An assessment of isotropic constitutive models for ductile fracture under high and low stress triaxiality

In this contribution, a numerical assessment of three isotropic constitutive models is performed in order to identify their applicability and reliability in the prediction of ductile failure under a wide range of stress triaxiality. The well established isotropic coupled damage models proposed by Gurson–Tvergaard–Needleman (GTN), which is based on micromechanical grounds and here extended with a shear mechanism, and by Lemaitre, which is based on continuum damage mechanics, are selected and investigated. Besides these, an uncoupled damage elasto-plastic model proposed by Bai and Wierzibicki, which includes the effect of three invariants of the stress tensor, is also selected and examined. All constitutive formulations are implemented in a quasi-static finite element scheme and applied to simulate the behavior of the 2024-T351 aluminum alloy, which is strongly dependent on both pressure and Lode angle. To assess the predictive ability of the constitutive models under different levels of stress triaxiality, specimens with different geometries and dimensions are used, such as: smooth and notched cylindrical bars, a plate hole specimen and a butterfly specimen. The evaluation of the models is initially carried out under pure tensile loading conditions and then under shear dominated deformation modes. In addition, a combination of both tensile and shear loading is also studied. Finally, the results obtained from the numerical simulations are analyzed and critically compared with experimental results available in the literature. The performance of each constitutive approach under each range of stress triaxiality is highlighted and the main observations are discussed.