Mechanical performance of carbon fibre-reinforced composites based on preforms stitched with innovative low-melting temperature and matrix soluble thermoplastic yarns

In order to achieve a superior overall mechanical performance of composites based on stitched preforms for demanding aircraft applications, innovative thermoplastic stitching yarns are comparatively evaluated in carbon fibre-reinforced epoxy composites. Low-melting temperature yarns based on polyamide and phenoxy in comparison to a standard polyester yarn allow prestabilisation of the dry preforms by thermobonding and lead to significantly reduced laminate disturbances following liquid composite moulding; thereby minimising the degradation of the resulting composite strength properties. While the softening polyamide yarns allow partial rearrangement of the carbon fibres during the resin injection process, the dissolution and subsequent phase-separation of the phenoxy can induce a further local toughening of the epoxy matrix. The improvements in overall composite performance when using stitching yarns are partly due to the particular yarn material but also depend on variations in linear yarn density. Last but not least, it is demonstrated that stitching seams close to a bolted joint have only little effect on the bolt bearing strength of the stitched composite whereas seams running directly through the hole and oriented in the load direction induce small degradation of the bolt bearing strength.