Numerical investigation of dynamic strain ageing and slant ductile fracture in a notched specimen and comparison with synchrotron tomography 3D-DVC

Abstract : In ductile tearing experiments, a flat crack is often observed to develop normal to the loading direction that subsequently turns into a slant crack. The underlying physical mechanisms are poorly understood. The numerical strategy for reproducing such kind of slant fracture remains a challenge. Recently, the strain field of a 2198 aluminium alloy CT-like specimen has been measured by lamino(tomo-)graphy combined with digital volume correlation (DVC) (Morgeneyer et al. (2014)). Multiple localisation bands were observed at notch area at early loading stages. The final fracture occurred within an inclined band. Experiments showed evidences of Portevin-Le Chatelier (PLC) effect in this alloy at room temperature. Previous simulations with von Mises, anisotropic plasticity or Gurson-Tvergaard-Needleman (GTN) models were not able to simulate these observations. As the PLC effect produces instabilities and multiple inclined localisation bands, it is considered to be a candidate for the underlying mechanism related to slant fracture. A fully coupled model (Rousselier and Quilici (2015)) combining polycrystalline, PLC, porous plasticity and Coulomb fracture implemented in FE code is used for simulating these phenomena. The PLC model gives intermittent and moving oscillations of the macroscopic plastic strain rate bands. The multiple strain localisation bands obtained by FE simulation for a thin sheet CT-like specimen are similar to the ones observed in laminography with 3D DVC. Crack propagation occurs during strain rate surges. A flat to slant fracture surface observed in laminography is reproduced successfully by the current FE simulation.