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Fracture constantly occurs during the plastic forming processes of thin-walled metal sheets due to the local uneven plastic deformation. During the processes, the sheets may experience different stress states, which are crucial factors related to the fracture. To predict fracture in different forming processes, various uncoupled ductile fracture criteria have been proposed due to their advantages of relatively simple expressions and less modeling parameters. Because each fracture criterion focusses on certain aspects of the deformation behavior, the ability of fracture prediction can be dramatically different. Thus, a systematical study on eight widely applied uncoupled ductile fracture criteria is carried out to evaluate their applicability under four different stress states, i.e. pure shear, dog-bone tensile, notched tensile and hydraulic bulging. Each criterion is numerically implemented and embedded into finite element models and is applied for fracture prediction of an anisotropic aluminum alloy sheet. The prediction results are compared to experimental results and the applicability of each criterion is analyzed.

A constitutive model that can describe the damage evolution of anisotropic metal sheets during the complex forming processes which experience wide stress triaxiality history is essential to accurately predict the deformation and rupture behaviors of the processes. In this study, a modified Lemaitre damage criterion which couples with the anisotropic Barlat 89 yield function is established. The effects of stress triaxiality, Lode parameter and shear stress on damage accumulation are considered in the constitutive model. The model is numerically implemented and applied to fracture prediction in tensile tests with different stress triaxialities and a complex deformation process with wide stress triaxiality history. The good consistency of predictions and experiments indicates that the modified Lemaitre damage model has excellent fracture prediction ability. Finally, the accuracy of the model is analyzed and discussed.

Ductile fracture always occurs in plastic forming processes of thin-walled metal sheets. Damage models, both coupled and uncoupled ones, have been extensively developed to predict fracture of the sheets. Among these, uncoupled damage models, owing to their simple form are widely used. While for an uncoupled damage model, there exist three forms for its fracture prediction: (1) damage threshold (DT), which is related to the damage accumulation; (2) fracture strain (FS), which is always a function of stress triaxiality and Lode parameter; (3) fracture forming limit diagram (FFLD), which is always related to the principal strains. However, for an uncoupled damage model, its three forms (DT, FS and FFLD) for fracture prediction have never been compared before, and thus scholars have always been confused about how to choose the right form of the three in different loading processes. To clarify this confusion, two popular uncoupled damage models of the MMC4 and DF2016 are embedded into an anisotropic yield function first. Then the above-mentioned three forms of the two models are separately applied for fracture prediction in shear test, dog-bone specimen tensile test, hole specimen tensile test, notched specimen tensile test, bulging test in proportional loadings, and a complex non-proportional loading process of spin forming, respectively. Finally, the predicted results with these three forms are systematically compared and a new explanation for their applicability under different stress states is displayed.