Manufacturing and mechanical characterisation of 3D biaxial weft knitted preforms for composites

Abstract

There is a growing interest in the use of near-net-shape manufacturing for composite preforms, where the dry fibre preform is produced in a single manufacturing step without the additional cutting and laying up processes. This research reviews existing manufacturing methods and identifies that weft knitting technology offers the highest potential to produce high performance near-net-shape preforms for composite applications. Weft knitting addresses the limitations of weaving and braiding machines, such as fixed fabric width and long machine set-up times. The method of weft and warp inlay insertion in weft knitted structures to produce biaxial weft knitted (BWK) structures also eliminates the crimp present in woven and braided fabrics. The thesis presents a modified mechanised flat-bed knitting machine which is capable of inserting inlay tows in the 0° and 90° machine directions to manufacture BWK fabrics. It addresses the limitations in conventional non-crimp manufacturing technologies which are unable to offer novel in-plane and out-of-plane shaping techniques. Using the proposed developments and modifications to the mechanised flat-bed knitting machine, an extensive study on novel shaping techniques is presented. It introduces ‘weft inlay positioning’, a concept which is capable of producing localised areas of thickness variation including tapered angles in the preform. This provides cost reduction in the preforming stage due to the elimination of the individual ply lay-up process. In order to optimise the mechanical properties of the near-net-shape preforms and composites, a study on the structural parameters of the BWK fabrics is presented. The knitted binding structure, which is the scaffold that holds the inlay tows, is a key parameter which determines the inlay packing density and inlay tow geometry. Both are factors which influence the mechanical properties. The in-plane shear behaviour of the preforms are characterised through a picture frame test to identify how the knitted binding structure affects the fabrics ability to resist shear deformation. This helps to identify the design parameters needed for manufacturing a preform capable of assuming the curve of complex shaped moulds without wrinkling. In addition, the effect of the knitted binding structure on the tensile properties of both the BWK fabrics and composites is identified. Findings conclude that the binding elements impart variations to the fibre volume fraction and inlay tow waviness of each composite.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Prasad Potluri (Supervisor) & Anura Fernando (Supervisor)