Aligned short fibre composites with nonlinear behaviour

Continuous fibre reinforced composites have high properties for structural applications but tend to fail in a brittle manner, unlike metals. However, when the fibre aspect ratio is less than the critical value and a high level of fibre alignment is obtained, discontinuous fibre composites could potentially achieve a ductile or pseudo-ductile tensile response caused by deformation and slippage at the fibre ends. A lot of modelling work on aligned discontinuous fibre composites shows nonlinear behaviour on the stress-strain curve with a limited reduction of modulus and strength. Despite the interesting results from analytical studies, there have been limited experimental results to validate the models as the required fibre length leads to difficulties in producing high-quality specimens with consistent fibre length, good alignment, controlled volume fraction and uniformly distributed fibres. In this paper, specimens with highly aligned discontinuous fibres and with fibre length close to the critical value, which can bring a brittle-ductile transition, are manufactured with the HiPerDiF method (High Performance Discontinuous Fibre method) and tested in tension. In particular, 1 and 3 mm carbon fibres are used to manufacture composite specimens with epoxy and polypropylene matrices. The analytical solutions are compared with experimental results.