Investigation Of Forming Limit Of 5083 Aluminum Alloy Sheet Using GTN Damage Model

Aluminum alloy has low density,high specific strength and stiffness.It is widely used in transportation,aerospace and other fields.However,due to its forming ability at room temperature,it is easy to crack and wrinkle during forming.Forming limit diagram(FLD)is a method to predict the failure of sheet metal in sheet metal stamping.Using finite element method,the forming limit diagram of sheet metal can be obtained quickly and cheaply.The core of predicting sheet FLD lies in the choice of instability theory or fracture model,which is directly related to the accuracy of prediction results.At present,the research on the forming limits of 5083 aluminum alloy mainly focuses on the classical plastic mechanics and macroscopic damage mechanics models,and there is a lack of research from micro and microscopic perspectives.In the mesodamage mechanics,the Gurson-Tvergaard-Needleman(GTN)damage model connects the microscopic pore structure with the macroscopic mechanical behavior of materials,and is effective in predicting ductile fracture of metals.This paper takes 5083 aluminum alloy sheet as the research object,the main research contents and conclusions are as follows:(1)The true stress-strain curve and thickness anisotropy index of sheet metal were obtained by several unidirectional tensile tests with different rolling directions.The results of plane anisotropy index Δr show that the anisotropy of 5083-H111 aluminum alloy plate is obvious.The deformation of sheet metal in the thickness direction is easier than that in the plane direction in the forming process.The parameters of Hill48 yield function are calculated according to the thickness anisotropy indices in different directions.(2)Scanning electron microscopy was used to observe the samples at different deformation stages,which confirmed the objective existence of micropores in the material and measured the range of damage parameters.Twenty-eight sets of unidirectional tensile finite element simulation experiments were planned by central composite design method,and the response values of different parameter combinations were obtained by using ABAQUS finite element analysis software.A response surface model between the characteristic values of tensile curves and the damage parameters was established.The influence of damage parameters on the mechanical properties of materials was analyzed by the coefficients of the regression equation and the trend of the response surface.NSGA-II genetic algorithm with band in Isight was used to optimize the damage parameters.The engineering stress-strain curve obtained by using the optimized damage parameters has a good agreement with the tensile test curve,and the relative error of the highest point is less than 5%,and the relative error of the breaking point is less than 4%,which proves the effectiveness of the finite element inverse method to obtain the damage parameters in this paper.(3)Based on Nakazima method,bulging experiment and finite element numerical simulation were carried out.Brozzo fracture criterion,Cockcroft & Latham fracture criterion and GTN damage model were used to predict the forming limit of 5083 aluminum alloy.The forming limit diagram with GTN damage model as failure criterion has good accuracy.The predicted fracture position of the plate and the load-displacement curve of the punch during the numerical simulation can be consistent with the bulging test results.The evolution law of pore volume fraction with equivalent plastic strain during bulging is analyzed.It is found that pore growth is affected by stress triaxiality,and high stress triaxiality can promote the growth of micropores and accelerate the failure of materials.Combining the GTN damage model with Hill48 anisotropic yield criterion,the prediction accuracy of forming limit can be improved by applying the GTN-Hill48 model.However,the prediction accuracy of Brozzo fault criterion and Cockcroft & Latham fault criterion is low,and the ultimate principal strain is underestimated in the double-tension deformation zone,and the deviation of Brozzo fault criterion is large.(4)Finite element analysis was carried out on the forming process of two kinds of surface covering parts,and the predicted FLD curve was applied to the forming analysis of surface parts.The prediction effect of thinning rate method and FLD method on fracture position of overlay forming was compared and analyzed.It is proved that the FLD obtained by using GTN damage model as failure criterion can be used to predict the fracture location of 5083 aluminum alloy curved parts.