Effect of Sheet Material Properties on Characteristic-Curve and Residual Stress Profile in Self-Pierce Riveted Joint

Self-Piercing Riveting (SPR) is a high-speed mechanical fastening technique which does not require pre-drilling. During SPR, a tubular rivet is driven through the top sheet, piercing the bottom sheet, accompanied by flaring of the legs in the bottom sheet under the guidance of a suitable die. The rivet material should have adequate hardness to pierce the top sheet and sufficient ductility to deform in the bottom sheet without cracking. An increase in strength or thickness of the ply materials narrows down the operating window in terms of joint quality and performance. It is important to know the residual stress distribution arising from the riveting process, and its dependence on the ply materials properties, to ensure a sound joint. It is also important to establish a relationship between the characteristic-curve of the riveting process and the residual stress profile.

In this study, two different joints were examined: aluminium-steel and steel-steel plies. A characteristic-curve was produced by measuring force and displacement directly during riveting. For both joints, the maximum residual stress was compressive and occurred in the rivet leg. A sharp rise in force was observed in the last stage of characteristic-curve for aluminium-steel joint, which corresponds to the higher residual stress for the same joint. An equivalent stress comparison revealed that the stress in rivet head was higher for a steel-steel joint, which also relates with the higher force required in the first two part of the characteristic-curve. The results are discussed in terms of the events involved in SPR process.