and failure mechanisms of mild steel spot welds under impact combined opening and shear loading conditions. Based on the experimental results, failure criteria are modified to characterize the failure loads of spot welds under static and impact combined loading conditions.; Experiments of spot welds in various types of specimens under cyclic loading conditions were conducted to understand the fatigue crack growth behavior and to develop a fatigue model. The paths of kinked fatigue cracks near the spot welds in various types of specimens were identified by the micrographs of failed spot welds. A fatigue crack growth model is developed for spot welds in various types of specimens based on the Paris law and kinked crack behavior. The proposed fatigue model characterizes the fatigue lives of spot welds well.