Strain localization and ductile fracture in single crystals: application to irradiated austenitic stainless steels

For their excellent mechanical and oxidation properties, austenitic stainless steels are widely used in the nuclear industry, in particular for structural applications inside the core of reactors. However the substantial neutron irradiation levels these materials can be exposed to can detrimentally affect their mechanical properties. A sharp drop of toughness is indeed observed as the irradiation dose increases. Depending on the irradiation conditions (temperature, dose), mainly two kinds of radiation-induced defects can be responsible for this behaviour: dislocation Frank loops at low irradiation temperature (∼300 °C) and nano-voids at higher temperature (∼600 ◦C). Since these defects exist and act at the subgrain level, it motivates to study their effects at the single crystal scale. First of all, this work aims at obtaining experimental data on the mechanical behaviour of austenitic stainless steel single crystals. Then, modeling of softening induced strain localization phenomena, as those taking place in irradiated materials, is investigated. The limitations of a reduced strain gradient crystal plasticity model regarding shear bands predictions are exposed on the grounds of analytical solutions and an enhanced theory accounting for internal length evolution is proposed. Thereupon attention is given to the numerical efficiency of the finite element implementation of the aforementionned strain gradient plasticity model. Micromorphic and Lagrange multiplier based formulations of the original theory are described and compared upon finite element simulations. Eventually, one of a kind ductile fracture model of porous single crystals is proposed – including both void growth and void coalescence – in order to investigate impact of radiation-induced nano-voids on the mechanical behavior of irradiated austenitic stainless steels. The model is set up in a strain gradient framework in order to regularize ductile fracture.
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