Research On Macro-Micro Mechanical Properties Of Cfrp And Tensile Failure Analysis Of Adhesively Bonded CFRP Joints

Carbon fiber reinforced plastics(CFRP)have many advantages such as high specific strength,high specific modulus,corrosion resistance,high temperature resistance,fatigue resistance and good designability.Therefore,they are more and more widely used in aerospace,automobile,wind turbine blades,sports leisure and other fields.CFRP are composed of carbon fiber and resin matrix,which are very different from each other in mechanical properties,in a particular design approach,and the macroscopic properties of CFRP are determined by the component materials and the microstructure.Both microstructure and macrostructure of CFRP have an effect on the damage and failure.Therefore,it is necessary to conduct the macro and micro scalespan analysis of CFRP.Bonding technology is one of the key technologies in the lightweight development of aircraft and automobile.Bonding can connect heterogeneous materials,having the advantages of wide application range,design flexibility,light weight,good fatigue resistance,simple process and so on.The investigation of the mechanical properties of adhesive joints and the improvement of the performance prediction method and theory will provide important guidance for the optimization design of adhesive joints in aircraft and automobile.This thesis aims to investigate the mechanical properties of CFRP and adhesively bonded CFRP joints,and provide theoretical basis for their application in engineering.The main contents of this thesis are as follows:The tensile,compressive and shear mechanical properties of 7901 epoxy matrix under different strain rates were tested,and the strain rate dependent elastic-plastic constitutive relation and continuum damage model(CDM)of the matrix were established.According to the component properties,the elastic properties and strength of CFRP with different fiber volume fraction and under different strain rates were predicted by micromechanics analysis.The finite element(FE)model of unit cell was established to predict the elastic properties of CFRP with different,different fiber distributions and under different strain rates.The FE model of representative volume element(RVE)was established to analyze the microscopic damage mechanism of CFRP under different loading conditions and the effects of the interface strength,,fiber distribution and strain rate on the strength and failure mode.The tensile and compressive tests of CFRP laminates with different stacking sequences were carried out under different loading rates to obtain the longitudinal and transverse properties in tension and compression under different strain rates,and the shear properties under different strain rates were obtained by the micromechanical model.Based on macromechanics of composites,the dynamic intensifying factor(DIF)was introduced to modify the modulus and strength of CFRP,and the modified Hashin failure criterion was applied in combination with CDM to establish a strain rate dependent progressive damage model.The model was embedded into the ABAQUS/Explicit by VUMAT subroutine to carry out simulation of CFRP laminates under different strain rates.The accuracy of FE model was verified by comparing the simulation results with the experimental ones.The user-defined cohesive zone model(UCZM)including elastic deformation,plastic deformation and exponential degradation was established by VUMAT subroutine to simulate the failure of adhesive.3D FE models of the double cantilever beam(DCB)specimen and the end notched flexure(ENF)specimen were established to carry out simulation,and the accuracy of UCZM was verified by comparing the results of simulation and experiments.The relationship between mechanical parameters of adhesive and strain rate was determined through experiments.After simplifying UCZM and considering the effect of strain rate,a user-defined strain rate dependent trilinear CZM was established by VUMAT subroutine.3D FE models of thick adherend shear test(TAST)under different loading rates were established,and the accuracy of strain rate dependent tri-linear CZM was verified by comparing the results of simulation and experiments.The tensile failure of CFRP single lap joints and scarf joints with different design parameters was investigated through experiment and numerical simulation.In the 3D FE models,the in-plane damage initiation and progressive failure of laminates were predicted based on modified Hashin criterion and CDM,the delamination of laminates was simulated by CZM,and the failure of adhesive was simulated by UCZM.The tensile performance of the joints with different parameters was investigated systematically.The influence of overlap length,overlap width,ply stacking sequence on the strength,failure mode and stress distribution of single lap joints,and the influence of scarf angle,ply stacking sequence and adhesive’s properties on the strength,failure mode and stress distribution of scarf joints were analyzed.The influence of loading rate on joint strength was also considered,and the failure of adhesive was simulated by the strain rate dependent tri-linear CZM.The simulation results are in good agreement with experimental ones.