Study On Multi-scale Modeling And Mechanical Behavior Of CNTs/Al Composites

Metal matrix composites(MMCs)have excellent properties such as high modulus,high strength and resistance to friction and wear,and have been widely used in aerospace,shipbuilding,automotive,electronics and other fields.Although the strength of metal matrix composites has been improved to a certain extent,its plastic deformation ability and elongation are greatly reduced,which leads to the metal matrix composites are not easy to be processed and formed,and increases the difficulty of manufacturing products.Therefore,the accurate description and prediction of deformation mechanism and damage evolution of metal matrix composites can provide guidance for the improvement of plastic forming process,which has important industrial application valueAccurate prediction of fracture strain and damage evolution of metal matrix composites has always been a hot research topic in the field of composite forming.In the simulation of metal matrix composites,the finite element method is the most widely used simulation method,which can accurately simulate the plastic deformation and damage evolution of materials.In view of the plastic deformation ability,elongation and damage evolution of metal matrix composites under complex loading conditions,this paper takes carbon nanotubes reinforced Aluminum matrix composites(CNTs/Al)as the research object.Cross-scale modeling and analysis were carried out from macro scale to micro scale,and the mechanical response mechanism and damage evolution process of composite materials were further studied.In the process of simulation,the accuracy of simulation results depends on the accuracy of the constitutive model.Therefore,the constitutive model which can accurately describe the mechanical properties of materials is very important to the accuracy of simulation results.At the macroscopic scale,based on the existing GTN(Gurson-Tvergaard and Needleman Model,GTN)model based on Gurson's porous plasticity model,this paper considers the growth rate of porous generated by shearing.An advanced shear damage enhanced GTN(AGTN)model was implemented and used.At the microscopic scale,based on the theory of strain gradient plasticity(SGP)and Taylor's model,A multi-mechanism hybrid strengthening strain gradient plasticity model(MMSG)based on multiple reinforcement mechanisms is proposed.Based on a multi-scale framework,the model relates the two dislocation densities of SSDs(Statistically stored dislocations,SSDs)and GNDs(Geometrically necessary dislocations,GNDs)to the plastic strain and strain gradient at mesoscopic scale.And this constitutive model can intuitively reflect the distribution of dislocation density in the process of plastic deformation of composites.The main research contents of this paper are as follows:(1)At the macroscopic scale,a fully coupled constitutive model considering stress triaxiality,Lode angle and growth rate of shear-reinforced porous was established based on Gurson's porous plasticity model.The model can truly reflect the plastic deformation and damage evolution process of CNTs/Al composites on the macroscopic scale.At the microscopic scale,the strain gradient plastic model based on multi-mechanism hybrid strengthening can reflect distribution of the stress-strain,dislocation density and various reinforcement mechanisms in the plastic deformation process of composite materials.These two constitutive models are implemented in ABAQUS finite element software by writing subroutines in Fortran program language.(2)In this paper,CNTs/Al composite was selected as the research object,and experiments with different loading paths were designed,which mainly included uniaxial tensile experiment(UTT),pre-notch tensile experiment(PNT)and butterfly wing tension-shear experiment(BFT).The mechanical parameters such as elastic modulus,yield strength,tensile strength and elongation of the material were obtained by uniaxial tensile test.The mechanical properties and damage evolution of CNTs/Al composites under different loading paths were compared and analyzed.(3)Based on the least square method,with the experimental results as the target,the experimental and numerical hybrid inversion method is adopted to realize the parameter identification.In Abaqus software,a simulation model with the size of 1:1 to the experimental sample was established at the macroscopic scale.At the mesoscale,based on EBSD grain diagram and Voronoi algorithm,a representative volume element(RVE)model similar to the microstructure of CNTs/Al composite was constructed.On the basis of this RVE model,the RVE models containing different CNTs volume fraction and Al matrix grain size were constructed simultaneously.(4)The AGTN model was used to simulate the UTT,PNT and BFT.And the Force-Displacement curve,fracture strain and simulation accuracy under different loading paths were compared and discussed.The strain gradient plastic model was used to simulate the RVE model under tensile and shear loading conditions,and the stress-strain distribution,dislocation density distribution and strengthening mechanism of CNTs/Al composites were discussed.