Failure modesThe major failure mode for the two-sided FSR by extrusion jointswas shearing of the aluminum extrusion at both edges of the steelalong with a breaking of the aluminum/steel bond. The aluminumshearing in most samples occurred at both the top and bottom of thesteel hole leaving a deformed, cylindrical section of aluminum inthe hole as seen in Fig. 5. The setup of the “rivet” in a perpendicularorientation to the tensile direction helped increased the ultimatetensile strength of the joint by doubling the surface area since therivet was in double shear. Other samples fractured only along oneedge of the steel, leaving the aluminum rivet attached to the oppo-site aluminum sheet. The failure in the aluminum/steel region is ashear fracture between the interface of the materials and was sim-ilar to results found by Uematsu in their study of dissimilar FSSWof A6061 and low carbon steel [20]. The lighter colored region inFig. 5b is aluminum that has been left behind on the steel from thefracture