Investigation Of Forming Mechanism And Mechanical Property Of Self-piercing Riveted Joints

As lightweight design is developing in manufacturing industries covering automobiles and aerospace components, lightweight materials such as aluminium alloy, magnesium alloy and fiber reinforced plastic material are increasingly being used. These lightweight materials are difficult or impossible to join by traditional joining methods. Self-piercing riveting (SPR) technology is a new joining method and has particular advantages. Based on experimental observations and Finite Element Analysis (FEA) of several types of joints, the research work was divided into several parts as follows:1. The deformation and fluxion state of substrate materials during the riveting process and the stress/strain distribution in shaped joints were analyzed using the ANSYS/LS-DYNA sofeware. Results showed that the interfaces between the rivet head and upper sheet and between the rivet shank and lower sheet were also the most vulnerable parts in shaped joints.Results of microscopic examination and hardness measure of SPR joints in AA5052showed that large plastic deformation of the joints resulted in the material structures of parts being stretched from the equiaxed grain into fibrous structures and also in significant thinning of the grain structure, which produced a work hardening effect. Compared with the hardness of parent metals (72.3HV), the hardness was improved by40.6%maximumly.2. The analysis results of static test data using the Student's t distribution showed that the shear strength of joints with0°preformed angle was largest and the strength of joints with30°preformed angle was smallest. With the increase of preformed angles continuously, the strength was magnified. The failure displacement and energy absorption of joints with60°preformed angle were highest. With the increase of thickness and width of substrates, the strength of joints was improved by40.1%and12.6%respectively,11.7%and14.9%responding to failure displacement and75.0%and11.7%for energy absorption. The strength of joints with two-rivet (SDL and SDT) was improved by88.2%and99.6%respectively,39.0%and25.8%responding to failure displacement, and154.8%and115.6%for energy absorption. The strength of joints with three-rivet (SMI and SMO) was improved by175.2%and163.4%respectively,17.6%and35.2%responding to failure displacement and187.9%and130.4%for energy absorption.The preformed angle, thickness and width of substrates, the number of rivets and their distribution had little influence on the macro-scale failure mode of joints. In all test cases the interlock structure failed, with the lower sheet separating from the upper sheet and rivet. Analysis of failure surfaces showed that the fracture started at the centre of the joint and propagated outward to the two sides. The locations of the initial failures were the'swell'parts of the lower sheets. Deformation characteristic of the upper sheet influenced directly the deformation displacement of stressed joints. The deformation displacement and the strength of joints were influenced by the deformation characteristic and its failure process in the lower sheet.3. The fracture of the lower sheet was the main fatigue failure mode. Black oxide debris caused by fretting fatigue damage was found on the fracture surfaces. When a bigger fatigue load was applied, the failure mode could be influenced by the width of substrates, the number and distribution of rivets, which led to the uppere sheet fracture.Based on the method of coefficient of variation, the effectiveness of fatigue test data was analyzed assuming a two-parameter Weibull distribution. The least squares best fit lines of fatigue data and its two-parameter power function equations were obtained, corresponding to NN=107.139F-4.799,NW=109.578F-7.158,NSDL=107.572F-3.397,NSDT=10A8.673F-5.112, NSMI=108.687F-4.531and NSMO=109.105F-5.005.The fatigue life and the slope of the F-N curves were influenced significantly by the width of the substrates and the number and distribution of rivets. Increasing the number of rivets reduced the extent to which the slopes of F-N curves and fatigue strength were affected by the rivet distribution.The existence of primary and secondary load bearing sequences was proposed. The fatigue fracture surface was the location of the primary load bearing sequence. An analysis of fatigue surfaces showed that for the upper sheet fracture, the fatigue crack started at the contact interfaces between the rivet head and the upper sheet. For the lower sheet fracture, the fatigue crack started at the interface between the rivet shank and lower sheet. The fretting damage region spread from the bottom of the rivet shank toward the rivet head.Results of element analyses of black debris on the rivet surface showed that the element content of Al was highest. This indicated that the violent fretting damage happened at those locations. It can be inferred that reducing the friction at those locations can improve the fatigue life of joints.4. Load bearing part in adhesive bond-clinch hybrids and adhesive bond-SPR hybrids joints was the clinching and the SPR interlock structure respectively.The analysis results of static test data using the Student's t distribution showed that compared with adhesive bonded joints in Y2, the strength of clinched, SPR and their adhesive bond hybrid joints was improved by8.0%,231.9%,21.3%and246.2%respectively,221.4%,1271.4%,178.6%and978.6%responding to failure displacement, and300%,4186.4%,254.6%and3800%for energy absorption. Compared with T-SPR joint, the strength, failure displacement and energy absorption of lap-shear SPR joints was enhanced by169.3%,-188.2% and-52.3%. Compared with clinched and SPR joints in Y2, the strength of clinched and SPR joints in AA5052was enlarged by117.6%and70.2%respectively,-25.2and9.5%responding to failure displacement, and45.5%and128.4%for energy absorption.