Static Characteristics And Fretting Fatigue Failure Analysis Of Self-piercing Riveted Butt Joints

In the context of energy saving,emission reduction and the rapid development of material science,a large number of lightweight materials are widely used in the industrial field.The self-piercing riveting technique has outstanding capacities in the connection of sh eet materials,which makes this technique have broad application prospects in the aviation field.Single strap butt joint is main form of wings and fuselage for the airplane.However,for the self-piercing riveting joining technique,most of the research only focuses on the single-lap structure,and there is relatively little research on self-piercing riveting joints for butt joints.Therefore,it is necessary to carry out research on the static s and fatigue properties of self-piercing riveting butt joints.Based on this background,this research uses 1420 lithium aluminiu m alloy and 5052 aluminium alloy as sheet materials to fabricate butt joints and lap joints made of the same material,and explores the mechanical properties of butt joints and the different between them in term of static properties.In addition,the influencing factors of the static mechanical properties of butt joints are further studied.Based on statics result,fatigue tests of butt joints and single lap joints with lithium-aluminium alloy are carried out to study the fatigue properties,stiffness,fretting,and dissipation of energy.The specific research conclusions are as follows:(1)The maximum average failure load of self-piercing riveting lap joints is higher than that of their corresponding self-piercing riveting joints butt joint,i.e.,the mechanical properties of single-lap self-piercing riveting joints are more outstanding.On the other hand,compared with self-piercing riveting single lap joints,self-piercing riveting butt j oints have better performance in energy absorption value and load bear capacity,but poor in joint stiffness.Both types of joints of the two materials failed due to the rivet being pulled out of the lower sheet.The length of the strap has a very limited effect on the static strength of the butt joint.The static failure strength of the butt joint is highly related to the rivet pitch,and the rivet pitch is almost linear with the failure load,that is,the greater the rivet pitch,the higher the joint fail ure load.The distance between the rivets and the length of the strap have a significant effect on the failure mode of the joint,but it has little effect on the energy absorption value and the failure displacement of the joint.(2)In the short and middl e fatigue life,the fatigue performance of butt joints is better than single lap joints.Overall,the butt joints have better performance in fatigue performance.Both types of joints fail due to the fatigue fracture of the lower sheets,and fatigue cracks both originate on the contact surface of the rivet tip and the inner lock structure and propagate in the thickness and width directions of the sheet materials.Under the continuous action of fatigue load,the cracks propagate in the direction of sheet thickness and sheet width,respectively.In the aspect of stiffness,the stiffness of the two types of joints could be divided into three stages:stiffness rise period,stiffness stability period,and stiffness fall period.Fatigue load amplitude and joint str ucture are the main factors that affect the stiffness of joints.In addition,the time of fatigue crack initiation can be approximated by the ch ange trend of joint stiffness.(3)The joints on both sides of the butt joint have fretting wear,and the fretting area is mainly located in four different positions between the rivet and the sheet or the sheets.The main reasons of fretting wear are the secondary bending effect of the joint and the mutual movement between the sheet or the rivet and the sheet.The fretting wear area of lap joints is similar to that of butt joints,but the degree of fretting wear is different,and there is a competitive relationship between different fretting wear areas.Both the fatigue load level and the joint structure will cause t he change of joint fretting wear,which is closely related to the dissipated energy.For butt joints,the load level and the joint structure have a greater influence on the fretting wear of the joint.Comparing with butt joint,the degree of fretting wear of single lap joints is smaller.The fatigue life of the two types of joints has a positive correlation with the total energy dissipation.Under the same fatigue life,the total dissipated energy of butt joints is greater.