Combining porous plasticity with Coulomb and Portevin-Le Chatelier models for ductile fracture analyses

For large void volume fraction, the Rousselier porous plasticity model transforms naturally into a coalescence model, in the whole range of stress triaxiality, in agreement with the necessary kinematic coalescence condition (NKCC). It enables to model slant fracture in notched tensile or cracked specimens. Nevertheless, void coalescence is not the single mechanism involved in slant fracture. That is why it is necessary to combine porous plasticity with other models. In this paper, the Coulomb fracture model and the Portevin-Le Chatelier (PLC) model (or dynamic strain aging: DSA) are formulated at the slip system scale. The Coulomb model combines the resolved normal and shear stresses for each slip plane and direction. For DSA, we postulate that each slip system has its own history of dislocation pinning and unpinning by solute atoms. The models are fully coupled in the framework of classical polycrystalline plasticity. A Reduced Texture Methodology (RTM) is used to provide the computational efficiency needed for numerical applications. The RTM approach involves a significant reduction of the number of representative crystallographic orientations. The models are applied to a notched tensile specimen taken from a 6260 aluminum alloy thin-walled extrusion. Fractographic examinations show a combination of dimples and large smooth areas on the slant fracture surface (mixed fracture). It highlights the need for combined fracture models. The PLC model gives very sharp oscillations of the macroscopic plastic strain rate, associated with moving plastic strain rate bands. It leads to a significant reduction of ductility compared to porous plasticity alone.