A Computational Thermo-Chemo-Mechanics Framework for the Large-Scale Simulation of Material and Structural Failure in Hypersonic Environments

Abstract

Materials and structures subjected to the extreme conditions of hypersonic flight undergo complex degradation and fracture processes. This thesis presents a theoretical formulation and a computational framework that enables the large-scale simulation of thermochemically fracturing solids exhibiting complex post-fracture interface response. The continuum theory is based on a general thermodynamically-consistent description of the coupled multiphysics problem, and the numerical formulation extends the scalable discontinuous Galerkin(DG)/Cohesive Zone Modeling paradigm to thermo-chemo-fracture mechanics. The approach is distinguished by its unified DG treatment of the coupled problems, which facilitates the analysis of fracture propagation, fracture-dependent heat and mass transfer as well as thermally-activated solid-phase chemical reactions. The framework is verified against two analytical solutions of boundary value problems drawn from thermo-poro-elasticity and thermally-driven delamination. Three-dimensional simulations of a benchmark thermochemically-driven fracture problem illustrate the parallel scalability of the fully-coupled computational framework. We utilize this framework to render models of passive oxidation-induced fracture in ultrahigh temperature ceramics computationally tractable. First, a rigorous constitutive theory is shown to capture the molecular diffusion of oxidant through the reaction product layer using only fundamental transport properties, i.e. without the need for calibration to reaction experiments. The physical processes observed on the diminutive scale of an oxide layer are explicitly resolved, but the approach is limited to microscale analyses by scale separation. We sidestep this limitation by specializing the general theory under specific phenomenological assumptions, thereby yielding a practical model that can reproduce oxidation experiments. We use this specialized model to analyze oxidation-induced swelling, fracture and delamination in SiC/coating systems, and unveil the coupled thermochemical response as well as fracture morphologies in the vicinity of critical flaws. Then, we conduct a parametric study of three-dimensional coatings that exposes the channeling mechanisms above penny-shaped delaminations of various sizes. The computational analyses identify a transition from decussating to circumferential channel cracking that explains the wide variety of surface channel cracks observed in experiment. The physical mechanisms and fracture morphology regimes are corroborated by a simple structural theory. Finally, cohesive fracture models, splitting methods and thermal solvers are developed specifically for applications to thermally shocked ceramics. Simple and rigorous calibration procedures are proposed which facilitate the direct analysis of fragmentation and comminution in brittle solids subjected to extreme advective heat transfer. The presented examples serve as evidence that the framework can successfully enable three-dimensional, thermochemically-coupled fracture analyses of unprecedented physical fidelity, which furnish new insights into complex hypersonic thermal protection system response.