Multiscale predictive modelling of ultra-high temperature structural ceramics

Abstract

Components in high-flow environments such as hypersonic vehicle nose cap and leading edges are subject to high temperatures and thermal gradients, with the requirement that they must maintain their geometric and structural integrity. A substantial increase in the capabilities of ceramic materials at the limits of temperatures and operational loads is being pursued, in order to use them in advanced hypersonic vehicles. Ultra-high temperature ceramics (UHTCs) are considered attractive candidates for nose cones and sharp leading edges of next generation hypersonic re-entry vehicles thanks to their high melting point, good stability at high temperature and high thermal and electrical conductivity. This project, funded by the UK’s Defence Science and Technology Laboratory (Dstl), aims at developing computational methods to assess damage and failure in UHTCs to aid in the design of laboratory tests, thus reducing time and costs associated with experiments. In fact, UHTC components may undergo creep and surface oxidation while operating at high temperature in aggressive environment, as well as fail prematurely due to thermal shock. It is therefore essential to be able to predict such phenomena to provide a powerful tool that can be useful for hypersonic vehicle design. A continuum damage mechanics-based model running as a custom user subroutine in the finite element (FE) software Abaqus is hereby proposed to account for damage due to creep and time-dependent material oxidation. Following an initial validation on a representative three point bend geometry to illustrate the methodology proposed to predict creep damage, a simple novel approach to estimate oxidation damage was incorporated in the user subroutine. Additionally, a microstructural model was introduced in the FE code to be able to treat grains and grain boundaries separately in the analysis, making it possible to simulate enhanced diffusion to grain boundaries. The same approach was also used to predict the heat flow, temperature distribution, phase transition and stress distribution during high heat flux laser tests and compared with experimental results.