Abstract
arbon Fibre Reinforced Polymers (CFRPs) have become increasingly used for high-performance lightweight structures due to their exceptional specific stiffness and strength. Conventional composite structure analysis relies on design practices and strength predictions based on ply-level analysis or macroscale testing, which typically oversimplify failure mechanisms at the microscopic level. The desire to better understand the relationship between damage and mechanical properties in composites at the microscale has grown rapidly, as these insights offer the potential to optimise composite design and maximise performance. Although micromechanical characterisation has become increasingly well established in metals and ceramics, to date limited techniques have been used to study CFRPs.This thesis aims to apply micoscale mechanical characterisation techniques to CFRPs, with a primary focus on novel synchrotron techniques including Synchrotron X-ray Diffraction (SXRD) and Synchrotron Computed Tomography (SCT). These techniques have been used to characterise the composite lattice strain at an unprecedented micrometre length scale. To demonstrate the utility of these techniques, a variety of engineering problems such as interlaminar shear stress, fibre kinking evolution, and the effects of micro-defects such as voids and fibre misalignment have been investigated. Multi-scale modelling has also been developed to provide numerical validation and to verify these novel experiment techniques. The overview and limitations of these techniques on composite materials are highlighted, and alternative methods and improvements are proposed to overcome the limitations.
SXRD has enabled the evaluation of lattice strain in a PolyAcryloNitrile (PAN)-based carbon fibres embedded in a CFRP structure for the first time. This analysis was complemented with in situ loading, revealing the fundamental two-dimensional (2D) averaged bulk material deformation mechanisms in this composite system at unprecedented resolution. One of the main advantages of SXRD is its ability to assess the thermal residual strain that is generated in the fibre during the curing process. This residual strain is recognised to have an impact on the properties of the composite and can result in premature failure, but cannot be easily measured by other well-known measurement techniques. This analysis has been completed with SCT and Digital Volume Correlation (DVC) which have proven to be a powerful tool for the analysis of volumetric material deformation in textured materials. It should be noted that the similar densities of the fibre and matrix pose challenges when reliably using this approach to study CFRPs, and therefore this work adds to the relatively few studies that have been able to successfully implement this approach. Metal doping of CFRPs is often used to increase the phase contrast in this class of materials, and this approach has also been used to provide complementary insights into the analysis performed in this thesis. It should be noted that the combined insights offered by these methods have been crucial in validating the results obtained.
This thesis provides valuable perspectives on these techniques that can be extended to analyse CFRP structures that better reflect industrial applications. This additional information at different length scales will support the advancement and verification of multi-scale modelling which is essential in enhancing the efficiency of these systems and accelerating the design process.
| Date of Award | 13 Nov 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Alexander Lunt (Supervisor), Richard Butler (Supervisor), Jean Benezech (Supervisor) & Chris Bowen (Supervisor) |