Research On Testing Methods For Mechanical Properties Of Body Adhesion Of Body Adhesion

Nowadays,more and more light materials such as aluminum alloy and the composite material,are used in automobile body structure.Materials such as carbon fiber,aluminum alloy and high-strength steel have the potential to make lightweight cars by virtue of their lightweight and high strength ratio.Besides,the mixed use of various materials is the inevitable trend of future development.Because the traditional connection such as welding,riveting and other connection methods can not be satisfied with the same or dissimilar materials between the connection,automotive structural connection technology has gradually become the key technology that needs to break through urgently in automobile body manufacturing.Bonding technology as a new connection technology,has many advantages.It can fully use its characteristics to connect new composite materials.At present,domestic and foreign scholars for the study of bonding technology,more focused on the joints structure size,force form,surface treatment,temperature and humidity caused by the adhesive aging and the load caused by the adhesive fatigue.However,the adhesive strength of composite body adhesive structure is the basis of joint strength check,and it is necessary to test its mechanical properties to meet the practical needs before the application of the new composite bonding structure.The results show that the adhesive layer of different adhesive layer thickness will appear different degree of necking before failure.Because of the boundary effect caused by the necking of the rubber layer,the strength of the adhesive specimen is not reflected in real,therefore,it is necessary to study the influence of the boundary effect of the necking on the bonding strength.This paper is based on the Sikaflex-265 adhesive provided by the Cooperation Institute of a company and our research group.The mechanical properties of adhesives were studied by reference to relevant test standards to analyze the stress-strain problems caused by necking of adhesive specimens,and the effects of thickness and boundary effects on mechanical properties.Aiming at the above problems,the paper studies the improvement of the test method of mechanical properties of the bonded specimens.Through experiments and finite element simulation,the paper studies the strength and stress-strain distribution of the adhesive specimens under different thickness of adhesive layer,and puts forward the improved method of testing the mechanical properties of the bonded specimens,which can reduce the effect of the boundary effect after necking on the bonding strength of the specimen.According to the above research ideas,this topic mainly completes the following several contents:(1)The test experiment was designed to study the influence of the adhesive thickness and stress state of the adhesive layer on the mechanical properties.The stress state of the adhesive structure is analyzed mainly by tensile stress,shear stress and tensile-shear combined stress.The paper chooses the Sikaflex-265 polyurethane adhesive as the research object,makes the dumbbell-type specimen of the adhesive,and designs the adhesive test plan to make the adhesive specimen with different thickness and stress form.(2)The test data of the mechanical properties were obtained by the experiment of the adhesive dumbbell type specimen and the adhesive specimen.By observing the performance parameters of the adhesives and the load-displacement curves of different adhesive layers under different stress states,it was found that the failure strength of the bonded specimen decreases with the increase of the thickness of the adhesive layer and the failure displacement increases with the thickness of the adhesive layer.By comparing the failure strength values of the adhesive specimens under different stress states,the scarf joints decreased more with the increase of thickness.The strain of the adhesive layer and the displacement in the axial direction are measured by using the DIC non-contact strain measuring system and the electronic strain extension instrument.(3)Simulation analysis and calculation of the finite element model.Firstly,the finite element simulation model with different adhesive layer thickness is established,and the failure load distribution of specimen tensile test is applied to the finite element model.The stress state and stress distribution of the adhesive specimens are compared with different adhesive thickness,then the influence of thickness and stress state on the adhesive strength is judged by comparing the finite element simulation with the experimental data.According to the analysis model,it is found that the maximum axial stress of the FEM simulation model is consistent.It is indicated that the variation of the strength of butt joints is mainly affected by the boundary effect,which is caused by the necking of the adhesive layer.What's more,the SLJ and the SJ show a significant stress concentration in the edge area of the adhesive layer,that is,the crack propagation begins at the end of the adhesive and the necking is not obvious.(4)An improved method for testing the mechanical properties of adhesive is presented.By analyzing the effect of the boundary effect on the strength of the adhesive specimen,the corresponding correction scheme is proposed and the correction equation is established.Then,the strength of the modified specimen was obtained by modifying the necking area of the adhesive layer.In view of the effect of the boundary effect of the necking on the strength,a new method for improving the mechanical properties of adhesive bonding was proposed.On one hand,the strength of the modified specimen can be obtained by dividing the failure load by the cross-sectional area after necking.On the other hand,the strength of the adhesive layer can be obtained by taking the failure load of the experimental test into the analysis model.Finally,by combing the experiment and the finite element analysis,the effect of boundary effect on the strength can be reduced.