Residual stress distribution in self-piercing rivet joint of high strength steel

Abstract

Abstract Manufacturing industries throughout the world are constantly facing the pressure to reduce carbon emissions. The automobile industries strategy for reducing carbon emissions are targeted at reducing the overall weight of the car body. Efforts to accomplish this haven been based on clever materials selection that suggests using materials like aluminium and magnesium with high strength to weight ratio in place of the traditional steel. However, these lightweight alloys are difficult or sometimes impossible to join by the traditional joining methods such as welding. As a result, self-piercing riveting (SPR) is increasingly used for joining required for assembly of components in automobile industries. In SPR, a mechanical interlock is created by the rivet. However, the rivet material should have enough hardness to pierce the top sheet and have enough ductility to deform plastically in the bottom sheet. Due to the rapid introduction of lightweight high strength alloys, joining ability of a rivet becomes more limited. It is necessary to understand the residual stress in SPR joints. Residual stress was measured in rivets of two different SPR joints using neutron diffraction technique. It was observed that the stresses that developed inside the SPR joints was compressive and the magnitude of the residual stress was low in the softer material in comparison with the harder material. It was also evident that the neutron diffraction can detect a crack in the rivet tip which was not evident from a cross-section. The results are discussed based on the physical movements of materials in SPR joining process.