Research On Damage And Mechanical Properties Of Warm Self-piercing Riveted Joints In Carbon Fiber Reinforced Composite And Aluminum Alloy

In recent years,the popularization and application of new energy vehicles have put forward higher challenges to automobile lightweight technology,and the research on automotive lightweight has become a research hotspot.Multi-material body structure is one of the most effective ways to realize automotive lightweight.Carbon fiber reinforced composites(CFRP)have been favored by automotive manufacturing industry in recent years because of its high specific strength and specific stiffness,easy molding and strong structural design ability.The joining technology is a key problem in application of multi-material car body.As a new mechanical joining technology of light-weight sheet materials,self-piercing riveting(SPR)has advantages in joining dissimilar materials and non-metallic materials,and has been widely used in car body manufacturing.When fiber reinforced composites(FRP)are joined by SPR,the deformation of the composites will cause macro cracks on the surface of the laminate sheet and serious delamination damage inside the laminate.Therefore,it is urgent to develop a new SPR technology to reduce the joining damage of FRP.In this dissertation,unidirectional carbon fiber reinforced epoxy resin composites were taken as the research object.In order to reduce the joining damage of SPR joints in CFRP and aluminum alloy,the warm self-piercing riveting(WSPR)process and joining damage were studied,and the effects of ply structure,sheet thickness of CFRP and service temperature on the mechanical properties and failure process of the joints were analyzed.Firstly,a prediction model of the mechanical properties of CFRP considering the temperature effect was established.Considering the anisotropic mechanical properties of CFRP and the high elastic properties of thermosetting epoxy resin matrix at the glass transition temperature,the tensile and compression tests,shear tests and interlaminar fracture toughness tests of CFRP were carried out in the temperature range from room temperature to glass transition temperature of the matrix.Based on the experimental data obtained,the mechanical properties of CFRP at different temperatures were predicted by arcsine function with two parameters.Secondly,the damage constitutive model for composite material considering shear effect and temperature effect was established.As the CFRP is mainly subjected to shear stress during the SPR process,and the longitudinal-transverse shear mechanical response is nonlinear,the in-plane shear failure criterion and the out of plane shear failure criterion were added based on the strain based Hashin failure criterion.Based on the continuum damage mechanics theory,coupling the damage variables into the stiffness matrix can realize the progressive degradation of the mechanical properties of materials under the corresponding failure modes.The influence factor of temperature was coupled into the damage constitutive model to simulate the mechanical properties and failure of materials at different temperatures.Thirdly,the simulation model of SPR in CFRP and aluminum alloy was established,and the joining damage mechanism of joints was understood.The macro-crack defects in CFRP surface at different joining temperatures were studied by WSPR experiments.The influence of ply angle and joining temperature on the deformation and damage of CFRP was studied by the numerical simulations.Then,a simulation method for tensile simulation of SPR joints in CFRP and aluminum alloy was proposed,and the tensile-shear mechanical properties and failure mechanism of WSPR joints were studied.Considering the advantage of the designability of CFRP sheet,three typical ply structures and three thicknesses of CFRP sheets were selected for WSPR with aluminum alloy.Tensile-shear tests were carried out on the joints.The effects of ply angle and sheet thickness on the mechanical properties and failure modes of joints were investigated.MMC failure criterion was used to determine the failure parameters of aluminum alloy,and the tensile failure mechanism of joint and the crack growth trend of CFRP sheet were studied.The adhesive-SPR hybrid joints of CFRP and aluminum alloy were prepared,and the effect of adhesive on the mechanical properties of the SPR joints was studied.Finally,based on the previous research results,the influence of service temperature on the tensile-shear mechanical properties of joints with 0°/90° ply structure was analyzed.The non-contact strain measurement system was used to analyze the changes of strain field in the overlap region,and the tensile failure mechanism of the joints under different service temperatures was studied qualitatively and quantitatively.Scanning electron microscope was used to observe the failure area of riveted holes on the upper sheet,and the difference of damage morphology of riveted holes under different tensile temperatures was studied.Based on the research above,a WSPR method for joining CFRP and aluminum alloy was proposed,which effectively eliminated the macro cracks on the surface of the upper sheet induced by the joining process,and reduced the joining damage of CFRP.The variation law of the basic mechanical properties parameters of carbon fiber reinforced epoxy resin composites at different temperatures was obtained by experiments.The damage constitutive model of FRP was established,and the deformation and damage prediction of CFRP at different temperatures were realized.This study will promote the application of SPR technology in joining the composite automotive body.