Study on the Mechanical Properties of Fibre-steered Composites for Aerospace Design Allowables

: Investigating the mechanical properties of CTS and AFP manufactured laminates

Abstract

Traditionally, composites have been limited to balanced, symmetric, and quasi-isotropic laminates composed of the standard angles 0°, 90°, and ±45°. Whilst convenient and easy to work with, these rules limited the design space to ’black metal’ laminates whereby the laminates take on the same shapes and forms that their metallic counterparts once filled, albeit at a reduced weight. To unlock the full performance advantage, a fundamental shift in how structures are designed is required: rather than using only the traditional fibre angles, structures can be designed with curvilinear paths made via fibre-steering. Fibre-steering unlocks many new possibilities such as aeroelastically tailored wings, panels that redirect loads around holes, or optimised pressure vessels. This shift in design is one that was realised recently via Automated Fibre Placement (AFP) and Continuous Tow Shearing (CTS).
AFP has been used extensively in the aerospace sector for producing fibre-steered structures.
However, inherent to AFP fibre-steered laminates are a number of process-induced defects such as wrinkles, gaps, and overlaps which all form due to the in-plane bending that is caused by AFP. An alternative is CTS, which can produce mostly defect free laminates via its in-plane shearing mechanism. Although, fibre waviness can be introduced due to both the stochastic variability in fibre orientation within the material itself, and unoptimised CTS processing conditions. Thus, when material is sheared, fibre waviness may occur. The effect of shear-induced fibre waviness is not well studied.
This research explored the impact that fibre waviness has on the tensile and compressive
mechanical properties of CTS produced laminates. A novel image analysis technique was developed for measuring fibre orientation. Tension and compression testing on specimens produced at constant shearing angles were performed on steered laminates, and the impact of waviness was quantified. Finally, a novel fibre-steered testing method was created for directly comparing the tensile properties of AFP and CTS produced coupons. From these tests, the advantage of CTS over AFP was shown experimentally for the first time.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Sponsors	Engineering and Physical Sciences Research Council & Embraer (Brazil)
Supervisor	Byung Chul (Eric) Kim (Supervisor), Jonathan E Cooper (Supervisor), Pedro H. Cabral (Supervisor) & Alex P. Prado (Supervisor)