Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 1: Welds made by a concave tool

Failure modes of spot friction welds made by a concave tool in lap-shear specimens of aluminum 6111-T4 sheets are first investigated based on experimental observations. 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 quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, the failure mainly starts from the necking of the upper sheet outside the weld. Under cyclic loading conditions, the experimental observations indicate two types of fatigue cracks. One type initiates and grows into the lower sheet outside the stir zone and another type initiates from the bend surface of the upper sheet outside the weld. The dominant fatigue cracks for the final failures of the welds are the kinked fatigue cracks growing in the lower sheet outside the stir zone. 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 spot friction welds. The global stress intensity factors and the local stress intensity factors for kinked cracks based on the recent published works for resistance spot welds and the Paris law for crack propagation are used in the fatigue crack growth model. 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.