Fracture and Fatigue Behaviors of Swept-Fssw's in Lap-Shear Specimens of Alclad 2024-T4 Aluminum Sheets

Fracture and fatigue behaviors of swept friction stir spot welds (Swept-FSSW’s) in lap-shear specimens are first investigated based on experimental observations.  Alclad 2024-T4 aluminum sheets with the thickness of 1.6 mm were used.  A tool with a triangular probe along a circular tool path was used to make the Swept-FSSW’s investigated here.  Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined.  The micrographs show that the failure modes of the welds under different loading conditions are quite different.  Under quasi-static loading conditions, the failure mainly starts from the necking of the upper sheet outside the stir zone.  Under cyclic loading conditions, the experimental observations indicate two types of fatigue cracks.  One type initiates and grows into the upper sheet near the boundary of the stir zone.  Another type initiates and grows into the lower sheet outside the stir zone.  Under low-cycle and lower load ranges of high-cycle loading conditions, the dominant kinked fatigue cracks are the first type.  Under higher ranges of high-cycle loading conditions, the dominant kinked fatigue cracks are the second type.  Based on the experimental observations of the paths of the dominant kinked fatigue cracks, a fatigue crack growth model is then adopted to estimate the fatigue lives of the Swept FSSW’s.  The global stress intensity factors and the local stress intensity factors for kinked cracks and the Paris law for crack propagation are used.  The fatigue life estimations based on the fatigue crack growth model with the global and local stress intensity factors as functions of the kink length and the experimentally determined kink angles agree well with the experimental results.