Fatigue behavior of aluminum 5754-O and 6111-T4 spot friction welds in lap-shear specimens

Fatigue behavior of aluminum 5754-O and 6111-T4 spot friction welds in lap-shear specimens is investigated based on experimental observations and two fatigue life estimation models. Optical micrographs of the 5754 and 6111 welds made by a concave tool and a flat tool, respectively, before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show that the failure modes of the 5754 and 6111 welds under quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, both types of welds mainly fail from the nearly flat fracture surface through the nugget. Under low-cycle loading conditions, both types of welds mainly fail from the kinked crack through the upper sheet thickness and the fracture surface through the nugget. Under high-cycle loading conditions, both types of welds mainly fail from the kinked cracks through the upper and lower sheet thicknesses. A kinked fatigue crack growth model based on the stress intensity factor solutions for finite kinked cracks and a structural stress model based on the closed-form structural stress solutions at the critical locations of the welds are adopted to estimate the fatigue lives of both types of welds. The fatigue life estimations based on the kinked fatigue crack growth model and the structural stress model appear to agree well with the experimental results for both types of welds.