Fracture Prediction Of 5A06 Aluminum Alloy Sheet In Different Stress States

Fracture is one of most common defects in stamping. Fracture prediction is very important for correcting mold and adjusting forming process parameters timely, which is helpful to reduce rejection rate and enhance efficiency. The forming limit diagram(FLD) is the main conventional method of fracture prediction in stamping processes. However, the strain path dependence of the FLD has an effect on the accuracy of the fracture prediction of the complex strain path.In recent years, fracture criteria based on the damage mechanics are widely used for fracture prediction, but these criteria can’t be accurately applied in all stress states. The research on the applying scope of these damage fracture criteria is significant for the fracture prediction in different metal forming processes.In this paper, four types of tensile specimen with different stress states were designed and stress triaxiality was selected as the characterization parameter of stress state. The distribution of stress triaxiality was researched by the experiments combined with numerical simulation. Fractures were observe by SEM and the fracture mechanisms were also analyzed.Parameters of Johnson-Cook fracture criterion and GTN fracture criterion were identified by experiment and simulation. Fractures of four types of tensile specimens were predicted by these two fracture criterions. The predicted results from simulation were compared with the corresponding experimental results. The results show that, Johnson-Cook fracture criterion is suited to predicting the fracture in the condition of middle and low stress triaxiality. On the contrary, GTN fracture criterion is suited to predicting the fracture in the condition of middle and high stress triaxiality.Experiment of hydro-bulging half ellipsoid part was conducted and its fracture was predicted by Johnson-Cook and GTN fracture criteria. The results show that, the stress triaxiality in the fracture position during hydro-bulging half ellipsoid parts is between 0.55 to 0.7, which belong to the scope of high stress triaxiality. The predicting result of Johnson-Cook fracture criterion exist large deviation while GTN fracture criterion can give a good prediction, which can precisely predict the fracture position, fracture pressure, the bulging height and the minimum wall thickness of half ellipsoid part. The results from hydro-bulging half ellipsoid part confirm the results from tensile tests, which Johnson-Cook and GTN fracture criteria is suited to the different level of stress triaxiality. This conclusion can be used to predict the fracture behavior in common stamping.