The Complex Deformation Law Of Sheet Metal Under Macroscale And Mesoscale Condition

The deformation of sheet metal in the actual forming process is very complicated.After the material has the plastic deformation,the strain path of the material point will change.This will affect the mechanical behavior,flow rule and forming ability of the material,and the material will have transient effect,bauschinger effect,cross effect,permanent softening and other phenomena.Due to the existence of these material characteristics,the material will also show a variety of complex deformation laws.Therefore,it is of great theoretical significance and practical value to establish a mechanical model that can accurately describe the plastic deformation behavior of sheet metal and embed it into the finite element simulation software to accurately analyze the plastic forming of sheet metal.In this paper,the complex deformation law of sheet metal in the actual forming process is studied through the combination of theoretical research,experimental verification and finite element analysis at macro and meso scales.Macroscopically,the anisotropy index(R-value)in different loading states(uniaxial and biaxial tension)is defined uniformly.Then,according to the relationship between forward loading and reverse loading proposed by Barlat,the influence relationship based on stress or R-value with the Hill48 yield criterion is established to determine the subsequent yield locus at different strain moments in real time.The results show that this method can be combined with different yield criteria,and the coefficients of the yield criteria can be solved by different methods according to the actual requirements,in order to predict the subsequent yield trajectories of different materials more accurately.As an important tool for sheet metal forming performance research and defect prediction,the forming limit diagram(FLD)has been widely used for a long time.Studies have shown that using the experimental method to obtain the FLD is very time-consuming,especially for experiments under non-linear loading paths.Therefore,a theoretical prediction model of forming limit under complex loading conditions is needed.In this paper,a method for predicting the forming limit under complex loading paths using the FLD under linear loading condition based on the thickness reduction rate is proposed.The results show that the method proposed in this paper can accurately predict the forming limit under different pre-strains within a given small safety margin.What's more,the thickness of the sheet can be easily obtained in the experiment,which is not affected by the hardening model and constitutive relationship,so it is more convenient for practical application.On the meso scale,compared with the phenomenological theoretical model based on the macroscopic plastic deformation behavior of materials,the crystal plasticity theory is considered to be a theory based on physical mechanisms considering the mesostructure and meso-deformation behavior of materials.In this paper,the crystal plastic finite element method(CPFEM)is used to combine crystal plasticity theory with finite element numerical simulation.Taking a polycrystalline metal material(5754M aluminum alloy)with a typical FCC structure as the research object,the initial and subsequent yield loci of 5754 M aluminum alloy are obtained.The results show that the simulation results can better reflect the experimental results,and then the appropriate macroscopic phenomenological model can be selected to predict its subsequent yield loci under different loading conditions.The relationship between the macro-mechanical properties and microstructure can be established.Then,the experimental process and the cost can be reduced by using CPFEM.It provides a new idea and basis for getting and predicting subsequent yield loci under complex loading conditions.