Research On Damage Behavior Of AA6061 Under Complex Loading Conditions Via Void Evolution Analysis In The Forming Process

The fracture of metal is the result of damage accumulation.The damage accumulation process is also the process of void evolution during deformation,which includes four stages:void nucleation,growth,coalescence and microcracks propagation to macroscopic cracks.Metals are often under complex variable path loading conditions in the forming process.Changes in the loading path have a negligible effect on the damage behavior of material.In order to improve the prediction accuracy of finite element simulation for damage behavior of material in the forming process,the law of void evolution under variable path loading conditions should be studied in depth.GTN damage model is a coupled fracture criterion based on void nucleation,growth and coalescence,which can accurately describe process of void evolution and predict crack initiation and propagation of materials during monotonic loading.However,few studies are currently focused on the model and its parameters under variable path loading conditions.There are still many difficulties in the application of GTN model under complex loading conditions.Therefore,research on the law of void evolution under variable path loading conditions is carried out,then the effect of prestrain and loading direction change on the prediction of GTN model for the void evolution process,and the relationship between prestrain and model parameters are carried out,which is of great significance for broadening the applicability and improving the prediction accuracy of GTN model in the forming process.In this thesis,the variable path loading experiment of the annealed AA6061 material is designed,including the pre-loading experiment and the second-stage loading experiment.Experimental variables are the prestrain caused by the pre-loading stage and the loading direction of the second-stage experiment.By the observation and statistics of different regions of deformed specimens,effects of prestrain and loading direction on the void evolution of annealed AA6061 which is induced by subsequent deformation are studied.Results show that the larger the pre-deformation in the pre-loading stage,the greater the influence on the void evolution in the second loading process,which means that the void volume fraction decreases with the increase of the prestrain value;if the prestrain is certain and the pre-loading direction is consistent with the second stage along the sheet rolling direction,the void volume fraction is higher under the same equivalent plastic strain.GTN model is used to simulate the process of void evolution in the variable path loading experiment.The NSGA-II optimization algorithm combined with the parameter inverse method is used to determine model parameters,in order to study the effect of prestrain and loading direction change on the prediction of GTN model for the void evolution process and the relationship between prestrain and model parameters.Results show that the same set of f_n,?_n,f_c and f_F values can not accurately simulate the void evolution during deformation under different variable path loading conditions.As the prestrain increases,f_n,f_c,and f_F decrease gradually,?_n increases.If the prestrain value is constant and the loading direction changes from parallel to vertical,f_n,?_n,andf_F decrease,f_c rises.Changes of parameters reflect the contribution of the void nucleation process to the increase of the void volume fraction and the change of material carrying capacity.What's more,there is an exponential function relationship between prestrain and GTN model parameters,which can be used to determine model parameters corresponding to a certain prestrain value,so that the GTN model can accurately predict the void evolution process under different prestrain conditions.