Colloquia

Summary

The increased use of fibre-reinforced plastic composite materials has significantly enlarged their ecological impact, underscoring the need for more sustainable waste management practices. Thermoplastic composites offer a promising solution as a result of their ability to be recycled while largely maintaining their properties that make them a favourable material. However, efficient recycling and the production of high-quality products from recycled materials with predictable mechanical behaviour remain a challenge.

This research focusses on the mechanical characterisation of compression-moulded, chopped single-layer and unidirectional carbon-fibre-reinforced poly-ether-ketone-ketone (CF/PEKK) flakes. These materials, similar to sheet moulding compounds, include randomly orientated strands with varying flake sizes. By investigating the influence of flake size and flake size distribution on mechanical properties, this study aims to optimise the recycling process for composite materials in terms of material input characterisation, reduced recycling process steps, such as sorting and sieving irregular sized flakes, and increased predictability of the mechanical performance of the recycled material.

Tensile tests combined with the results of the digital image correlation technique show that tensile strength and variance increase with flake size, while a varied flake size distribution enhances overall tensile strength and reduces spread. This is likely due to an improved load transfer between flakes and a more uniform stress distribution throughout the specimen. The results of the drop-weight impact test indicate that the impact strength correlates with the stiffness of the material and is less influenced by the flake size distribution. However, the overall homogeneity of the flake size distribution and the flake orientation distribution throughout the sample has a significant influence on individual specimen results. It is recommended that future studies incorporate a more controlled and homogeneous flake orientation distribution during manufacturing of the tensile specimen. In addition, increasing the sample size will provide a more comprehensive understanding of mechanical behaviour.