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.