Study On Tensile Properties Of Aluminum Alloy-Composite Material Bonded Structure

In the past few decades,the proportion of composite materials used in the aviation field has been increasing,especially carbon fiber reinforced composite materials gradually shift from secondary load structure to main load structure.It is inevitable to connect composite materials with metal.The common connection method is adhesive connection.Adhesive connection has the advantages of light weight,low cost and no galvanic corrosion,which can effectively avoid the problems of stress concentration in traditional connection methods（bolt connection,rivet connection,etc.）.However,there are differences between metal and composite materials in material surface and stiffness,and there are many factors affect the strength of adhesive joints,and the failure mode of joints is relatively complex.Therefore,it is necessary to systematically and deeply study the failure and mechanism of metal and composite bonded structures,which is of great significance to the application of such structures in aircraft.In this paper,aluminum alloy and carbon fiber composites were used to prepare adhesive joints with different lap lengths,layers and joint configuation.Firstly,the load-displacement curves,surface strain distributions and failure morphology of the bonded joint were obtained by experimental methods.Secondly,the simulation model was established based on the experimental data.The 3D Hashin failure criterion was written by VUMAT subroutine to simulate the damage of composite,and the Cohesive Zone Model was used to simulate the damage of adhesive layer and resin layer.Finally,the experimental results were compared with the simulation results to verify the effectiveness of the simulation model.The internal stress distribution law of heterogeneous material adhesive joints in the tensile process is analyzed,the fracture process and failure modes of adhesive joints are studied,and the failure mechanism of adhesive joints is explored.The main conclusions are as follows:The tensile properties of CFRP-CFRP bonded joints with different lap lengths were analyzed,and the stress distribution of CFRP homogeneous material bonded joints under tensile load was studied.The results show that under tensile load,the peeling and shear stress in the adhesive layer are mainly concentrated at both ends of the lap area,and the failure of the adhesive layer is mainly caused by the combined action of peeling and shear stress.The initiation of joint damage mainly occurs at the end of the lap area of CFRP laminate of 90°layered,and the cracks grow obliquely under the peel and shear stress,resulting in the coexistence of interface failure and thin-layer fiber tearing.Mixed failure modeThe tensile properties of CFRP-Al single-lap joints with different lap lengths were analyzed,and the internal crack propagation and failure morphology of hetero-material single-lap adhesive joints were explored.The results show that as the load increases,the joint is subjected to shearing and peeling forces.Firstly,cracks occur in the adhesive layer of the composite material end,and then expand to the interior of the composite plate,and finally the adhesive layer and the composite material break suddenly.The boards were severely damaged.When the fiber is sheared and damaged,a large number of fiber breaks will occur,which can absorb more energy.The tensile properties of CFRP-Al double-lap joints with different lap lengths were analyzed,and the typical stress distribution rules and failure characteristics in the double-lap joints of heterogeneous materials were explored.The results show that under the action of tensile load,the upper and lower CFRP expand outwards,and the CFRP is subjected to shearing and peeling forces.The cracks are first generated on one side,and gradually expand to the end of the metal under the action of shearing and peeling forces.When the double-lap structure is transformed into the single-lap structure,the instability failure occurs suddenly.The tensile properties of CFRP-Al single-lap joints with different layup methods were analyzed,and the internal stress distribution and failure morphology of the single-lap heterogeneous materials were studied.The results show that the proportion of 0°ply has a greater influence on the stiffness degradation of the composites materials.The higher the proportion,the greater the load that can be borne,and the smaller the stiffness degradation area.[45/-45]_4s composite plate has the largest stiffness degradation area and can bear the smallest ultimate load.On the contrary,[0/90]_4s composite plate has the smallest stiffness degradation area and can bear the largest ultimate load.