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Title: Toughness and strength of recycled composites and their virgin precursors
Author: Pimenta, Soraia
ISNI:       0000 0004 2737 7158
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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With the exponential growth in carbon-fibre use, establishing recycling routes for the composite waste is now imperative. As recycling processes are maturing, it is vital to introduce the recyclates in non-safety-critical structural applications. This work aimed at studying the mechanical response of recycled composites and developing analytical models for predicting their failure, and also that of their virgin precursors. The effect of recycling on the mechanical response of composites was assessed by comparing virgin and recycled materials with identical woven architectures. The performance depended on the reclamation cycle and loading mode; under optimal recycling conditions, above 75% of strength and virtually 100% of stiffness were recovered. An experimental study of three state-of-the-art recycled composites revealed complex microstructures, featuring fibres and bundles (held together by residual matrix) of different sizes; this microstructure was key for the fracture process, as bundles significantly toughened the materials. The unique multiscale architecture and mechanical behaviour, while making these recyclates suitable for structural applications, created a need for the development of specific mechanical models for design. In order to predict the intrinsic properties of composites with such multiscale reinforcement, original models for size effects on the tensile strength and fracture toughness of composite bundles were developed. These models combine statistics, micromechanics and self-similar processes; while their development was originally motivated by recycled composites, they are equally relevant for unidirectional virgin composites. Subsequently, a micromechanical model for the fracture toughness of recycled composites was developed; this accounts for fracture, debonding and pull-out of reinforcing units with different sizes and orientations, and was successfully validated against the wide range of toughnesses experimentally measured in the recyclates. This work shows that recycled composites offer an environmentally-friendly and performance-competitive alternative to current structural materials. The models developed can support the optimisation of recycling processes towards damage tolerant materials, as well as their application for eco-design of non-safety-critical structures.
Supervisor: Pinho, Silvestre Sponsor: Fundacao para a Ciencia e a Tecnologia ; Programa Operacional Potencial Humano ; European Union
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral