Fatigue Behavior of Spot Friction Welds in Cross-Tension Specimens of Aluminum 6061-T6 Sheets

Failure modes of spot friction welds in cross-tension specimens of aluminum 6061-T6 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. Experimental results 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 low-cycle loading conditions, the dominant fatigue cracks are the kinked cracks growing into the upper sheet from the crack tips; hence, the upper nugget pullout failure mode can be seen. Under high-cycle loading conditions, the dominant fatigue cracks are kinked cracks growing into the lower sheet from the crack tips; subsequently, the lower nugget pullout failure mode can be seen. A fatigue crack growth model based on the paths of the dominant kinked fatigue cracks is then adopted to estimate the fatigue lives of the spot friction welds. The accurate global and local stress intensity factors for finite kinked cracks based on the three-dimensional finite element computations and the Paris law for fatigue crack propagation are utilized in the fatigue crack growth model. The fatigue life estimations based on the fatigue crack growth model correlate well with the experimental results.