Crystal plasticity modelling of thermomechanical fatigue in ITER relevant tungsten

This work contributes to a better understanding of the micromechanics of tungsten during cyclic heat loads of plasma-facing components in the ITER fusion reactor. Colossal energy will have to be extracted from the reaction chamber and the temperature of the walls will oscillate together with periodical changes of plasma intensity. This research addresses thermomechanical fatigue, i.e. microscopic cracking due to repeated cycles of thermal expansion and compression. The effect of neutron irradiation damage is not accounted for, but the proposed model applies to thermal shock tests, which are used for the qualification of ITER-relevant tungsten grades. The computational model relies on crystal plasticity theory in order to account for the anisotropy of individual grains. The sensitivity of the mechanical response to strain rate and temperature is reproduced by considering thermally activated dislocation slip. The influence of internal stresses during repeated elastic-plastic transients is investigated using a simplified modelling of the kinematic hardening due to dislocation pile ups at grain boundaries. The model is implemented as a user-defined material law for the Abaqus finite element code, allowing crystal plasticity based finite element modelling (CPFEM). Fatigue indicators are defined to predict the onset of damage whereas cohesive elements are used to simulate the propagation of intergranular cracks. Based on an inverse finite element analysis, it is shown that experimental tensile test data can be reproduced with a limited set of parameters and then applied to as-received and recrystallized tungsten up to 1700 °C. Recrystallized tungsten shows significant asymmetry in the mechanical response during the heating and cooling phases. CPFEM predicts substantial inter- and intragranular heterogeneity and the fatigue indicator values probed at grain boundaries depend largely on the amplitude of backstresses. The occurrence of intergranular cracks is influenced by grain shape as well as the orientation with regard to the thermal flux. Experimentally observed foil delamination is properly reproduced only when accounting for crystalline anisotropy. The model predictions agree qualitatively with the outcome of thermal shock tests.(FSA - Sciences de l'ingénieur) -- UCL, 201