Fibre-Matrix Interface Longitudinal Debonding and Translaminar Fracture of Fibre-Reinforced Composites: Model Development and Experimental Validation

Fibre-reinforced composites (FRCs) have brought about a significant transformation in the realm of materials science, redefining the possibilities of engineering and design. The exceptional mechanical properties of FRCs, including their high stiffness- and strength-to-weight ratio, have enabled their widespread adoption in industries ranging from aerospace and automotive to renewable energy applications. As we look to the future, FRCs offer a tantalising prospect of a sustainable and eco-friendly alternative to traditional materials. Furthermore, the ongoing development of novel manufacturing techniques, computational tools, characterisation methods, and material chemistries is expected to unlock the full potential of FRCs. In this regard, carbon FRCs (and their hybrids) serve as a beacon of ingenuity and possibility, heralding a significant contender among the vast array of materials to choose from.

The failure of FRCs is a complex phenomenon that involves various intricate stress distribution and damage propagation mechanisms. The multifaceted nature of this process is rooted in the interplay of numerous factors, such as the nature of the reinforcing fibres and matrix, their interface, the microstructural design of the FRC, and the loading conditions. Furthermore, the failure mechanisms can operate at multiple length scales, from the microscale, where the individual fibres and their interactions with the matrix come into play, to the macroscale, where the overall structural behaviour of the FRC manifests itself. Unravelling this complex failure behaviour requires a multifaceted approach integrating advanced experimental techniques, theoretical models, and numerical simulations.

The overarching goal of this thesis is to advance the field of fibre-matrix interface and mesoscale translaminar fracture modelling through approaches that incorporate a comprehensive spectrum of overlooked intricacies as well as the observed phenomena derived from cutting-edge experiments. At first glance, the title of this dissertation may imply that it pertains to two different topics. However, the first component of the title – fibre-matrix interface longitudinal debonding – concerns a major energy-dissipating mechanism during translaminar fracture of FRCs, which governs their damage tolerance and notch sensitivity, and is the focus of the second component.

Accurate prediction of fibre break accumulation and FRC ply failure necessitates a thorough understanding of the stress redistribution in the presence of a broken fibre and the accompanying interfacial failure. Following a comprehensive review of various methods for characterising the fibre-matrix interface, the research begins with finite element simulations at the fibre level. These simulations explore the debonding of a single broken fibre from the elastoplastic matrix and the consequent axial stress redistribution. Upon model validation via laser Raman spectroscopy stress/strain data from the literature, subsequent models are analysed, where a broken fibre is encompassed by intact fibres arranged in either hexagonal or random configurations. The use of multifibre models has enabled the accurate prediction of reallocated stress state and the shape of stress concentration factor (SCF) profiles in response to the applied strain. This approach effectively addresses the modelling challenges and overestimation of SCFs observed in earlier, fully-bonded finite element models. The parametric investigations conducted for each of the three cases indicate that higher values of interfacial friction coefficients, shear strength and fracture toughness, and a greater post-cure cooling gradient reduce the extent of debonding. Such shortened debond lengths correspond to amplified stress concentration factors on adjacent fibres. Moreover, within the representative volume element, lower fibre volume fractions coincide with shorter debond lengths and, consequently, shorter ineffective fibre lengths. These refined stress redistributions, in conjunction with the probabilistic attributes of fibre strength, can be leveraged by longitudinal strength models to yield robust strength predictions.

Using the novel double-edge notch specimen for a single-fibre fragmentation test, the intricate interfacial failure behaviour of single-fibre composites, doped with ceramic (barium titanate) particles, is thoroughly investigated. The integration of digital volume correlation (DVC) into synchrotron computed tomography enables quantitative strain mapping in the proximity of the fibre break(s) and offers perspective onto the real-time development of microscale damage and failure phenomena. According to the global DVC approach results, fibre break(s) causes a peak in the axial strain profile due to the opening of the break, and two peaks in the shear strain profile owing to elevated shear stresses at either side. These datasets are valuable assets for validating numerical models of single-fibre composites. By comparing the predicted and experimentally obtained axial strain/stress recovery lengths and the extent of the shear-dominant zones, a more precise estimation of interfacial properties can be made.

Fibre hybridisation in FRCs enables the customisation of materials to fulfil particular specifications regarding strength, stiffness, weight, ultimate failure strain or other characteristics. The analysed fibre-hybrid composites combine two distinct types of carbon fibres, one with a high strain-to-failure and low modulus, and the other with a high modulus and low-strain-to-failure. To examine the failure mechanisms underlying translaminar fracture in interlayer/intrayarn hybrids and non-hybrid laminates, in-situ experiments utilising synchrotron radiation computed tomography were conducted on novel downsized compact tension specimens with 0° and 90° thin plies. The major results highlight the in-situ discrepancy in the progression of cracks across different plies and delineate fibre (or bundle) pull-outs, as a significant energy dissipation mechanism during translaminar fracture. With the aid of literature data and reasonable assumptions, it was feasible to develop a mesoscale finite element model that incorporates separate cohesive failure definitions for the 0° and 90° plies. The quantitative data from the downsized specimens can then be utilised in constructing and validating mesoscale finite element models for translaminar fracture.

In summary, the development of models for microscale longitudinal fibre-matrix debonding and mesoscale translaminar fracture, as presented in this thesis, represents a notable stride forward in the characterisation, prediction, and enhancement of failure behaviour in FRCs. The utilisation of advanced in-situ imaging technologies, in combination with the development of novel specimens, has opened new perspectives in understanding the failure behaviour of the model and unidirectional carbon FRCs.