Simulation Study On Failure Of Self-piercing Riveted Joints Between Steel And Aluminum

Lightweight body is an effective means to improve the fuel economy of the vehicle.The replacement of steel structures with lightweight materials such as aluminum and magnesium will significantly reduce the mass of the car body,thus steel-aluminum hybrid structures are increasingly used.Spot-welding is currently the most widely used connection method on the body,but it is only applicable to the connections between steel and steel,not for the connections between steel and aluminum.Self-piercing riveting can connect materials that are not easy to weld,and can adapt to assembly lines and automated production.It is more and more welcomed and valued in mass-produced industries such as automotive industry and electronic products.With more and more applications of high-strength materials such as high-strength steel and aluminum alloys,the possibility of failure of the body structure itself is getting lower and lower.The problem of connection joint failure between structural parts has gradually become the focus of research.The finite element method is an effective method to study the failure of joints.There are two kinds of finite element models for joints.One is the detailed model,which studies the stress state and deformation process of each part of joints through careful mesh.The other is the coarse model,which simulates the transmission characteristics of the joints in the whole structure through less grids,and doesn’t care about the stress state of the joints.In this paper,the failure mechanism of self-piercing riveting joints is studied by using the detailed model firstly.Secondly the method of demarcating the failure parameters of the coarse model based on the detailed model is proposed.Then the coarse model is applied to the collision simulation of the body structure.The material constitutive models of 5754 aluminum alloy and Q235 low carbon steel were established.The hardening curves of two kinds of materials are fitted by the fitting formula of Swift.The GISSMO criterion is used to simulate the softening of the material in the necking stage.The failure parameters of MMC criterion are calibrated by single pull specimen,shear specimen and two notched specimen.The failure strain of the material under different stress triaxialities and Lode angle parameters of each stress is determined.A tensile test fixture for self-piercing riveting joint is designed.The drawing,shearing,stripping and 45 degree quasi static tensile tests of self-piercing riveting joint are studied.The common failure modes of the joint at different angles are obtained.The detailed model of the self-piercing riveting joints is set up,and the failure behavior of the joints is simulated and analyzed by using the material constitutive models introduced in the second chapter.The failure mechanism of the joint under various tensile conditions is analyzed by simulation.The simulation analysis of the self-piercing riveting joints of different orders of plate parts and different lengths of rivet is carried out by using the detailed model of the joints.The best order of 5754 aluminum alloy and Q235 plates is that the aluminum is on top of the steel.The optimum rivet length is 6mm.A method to calibrate the simplified model parameters by the detailed model of joint failure is raised.The models of the simulation of joint failure are established by using hexahedron solid spot-welding unit and the SPR2 constraint model separately,and the parameters are calibrated.The simulation effect of two coarse models on the failure of self-piercing riveting joint is compared.The research shows that the SPR2 constraint model can accurately simulate the failure of self-piercing riveting joints under drawing,45 degree tensile,shear conditions and peeling condition.The error of hexahedron solid welding point model in simulation of self-piercing riveting joint failure is larger.The SPR2 model is an ideal coarse model to simulate the failure of self-piercing riveting joints.The SPR2 constraint model is used to simulate the collapse behavior of the cap beam under collision condition.The influence of rivet spacing on the energy absorption characteristics of the cap beam is studied.