Micro-scale characterization of brittle fracture in UO

This work is conducted within the framework of nuclear fuel safety analyses involving multiscale modelling of UO2 fuel pellet fragmentation process. In this paper, a Finite Element (FE) methodology is proposed in order to enable 2D and 3D simulation of brittle fracture in micro-scale specimens used for the characterization of irradiated nuclear fuel. More specifically, these developments address Finite Element simulations using Cohesive Zone Modelling with complex shape of defects, where a direct application of Griffith’s criterion with the fracture mechanics approach is not always possible. In the case of UO2 ceramic material, the mesh refinement needed for the FE simulation of brittle fracture requires a specific attention due to the fact that the damage process zone has a characteristic size smaller than 10 nm. The main objectives are first to discuss the questions related to mesh convergence in the case of stress singularity, and secondly to enable the use of coarser meshes while maintaining accuracy compared to the converged mesh solution. The mesh convergence analysis and coarse mesh approach, established on a 2D configuration, are based on a Cohesive Zone Model (CZM) using two parameters: the critical cohesive stress σc, and the fracture energy Gc. The coarse mesh approach introduces a numerical critical cohesive stress, σc*, replacing the physical one, σc. Thanks to this approach, the relative mesh size can be increased by a ratio of ten with a deviation of the fracture load assessment smaller than 1% compared to the Griffith’s solution. The FE methodology and the coarse mesh approach are then tested with 2D and 3D simulations of a micro-cantilever bending test on a notched specimen. The simulation results are consistent with the experiment where the parameter Gc controls the fracture load. The critical stress, σc, ahead of the notch tip, can vary in the range 5–20 GPa with no effect on the fracture load. The coarse mesh approach is validated with a deviation lower than 2% compared to the fine converged mesh.