Optimisation of Interlaminar Stitching for Textile Composites

Abstract

Fibre reinforced polymer composites are frequently replacing metals and alloys in structural applications for the aerospace, automotive and marine industries due to their high specific strength and stiffness and excellent corrosion resistance. However, a key limitation of composite laminates is their low resistance to out-of-plane loading and susceptibility to delamination. Improving the out-of-plane resistance to delamination in composites is achievable through the insertion of a z-direction reinforcement. There are several available techniques to achieve this, of which, through-thickness stitching is considered an effective, low cost option. Despite its popularity, limitations to the current stitch geometries employed for composites often result in significant geometrical defects and degradation of the in-plane properties. The aim of this research was to optimise the stitching geometry in order to effectively improve the impact resistance of textile composites without causing excessive damage to the in-plane tensile properties. This research details the employment and optimisation of a novel stitch type for the composite industry, the ISO-401 double-thread chain-stitch. Although this stitch type is used regularly throughout the textile industry, no research to date considers its suitability to composite reinforcement. Adjustment of the junction positon of the upper and lower stitching ISO-401 threads demonstrated that defects such as resin pocket size and fibre waviness can be reduced through optimisation. Thus, at high density stitching, which is reported as beneficial to improving the out-of-plane properties, the tensile properties are not significantly affected and therefore maintained. Under low velocity impact, stitching was found to significantly reduce the damage area through an arrest and bridging technique. This resulted in improved compressive strength after impact in stitched composites due to the smaller initiated damage size and further bridging effects of the stitches under loading. Considering the importance of the ISO-401 junction location, a final case study also revealed that the preform fabric characteristics can significantly affect the junction position under the same stitching conditions. Therefore, careful consideration of the sewing conditions and parameters must be taken to achieve the desired geometry. The work completed for this thesis demonstrates that ISO-401 shows good compatibility for the through-thickness reinforcement of textile composites and that it can address some of the limitations of other popular stitch types.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Prasad Potluri (Supervisor) & Steve Hayes (Supervisor)