Modelling micromechanical damage of fusion reactor materials

Thesis

Nuclear Fusion presents a promising long-term solution to global energy demands. However, the successful operation of a fusion reactor depends on developing advanced materials capable of withstanding extreme operational conditions. This thesis employs Finite Element Models (FEM) to study the fracture properties of key candidate materials. It investigates the damage mechanisms from brittle fracture to complex orientation-dependent ductile-brittle fracture.

The first chapter focuses on the fracture mechanics of Silicon Carbide (SiC) fibre-reinforced composites with PyC interphase. FEM simulations of fibre push-out experiments characterised interfacial properties and determined the optimal length in fibre tests to be more effective for isolating interfacial debonding stress from frictional effects. This establishes the debond stress as a consistent and predictable material property, whereas friction was found to be dependent on fibre geometry and residual stresses.

Subsequently, a crystal plasticity open-access code (OXFORD-UMAT) was developed to model plastic deformation through Geometrically Necessary Dislocation (GND) densities. The code includes internal physical mechanisms for a more accurate GND calculation, which prevents erroneous hardening.

This framework was then applied to model the Bauschinger effect in copper, a material employed for stress relief during the cyclic fusion reactor operations. The results showed that existing kinematic hardening models fail to capture initial loading cycles. A new proposed model proved to be successful by incorporating the conversion of part of the GNDs into statistically stored dislocations, consistent with experimental evidence.

Finally, a comprehensive fracture model in single-crystal beryllium was developed by integrating the OXFORD-UMAT with eXtended Finite Element Method (XFEM). This approach simulates the entire failure process from plastic deformation to fracture, successfully reproducing experimental data for different crystal orientations. The model reveals that the material’s strong orientation-dependent failure is governed by the combined effect of Mode I (elastic) and Mode II (plastic) fracture.

Authors: Martinez Pechero, A

• DOI: 10.5287/ora-w4jbk0ybe
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fusion energy; GND; Beryllium; fibre-reinforced SiC; copper; FEM; OXFORD-UMAT; fracture mechanics
• Subjects: Material for Fusion Energy