Investigation On Subsequent Anisotropic Behavior Of Sheet Metals Based On Mechanism Of Mesoscopic Plastic Deformation

With the development of industry and engineering technology,metal and metal matrix composites have been applied to complex load-strain paths due to their excellent mechanical and geometric forming properties.In view of the complicated mechanism of plastic deformation and subsequent plastic deformation of sheet metal,and considering the limitations of the macroscopic phenomenological constitutive model and the existing experimental conditions,it is difficult for the current domestic and foreign studies to predict the evolution of yield surface and subsequent yield surface under complex paths and non-proportional loadingsIn this study,the typical single-phase structure of high-strength aluminum alloy,dual-phase structural high-strength dual-phase(DP)steel and carbon nanotube-reinforced aluminum(CNT/Al)composite sheet metal were selected as the research objects.The user material(UMAT)subroutine considering the mesoscopic constitutive model based on the classical mesoscopic crystal plasticity(CP)theory and strain gradient plasticity(SGP)theory is combined with the mesoscopic finite element model established by the representative volume element(RVE)method.Combined with multi-scale experiments,the plastic deformation and subsequent anisotropic behavior under the changing loading path are numerically studied.The research work of this paper has made the following progress:(1)Multiscale experimental characterization on metal and metal matrix composites is investigated.Which are mainly based on high-strength aluminum alloy 6061,high-strength dual-phase steel DP600 and CNT/Al composites as the research objects,then carry out macroscopic experimental tensile and mesoscopic EBSD experiments respectively.Macroscopic experimental test,especially uniaxial tensile experiments,can provide a fitting basis for the constitutive parameters in subsequent simulations.And the mesoscopic characterization of grain morphology via EBSD shows that the size of AA6061 grain is uniform and statistically random,and the morphology of dual phase F/M is quite different.The ferrite is large and regular,and the martensite is slender and surrounds the ferrite.Which can provide a reliable topological geometry structure for mesoscopic finite element modeling.In view of the characteristic of high strength DP600 with large difference in mechanical properties of two phase,an additional micro-mechanical Nano indentation was carried out,which provided an effective data guarantee for the determination of properties of martensite and ferrite via parameter inversion of constitutive model.The mechanical properties,especially the yield strength of CNT/Al composite after the addition of carbon nanotube(CNT)-reinforced phase,have been significantly improved.With the increase of the gradient of grain size,the mechanical response of the composite has a significant decline.Mesoscopic EBSD test can only characterize the morphology of the matrix grains in the CNT/Al,while CNT cannot be characterized due to its too small size.(2)In this paper,the rate-dependent crystal plasticity and strain gradient plasticity constitutive model considering the effect of deformation gradient are established to describe the real flow behavior of sheet metals based on the theory of continuum mechanics and solid mechanics.The two kind of constitutive model was deduced at the same time through the numerical implementation of integral formula.The former calculates shear strain increment of nonlinear equations to update other stress value and status variables,the latter is based on the methods of calculating average plastic strain and plastic strain gradient.The user subroutine of finite element software ABAQUS was developed in combination with Fortran programming to provide accurate mechanical properties for the subsequent mesoscopic plastic deformation simulation calculation of metal plates.(3)Considering the different mesoscopic structural morphologies of high-strength alloys and CNT/Al composites characterized by the mesoscopic experimental test,different numerical algorithms are chosen to establish their mesoscopic geometrical finite element models.Then the mesoscopic constitutive parameters of three typical alloys are accurately fitted.The morphology of aluminum alloy AA6061 grains are random and conform to the natural growth law.Therefore,the reasonable mesoscopic RVE model is constructed based on the Voronoi algorithm,Python programming and sensitivity analysis.Then combined with the sensitivity analysis of crystal plasticity parameters,the AA6061 curves are accurately fitted.As for the DP steel,the RVE model is built based on EBSD image processing due to the two-phase structure and significant difference in morphology of two grains.Then the CP parameters of two phase are obtained through the inverse identification of Nano indentation test.By inputting the CP parameters of two phase to the RVE model based on EBSD,the tensile simulation results will be compared with the macro mechanical experiments to further verify the CP parameters.The mesoscopic modeling and calibration of the CNT/Al composite takes into account the interaction between the matrix and the reinforcement phase.The matrix grain adopts the crystal plasticity constitutive and the CNT reinforced phase adopts the strain gradient plasticity constitutive model to finally fit the macro-mechanical experimental curve and obtain accurate mesoscopic constitutive parameters.(4)In order to consider the full-path load conditions of the RVE model,an innovative simulation and data processing method of ABAQUS-MATLAB-PDE combined application is proposed.The yield surface of high-strength aluminum alloy are described by setting variable path loadings and submitting simulation jobs in batches and batch processing of ODB files based on Python scripts.The study shows that the initial yield surface of single-phase aluminum alloy AA6061 is approximately isotropic and approximately circular,expands uniformly with the increase of offset strain.The constitutive parameters,yield definition,pre-deformation mode,amount of pre-deformation,combined pre-deformation mode and other factors are strongly related to the description and evolution of the subsequent yield surface.At the same time,the pole figures at different forming stages are given,and the evolution of polycrystalline texture is analyzed and predicted.It can be concluded that some strong relationship between the texture evolution,the inhomogeneous stress field,and the subsequent yield surface evolution are proved and discussed.(5)As for DP 600 steel,the effect of phase distribution topology on the plastic deformation and subsequent anisotropic behavior of dual-phase steels by changing the distribution and fraction of Martensite can be studied.The study shows that the local stress in the martensite is significantly higher,while the local stress in the ferrite is lower,and there is obvious strain localization and shear bands in ferrite.The addition of martensite phase and its cluster effect can greatly increase the effective elastic modulus and yield strength of alloy DP600,and have little effect on the subsequent yield mechanical response under a single path.The unique heterogeneous structure and the difference of F/M distribution in the RVE model make the yield surface expand differently in complex loading paths.The more serious the distortion of the yield surface,the greater the effect on the loading path in the stress space.And the sensitivity to the loading path in the stress space is not the same.(5)As one of the most potential reinforcements in the metal matrix composite,carbon nanotube owns excellent mechanical properties and unique nanofiber morphology.Through its introduction and its attached process conditions,it is bound to cause changes in the microstructure and anisotropy in performance of the composite.This paper considers the influence of the reinforcement phase topology,size of matrix grain,grain gradient and other microstructure factors on the mechanical response and subsequent anisotropy of the CNT/Al composite.The study shows that the reinforced phase affected zone of the CNT/Al composite is mainly controlled by recrystallization and ball milling conditions.The CNT/Al composite treated by the ball milling speed of 160rpm/8h has a larger thickness of the reinforced phase affected zone.Finally,the strength and ductility have reached an extraordinary combination.The distribution of GNDs and SSDs in the composite is significantly different,and GNDs showed a strong strain path and historical correlation.With the increase of CNT volume,the greater the irregularity of the matrix grains will increase the elastic modulus and tensile strength of the composite.Which will also affect the hardening evolution and result in increasing the strain hardening rate.The subsequent anisotropic yielding under different pre-deformations is more obvious.With the increase of the matrix grain gradient and the addition of large grains,the Trimodal model with a tertiary grain size gradient plays an important role in reducing the overall internal stress of the RVE model and the sensitivity of subsequent anisotropy.In a word,by summarizing the influence of the micro-microscopic mechanism of polycrystalline coordinated deformation under changing loading paths on the micromechanics and subsequent anisotropic properties,it can provide a theoretical basis for precise control of sheet metal forming.