Innovative characterization and elastoplastic modelling of metals under static and dynamic conditions

Material characterization and modelling are fundamental in both Industry and Research worlds in order to have reliable FEM simulations. In this research, such procedures are investigated regarding quasistatic and dynamic tests on metals. According to several experiments reported in the literature, the static elastoplastic behaviour of such materials depends not only on the first stress invariant (triaxiality) for the ductile damage and on the second stress invariant (equivalent von Mises stress) for the yield, but also on the third stress invariant (normalized Lode angle X) which may affect at the same time the yielding and the ductile failure. In this research, a new accurate and easy-to-calibrate yield model is presented, in which the yield surface depends on the Lode Angle and, eventually, also on the triaxiality ratio. The proposed model has been tested and validated against experimental data from the literature on the Titanium alloy Ti6Al4V. Moreover, in this research, a new methodology is proposed and experimentally validated for translating the engineering curves, coming from tensile tests, into the true curves via material-independent mathematical tools named MVB functions, which only depend on the necking initiation strain and on the aspect ratio of the undeformed cross section. Regarding the dynamic behaviour of metals, this research is aimed at the quantitative evaluation of the error levels in the characterization via Hopkinson bar tensile tests, ran according to the classical strain-gauge-based experimental procedure and to the enhanced high-speed-camera-assisted procedure. In addition, the effect of the specimen slenderness is investigated for checking the sensitivity of both the above techniques to different specimen geometries. Lastly, this work analyses the necking-induced freezing of the strain rate effect via experimental data and numerical simulations, with reference to materials exhibiting both early and late necking initiation and the consequences of this phenomenon in the characterization process via Hopkinson bar tensile tests.