Interface Slip Characteristics And Fatigue Reliability Analysis Of Self-piercing Riveted Joints

The automotive vehicle weight affects directly vehicle fuel consumption and exhaust emission in the automotive industry. Hence lightweight vehicle become one important means to improve fuel economy and emission standards. While the way of automobile lightweight is that the light alloy, composites and other new lightweight materials replacing lightweight high-strength steel are widely used to reduce overall vehicle weight in automotive body manufacturing. Self-Piercing Riveting (SPR) is a new cold-press rivet connection technique as a replacement for traditional resistance spot welding and becoming the most potential connection technology since many alternative materials welding performance is poor, or not even welded in automobile manufacturing.However, because of the connection technology began to be used in the production of automotive structures in recent years, it lacks the corresponding theoretical support and practical experience as well comparing with the application practice of the resistance spot welding for decades. This paper takes SPR joints in dissimilar metal sheets of aluminum5052and copper H62(Al-Cu) alloy as the object of research to study the micro slip characteristics, mechanical properties and fatigue reliable properties of SPR joints from experiment and simulation. Concrete research contents are list as follows:(1) A double-half-section specimen of SPR joint was designed, and a microscopic measurement platform was set up to measure the slight slip of joints. The tensile shear tests and numerical simulation were carried. The results show that the slight slip in the contact interfaces worsens the failure of SPR joints. Good agreement was found between the experiment and simulation under tensile shear load. The concentration of stress was found in the contact position between the bottom of rivet leg and lower sheet, so did the interlock position between rivet and lower sheet. The slight slip characteristics of SPR joints could be qualitatively described through the special specimen, and the specimen could be used to calculate the displacement of the slip in mechanical interlocking region of SPR joints.(2) The engineering stress-strain curves of two alloys were tested for the characteristics of SPR joints in dissimilar metal sheets of Al-Cu alloy. The static strength of SPR joints were studied through the tensile shear and peeling experiments. The experimental results show that:the tensile shear strength is greater than peel strength. The Cu-Al is better for SPR joints in dissimilar metal sheets of Al-Cu alloy. The static strength of SPR joints are influenced by the length of rivet, thickness and property of sheets, and the effect of property of upper sheet is prominent. The fail displacement is related to the thickness of sheets, and the failure mode is the rivet pull-out from lower sheet. The stiffness is related to the failure mechanisms and varies with load of joints.(3) Based on the static tests, fatigue tests of SPR joints in aluminum-copper alloy dissimilar materials sheet were carried out, and loads-life (S-N) curves were obtained to characterize properties of joints. Fatigue failure modes, failure mechanism, the influence of the stiffness on fatigue performance of SPR joints have been studied. The results show that:The fatigue strength is influenced by the thickness and property of sheets, the rivet and the slight slip. The influence of the lower sheet on the fatigue strength and fatigue life is almost a linear relationship, and the effect of property of upper sheet is prominent than lower sheet. Joints in similar metal mainly fracture in the lower sheet, while joints in dissimilar metal fracture in the softer material. The failure mechanism of SPR joints are affected by the properties and thickness of the sheets, rolling direction, working conditions, slight slip, bending effect, and the residual stress in SPR process. The stiffness of joints is different because of using dissimilar material combination, and the fatigue performance improves with the stiffness of SPR joints.(4)The fatigue test of SPR joints in5052aluminum sheets with a thickness of2.0mm was carried out to study the fatigue reliability of the joints. The failure modes of joints were analyzed and equivalent cross section of joints were also calculated in present paper. The least square regression analysis method is used to construct S-N fatigue characteristic fitting equation of SPR joints. Both P-S-N probability fatigue characteristic equation and correlation coefficient of different reliability P with95% confidence level is deduced. The minimum fatigue limit of the SPR joints can be calculated based on the corresponding P-S-N curve, and can be then used for predicting the fatigue life of SPR joints.