Forming Limit Prediction Of AA5182-O Aluminum Sheet Based On Ductile Fracture Criterion

Aluminum alloy sheet has the advantages of low density,high specific strength and good impact performance,which is widely used in automotive lightweight materials.However,in the actual production process of aluminum alloy sheet stamping,the phenomenon of cracking failure often appears,which leads to reduced yield and greatly increased cost.The mechanical behavior of aluminum alloy sheet before fracture is significantly different from that of traditional steel sheet,with poor ductility and no obvious necking phenomenon before fracture.Therefore,how to accurately and effectively predict the forming limit of aluminum alloy sheet during the forming process has always been a key problem in the field of sheet forming research.In order to predict the forming limit of AA5182-O sheet metal in stamping process more accurately and effectively,four new uncoupled ductility fracture criteria(MMC,Lou-2013,Mohr and Huqi)were selected as theoretical basis in this paper.Five tensile specimens with different shapes and notch were designed,and the equivalent fracture strain was obtained by DIC method.The material parameters of each ductile fracture criterion were obtained by the combined method of test and simulation.The Mises yield criterion was used to derive the theoretical forming limit diagram,and the forming limit curves of four kinds of ductile fracture criteria were plotted.The correlation coefficient and average error between theoretical fracture strain and experimental fracture strain of the four ductile fracture criteria are analyzed and calculated,and the prediction and accuracy of the four ductile fracture criteria for AA5182-O plate are evaluated and analyzed.Theoretical and experimental studies show that,within the range of unidirectional tension-bidirectional stress state,the average error of fracture parameters of Lou-2013 ductile fracture criterion obtained by DIC method using(circular arc + circular hole + cup protrude)scheme is the smallest,which is only 3%.The mean error values of fracture parameters of the three ductile fracture criteria,MMC,Huqi and Mohr,were 34%,18%and 30%,respectively.The mean error values of fracture parameters of Lou-2013,MMC,Huqi and Mohr ductile fracture criteria were 6%,47%,32% and 54% respectively for the equivalent plastic strain obtained by finite element method.According to the equivalent fracture strain obtained by finite element method,the mean value of fracture parameters of Mohr ductile fracture criterion obtained by using the(circular arc + circular hole)scheme is the largest,with a height of 3%.The mean value of fracture parameters of Lou-2013,MMC and Huqi ductile fracture criterion obtained by using the same scheme is 23%,55%and 42%,respectively.The mean fracture parameter errors of Lou-2013,MMC,Huqi and Mohr were 13%,47%,25% and 58% respectively for the equivalent plastic strain obtained by DIC method.The forming limit diagram based on Lou-2013 Ductile Fracture criterion can accurately characterize the mechanical variation law of fracture equivalent plastic strain of AA5182-O sheet with stress triaxiality.Through the prediction of forming limit of typical sheet parts in the deep drawing test of cylindrical parts,the experimental results show that Lou-2013 can accurately predict the forming limit of AA5182-O sheet.In conclusion,Lou-2013 ductile fracture criterion established based on the microstructure mechanism of cavity nucleation,growth and aggregation is used to describe the tensile fracture mechanism,and the joint influence of cavity growth and continuous acceptance is assumed to achieve a better prediction result in the prediction of AA5182-O high strength sheet.However,when MMC Ductile Fracture Criterion is used to predict the forming of AA5182-O sheet metal,the prediction result is partial to safety.The prediction results of Huqi Ductile Fracture Criterialfor predicting the forming of AA5182-O sheet metal tend to be dangerous.When Mohr Ductile Fracture Criterion is used to predict the forming of AA5182-O sheet metal,the prediction result is too safe.The research in this paper can provide reference for the choice of theoretical prediction model for forming fracture of AA5182-O high strength aluminum sheet and the solution of parameters of ductile fracture criterion.