High-temperature Oxidation Models And Thermo-Mechanical-Oxygenic Coupling Behavior Studies For Advanced Thermo-structure Materials

Advanced thermo-structure materials are a kind of special materials which can sustain complex loading conditions under high temperature and chemical corrosion environments for a long time.Owing to their excellent physical and chemical properties under high temperature environments,they are widely used in national defense and economy priority areas,such as aeronautics,astronautics,etc.In this thesis,we focused on the oxidation behavior and thermo-mechanical-oxygenic coupling failure mechanisms of advanced thermo-structure materials.This research includes:?1?Within the framework of small deformation theory,a phase-field oxidation model was proposed for the inward diffusion predominant oxidation behavior.The oxidation diffusion process was described by the time-dependent Cahn-Hilliard equation.And the material deformation was predicted using the elastic-perfect plastic model and the Norton power law creep equation.The corresponding finite element formulations were derived using the standard Galerkin approach and the Newton-Raphson iteration scheme.Then,taking the oxidation process of zirconium as object,the influences of an initial rough surface and microvoids on the mechanical behavior of the oxide layer were analysed in detail.The mechanical failure mechanisms of the protective oxide layer were discussed,and compared with related experimental observations.?2?Within the framework of finite deformation theory,we presented a continuum-level thermodynamically consistent oxidation model that couples large viscoelastic deformation,diffusion and oxidation reaction.Constitutive equations were derived by means of the free energy inequality.The Eyring model for the shear stress dependence of viscosity was adopted to describe the viscous deformation.Meanwhile,through the introducing of a phase dependent penalty term in the diffusion coefficient,this model can automatically mark and track the sharp phase interface.Subsequently,taking the SiC fibers as object,we analyzed the formation mechanisms of surface cracks during the oxidation processes.?3?Based on thermal gravimetric analysis?TGA?,scanning electron microscopy?SEM?and X-ray computed tomography?CT?observations,the oxidation behavior and mechanisms of C/C-SiC and C/SiC composites were studied in depth.Based on this,a damage-induced short-circuit diffusion model was presented.For ceramic matrix,the initiation and evolution of micro-cracks were described by a damage constitutive model.Meanwhile,based on the simplified assumption for the actual physical processes,the diffusion coefficient of O₂molecules in the ceramic matrix was modelled as a function of the damage factor.For reinforcement materials,the oxidation reaction was taken into account,and an oxidation degree dependent orthotropic constitutive model was adopted.Finally,three typical coupled failure problems of C/SiC composites under different scales were studied in depth.?4?Referring to the three point bending fracture toughness experiment standard,we tested the fracture toughness of 2D woven C/SiC composites under different oxidation treatment conditions.Based on the SEM observations and comparing analysis for the load-displacement curves,we pointed out that the weight loss percentage is the key index for the variation of fracture toughness.Then,we established a linear empirical formula to describe the relationship between fracture toughness and weight loss percentage.It can be used conveniently for the reliability evaluation during the service processes of C/SiC composites.