INVESTIGATING THE EFFECT OF MANUFACTURING POROSITY AND DAMAGE GROWTH ON THERMAL DIFFUSIVITY OF CERAMIC MATRIX COMPOSITES

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Abstract

Internal porosities in ceramic matrix composites are generally produced during the manufacturing processes. These porosities usually appear in the form of voids and cracks which lead to the degradation of thermal and mechanical properties of the composite. Furthermore, in-service damage growth leads to a further degradation of the properties of the composite. The primary aim of the work reported in this thesis is to develop a new and efficient multi-scale finite element modelling strategy to investigate the effect of manufacturing porosities and damage growth on thermal and mechanical properties of ceramic matrix composites.This investigation concentrates on two new materials developed recently for nuclear and aerospace applications; a twill weave carbon/carbon composite and a three-dimensional orthogonal C-C/SiC composite. To study these materials, the manufacturing macro-porosities are characterised by using X-ray micro-tomography and SEM micrographs. This data is then used to develop novel multi-scale image-based finite element models of the composite to determine its thermal and mechanical properties with and without porosity. The influence of the stacking sequence of laminates on porosity distribution and through-thickness thermal conductivity of the carbon/carbon composite is also studied. For both materials, the finite element predictions and analytical solutions are compared with experimental results obtained by using the laser flash technique and tensile tests.The latter part of this thesis presents the effect of damage growth due to mechanical loading on the degradation of through-thickness thermal diffusivity of a ceramic matrix composite. This is also achieved by using multi-scale finite element modelling approach.

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