Fatigue behavior of dissimilar spot friction welds in lap-shear and cross-tension specimens of aluminum and steel sheets

Fatigue behavior of dissimilar friction stir spot welds or spot friction welds in lap-shear and cross-tension specimens of aluminum 6000 series alloy and coated steel sheets is investigated based on experiments and three-dimensional finite element analyses. The Al/Fe spot friction welds in both types of specimens were tested under quasi-static and cyclic loading conditions. Optical micrographs of the welds after failure under quasi-static and cyclic loading conditions show that the Al/Fe welds in both types of specimens mainly fail along the interfacial surface between the aluminum and steel sheets. Three-dimensional finite element analyses based on the micrograph of the weld before testing were conducted to obtain stress intensity factor and J integral solutions for the crack fronts along the nugget circumferences of the welds in both types of specimens. The computational results suggest that the J integral and in-plane effective stress intensity factor solutions at the critical locations of the welds obtained from three-dimensional finite element analyses may be used as fracture mechanics parameters to correlate the experimental fatigue data for the Al/Fe spot friction welds in both types of specimens. Finally, the computational results are used to explain the experimental observations of the fatigue crack growth patterns of the welds.