Modelling dynamic necking instabilities in metallic ductile materials: the roles of porosity, tension-compression asymmetry and anisotropy

Abstract

Ductile metals are commonly used in the manufacture of protective structures. These structures may be submitted to impact or blast loads and their aptitude to absorb the energy of the dynamic load is strongly related to their ability to delay the onset of plastic instabilities. Numerous investigations, in the last two decades, have been dedicated to the study of plastic instabilities leading to a deeper knowledge on the effect that individually inertia, strain rate, loading path have on plastic instabilities and failure. However, the coupled influence of the material behaviour and loading effects deserves further analysis. This doctoral thesis contributes to the understanding of the key role played by material behaviour on the formation of dynamic necking instabilities.We focus our attention on the role played by porosity, tension-compression asymmetry and anisotropy. On the one hand we consider flat tensile specimens initially at rest and subjected to dynamic uniaxial tension and elucidate, using finite element calculations, the role of porosity and the combined role of anisotropy and tension-compression, respectively, on the characteristics of the necking bands incepted in the specimens. On the other hand we consider specimens subjected to dynamic loading conditions consistent with expanding rings and tubes and bring to light, using finite element calculations and linear stability analysis, the role played by porosity and tension-compression asymmetry, respectively, on the formation of multiple localization patterns.