Advanced 3D Characterisation of Failure Mechanisms in Fibre-Reinforced Composites

This thesis focuses on developing the understanding of fibre fracture processes in unidirectional (UD) CarbonFibre Reinforced Polymers (CFRPs), by delineating aspects of the load shedding phenomenon. In this context, Digital Volume Correlation (DVC) has been applied, in concert with in situ Synchrotron Radiation Computed Tomography (SRCT) to representative double-edge notched cross-ply specimens subject to quasi-static loading, which allow observation of fibre fracture in the 0° plies. Analogous to surface-based Digital Image Correlation (DIC), DVC is a relatively novel volumetric method that can utilize Computed Tomography (CT) data to quantify internal three-dimensional (3D) displacements and implicit strain fields. The highly anisotropic and somewhat regular/self-similar microstructures found in well-aligned UD materials at high fibre volume fractions are shown to be intrinsically challenging for DVC, especially along the fibre direction. To permit the application of DVC to displacement and/or strain measurements parallel to the fibre orientation, a novel CFRP was developed, specifically tailored for the application of DVC and CT to experimental mechanics analyses of these materials. The strategy chosen was to dope the matrix with a sparse population of sub-micrometre particles to act as displacement trackers (i.e. fiducial markers). Barium titanate particles (400 nm, ∼1.44 vol. %) were found to offer the most favourable compromise between contrast in CT images and the ability to obtain a homogeneous distribution in 3D space with sufficient particle compactness for local DVC analyses. By comparing the micromechanical behaviour of the particle-adapted material alongside its particle-free counterpart, it was determined through the application of in situ SRCT that the macroscopic and micromechanical responses of the newly developed CFRP are consistent with standard production materials indicating its suitability as a model system for mechanistic investigations. Following an experimental demonstration and validation of high-resolution 3D experimental strain measurement using DVC on CFRPs, via through-thickness strain analysis under a state of pure bending, this study has shown that the distance (∼50-100 µm) over which strain is recovered in the broken fibres was consistent with previous estimates and model predictions from the literature. New evidence was also highlighted, whereby the recovery lengths were shown not only to increase with the applied force, but also with the number of broken fibres in a cluster