Synthesis of Experimental Testing and Fatigue Behavior of Laser Stake-Welded T-Joints on Medium-High Cycle Fatigue Range

The paper presents a synthesis of experimental fatigue tests and theoretical studies of laser stake-welded T-joints in steel sandwich panels. The experiments indicate that the slope of the fatigue strength curves varies significantly depending on the type of loading the joint experiences, but also on the geometry. Therefore, the focus is on the influence of crack tip plasticity when the joints are loaded under tension and bending. This is investigated by two different approaches based on square root of J-integral. With this parameter, the experimental fatigue curves meet at the fatigue limit for all of the loading modes and geometries. To investigate the change in slope in the medium-high cycle fatigue range, the suitability of stress gradient and plastic zone size as defined by Irwin are studied. It is shown that the gradient of principal stress correlates well with the variation of slope, but it does not give a practical tool for fatigue assessment of the joints. Instead, based on the plastic zone approach, a new method is proposed that permits the estimation of number of cycles to failure under bending directly from the tension fatigue curve. The method is verified with experiments.