Numerical simulations and modeling of the effective plastic flow surface of a biporous material with pressurized intergranular bubbles: application to irradiated uranium dioxide

In this work, the specific microstructure of the irradiated uranium dioxide (UO2) is modeled as a biporous material with 2 populations of bubbles: intragranular bubbles of spherical shape, and an intergranular population of bubbles elongated along the grain boundaries. The effective plastic flow surface of such a biporous material is investigated through full-field numerical simulations. The effect of the intragranular cavities is modeled by a GTN (Gurson-Tvergaard-Needleman) matrix. Full-field numerical simulations are performed with a FFT-based (Fast Fourier Transforms) method (originally developed by [Moulinec and Suquet, 1994]) on 3-dimensional and periodic cells. A particular attention is paid to this specific microstructure and the effect of the distribution of the intergranular bubbles on the effective plastic flow surface. Different microstructures with different porosities and sizes for the intergranular bubbles are considered here (the mean size of the grains being fixed). Three loading conditions are imposed on the cells: a purely hydrostatic overall stress, a purely deviatoric overall stress, and an ‘intermediate’ overall stress (triaxiality ratio of 4). It is shown that the effect of the relative size of the intergranular bubbles depends on the direction of the loading. Then, a comparison is made with the analytical model of [Vincent et al., 2014] and a correction of the intergranular porosity is introduced in this model and identified to take into account the non-isotropic distribution of the intergranular bubbles. The corrected intergranular porosity is composed of two terms: the first one is predominant for low intergranular porosities, and the second one is predominant for high intergranular porosities. It is checked that the correction introduced in the analytical model can also be used for pressurized cavities such as encountered in irradiated UO2.