Micro-scale Analysis of Progressive Damage in Ceramic Matrix Composites

Abstract

In this thesis a numerical framework for predicting progressive damage in Ceramic Matrix
Composites (CMCs) at room temperature has been developed. The first stage entailed the
development of a tool for the generation of Representative Volume Elements (RVEs) in a
finite element environment. This uses an algorithm for predicting the random fibres pattern
already implemented for Organic Matrix Composites (OMCs) and rearranged to account the
multiple microstructural phases of CMC materials. The code has also been complemented
with a spatial hashing algorithm to drastically decrease the convergence time. A series of
other algorithms have also been developed for accounting the presence of voids as well as
eventual dispersed particles in the densified matrix. The virtual RVEs are then used for the
extraction of homogenous mechanical, thermal and electrical properties via a new formulation of
Periodic Boundary Conditions developed herein. The characteristic debonding/sliding mechanisms
happening at the interface between fibre and interphase material, which provide the CMC
toughening behaviour, has been schematised through a cohesive-frictional law implemented
in a Fortran subroutine for both implicit and explicit solvers. law has been validated through
experimental results of pushout tests performed by R.M.G. De Meyere at University of Oxford.
Furthermore, the failure properties of such complex bi-material interfaces have been determined
modelling some recently developed tests at Imperial College London by O. Gavalda-Diaz. In
particular, mode I interfacial fracture toughness has been determined modelling a micro-scale
double cantilever beam experiment via the insertion of a cohesive law at the fibre/interphase
interface. Mode II fracture toughness along with Coulomb frictional coefficient have been instead
calculated using some classical VCCT analysis. After the investigation of the interfacial behaviour
and its related properties, a continuous damage mechanics model (CDM) for predicting the failure
of the microstructural material phases has been implemented in a Fortran material subroutine.
The CDM subroutine hinges on the Christensen failure criterion as well as a cohesive-like
damage progression projected on the principal stress plane at failure. The subroutine is based on
a smeared approach were material’s fracture toughness is diluted over the element volume. The
full virtual damage response comprising the material phases damage as well as the cohesivefictional
interfacial behaviour has been validated against minicomposite test data. The material’s
properties employed in the simulations have been derived from the initial chapters of the work.
Finally, to look into future development a model for the generation of virtual minicomposite has
been implemented to predict, with an higher degree of fidelity, the complex three-dimensional
fibre pattern. Along with the model an image recognition algorithm has been implemented to
extract the fibre pattern from real minicomposite CT-scans as well as a topological and spatial
metrics tool for the post-processing of data. The thesis details the development of each of the
modelling phases and highlights the successes, challenges and limitations of each method.

Date of Award	12 May 2022
Original language	English
Awarding Institution	University of Bristol
Supervisor	Giuliano Allegri (Supervisor), Antonio R Melro (Supervisor) & Stephen R Hallett (Supervisor)