Hardening Behavior and Prediction of Ductile Fracture during AA7075-T651 Sheet Metal Forming

This paper addresses the prediction of ductile fracture and hardening behavior of AA7075-T651 sheets under different stress states. Tests from shear to balanced biaxial tension are carried out up to fracture to determine ductile fracture. Considering the effect of different loading conditions on the plastic behavior of the material, stress triaxiality T, representing different stress states, and Lode parameter L are introduced to the Voce hardening model. After calibration by an inverse engineering approach, the improved hardening model is analyzed numerically, and an accurate description of the material strength under different stress states is verified. Based on the hardening model, four uncoupled ductile fracture criteria (Oh, Rice-Tracy, DF2012 and DF2015) are calibrated and utilized to predict the onset of fracture for the specimens. All fracture strains are obtained by a hybrid experimental-numerical method. A comparison of the predicted fracture strokes indicates that the DF2015 criterion predicts the fracture behavior in better agreement with the experimental results, especially for the Nakajima specimen.