Study On Crack Prediction And Fracture Criterion Of 7075-T6 Aluminum Alloy Sheet Bending Forming

Plastic forming is a widely used method of metal processing technology.Bending forming of sheet metal is a common plastic forming method.In the metal plastic forming process,crack problems can occur.Before product processing,predictive formability is of great significance to improve technical parameters,reduce manufacturing time,and reduce production costs.Aluminum alloy is a kind of high-strength metal material that is widely used in aviation,railway transportation and other fields.The main research content of this paper is the formation and prediction of cracks in the bending forming process of aluminum alloy sheet.The focus of the work examined in this paper includes the following:For cracks in bending forming,the Lemaitre ductile fracture criterion is applied,and the Lemaitre criterion is improved by considering the stress triaxial δ_m/（?）,the maximum principal stress ratio δ₁/（?）,and the influence of plastic strain on damage.The crack defects of sheet metal in bending forming can be predicted effectively.During the bending process of the plate,the force state on the upper and lower surfaces is asymmetric.In order to optimize the production process,it is necessary to analyze the force state of the plate in the thickness direction and the law of neutral layer migration during the entire deformation process of bending forming,and summarize the changes in the neutral layer migration before and after cracks.According to the neutral layer shift theory,the relationship between the easily damaged area and the parameters of the upper roll controlled by the process is established,which provides a certain reliable basis for the optimization of the production process.Using 7075-T6 aluminum alloy as the object of study,DEFORM-3D software was used to simulate the location of cracks in aluminum alloy under bending and forming process conditions,and the effect of different temperature,plate thickness,roll distance and pressing rate on the crack generation.The crack generation conditions of the alloy sheet were obtained.Based on the simulation data,the material parameters of the improved Lemaitre criterion were obtained,and the fracture threshold of the damage area during the bending process of the plate was determined.It was found that the crack first occurred in the center of the center of the convex side during the bending of 7075-T6 aluminum alloy sheet,and the limit equivalent stress of the crack at the center of the convex side is greatly affected by the change of temperature.The plate thickness,roll gap and reduction rate have little effect on the equivalent stress when the crack is generated,but they will affect the crack generation rate and the bending degree of the plate.A three-point bending experiment was done about 7075-T6 aluminum alloy sheets with different thickness and roller spacing,and the metallographic structure of the plates at key stress positions was observed.The accuracy of the improved fracture criterion for crack prediction is verified by comparing the reduction amount when the crack is generated with the theoretical reduction amount obtained by the fracture criterion.In this paper,the prediction of crack in bending forming has certain significance and application value for the study of forming limit of sheet metal,which is beneficial to the optimization of the bending forming process of aluminum alloy sheet.