Tailored fibre placement of commingled carbon-thermoplastic fibres for notch-insensitive composites

Tailored fibre placement (TFP) is an embroidery-based technology that allows the fibre tows to be placed exactly where they are most needed for structural performance and stitched into position on a compatible textile or polymer substrate. In this study commingled carbon-nylon fibre tows were utilised to produce thermoplastic cross-ply net-shaped preforms using TFP. Four TFP composite plaques were manufactured; baseline (blank), machined-hole, tailored-hole-1 and tailored-hole-2. Steering the tows was used to create the hole in tailored-hole-1 and tailored-hole-2. In comparison to the design of tailored-hole-1, a different fibre trajectory, with a circular reinforcement around the hole, was suggested for the tailored-hole-2. Fibre volume fraction, optical microscopy, X-ray-CT scans, tensile and open-hole tests were carried out. With the exception of the baseline sample, the modified design of tailored-hole-2 composite exhibited the highest axial strength and modulus compared to the machined-hole and tailored-hole-1 composites. Only the tailored-hole-2 specimens exhibited less than 10% reduction of the notched strength compared to the un-notched strength. This study highlights the importance of the stress/load-paths and associated fibre-orientations. While TFP can be an extremely valuable design tool for composite preforms and resulting structural components, a deep understanding of stress distributions is inevitable to achieve optimal TFP-design.