# Self-Piercing Riveted Joint of Vibration-Damping Steel and Aluminum Alloy
## 3 Results and Discussion
### Vibration-Damping Steel vs. SPFC590 DP  
In this study, the geometrical and mechanical characteristics of the SPR joint of vibration-damping steel were compared with those of SPFC590 DP (a skin material of the vibration-damping steel). The thickness of SPFC590 DP was chosen to be 1.4 mm, which was the same as the sum of thicknesses (0.7 mm) of the outer sheets in the vibration-damping steel. For the comparison, SPR joints were formed with the type A1 die. In the cross-sectional images of the SPR joints in Figure 14, the stack of joining the vibration-damping steel to the aluminum alloy (Al5052-H32) as the bottom material had a smaller cavity and rivet head gap than the stack of joining SPFC590 DP to the aluminum alloy. This was because the vibration-damping steel was more flexible than the solid SPFC590 DP, because of the viscoelastic adhesive layer in the vibration-damping steel (refer to Figure 14a,b). For the Al5052-H32 top-stack configuration, the rivet cavity is completely filled with ductile Al5052-H32 for both the SPR joints (refer to Figure 14c,d). However, slightly higher deformation of the vibration-damping steel is observed at the bottom-center of the joint due to the separation of the outer sheets of the vibration-damping steel. Figure 15 shows the result of the measured geometrical indices for the SPR joints. It was found that the interlock width and bottom thickness of the SPR joint of the vibration-damping steel were almost the same as those of the joint of SPFC590 DP when Al5052-H32 was used as a bottom sheet. However,  
Figure 14: Comparison of cross-sectional images of SPR joint (fabricated by type A1 die) for different stack-up combinations. (**a**) SPFC590 DP (top) and Al5052-H32 (bottom), (**b**) vibration-damping steel (top) and Al5052-H32 (bottom), (**c**) Al5052-H32 (top) and SPFC590 DP (bottom), and (**d**) Al5052-H32 (top) and vibration-damping steel (bottom). Figure 13: Failure modes for SPR joint of Al5052-H32 (top) and vibration-damping steel (bottom) with respect to SPR die type. (**a**) Type A1 die, (**b**) type A2 die, (**c**) type A3 die, (**d**) type B die, and (**e**) type C die. when Al5052-H32 was used as a top sheet, both the interlock width and bottom thicknesses of the joint made from the vibration-damping steel were larger than those of the joint made from SPFC590 DP. The tensile-shear loads of the SPR joints of the stack configurations are presented in Figure 16. For the Al5052-H32 bottom configuration, the tensile-shear load of the joint made from SPFC590 DP was 22.6% larger than that made from the vibration-damping steel, despite the interlock width of the two joints being similar. This difference (\(\sim\)1 kN) in the tensile-shear load of the two joints was attributable to the difference in the intrinsic mechanical properties of the base materials. The flexural rigidity of the vibration-damping steel was less than SPFC590 DP, so more bending of the vibration-damping steel occurred than the SPFC590 DP as the top sheet in the lap shear test, which led to inter-locking failure in the earlier stage [19; 24]. However, the tensile-shear load of the joint made from the vibration-damping steel was slightly larger than that of the joint made from SPFC590 DP when Al5052-H32 was used as a top sheet. This was mainly due to the larger interlock width of the joint made from the vibration-damping steel. In this study, the mechanical performance of the SPR joint between the vibration-damping steel and Al5052-H32 was always improved when the thinner Al5052-H32 was used as a top sheet, due to the increase of the interlock width. However, for the SPR joint in the SPFC590 DP and Al5052-H32, using the thicker SPFC590 DP as a top sheet gave more tensile-shear load by 3.9%, despite the smaller interlock width.