Abstract
The application of advanced fibre-reinforced material systems to aerostructures is not novel and as such, for certain design problems, efficiency limits are faced. Current straight fibre design methodology is well understood and implemented by adherence to industrial guidelines. Future airborne systems will require lightweight structures to, in the short-term meet ever-stricter legislation and in the long term enable vehicles for next-generation operations. The concept of fibre-steered composites, those in which the fibre angle is spatially dependent, has existed in the literature for decades with authors exploring their potential. However, often studies do not consider the physical process used for manufacturing such structures and consider an idealised form of design. This idealisation misses both structural complexity, and more pertinently, possible additional benefit if the manufacturing and structural design process are intertwined as done in this thesis. Herein the potential for elastic tailoring of laminated plate and shell structures is considered by application of the novel Continuous Tow Shearing (CTS) process. Notably, the CTS process is subject to a mechanically nonlinear orientation-thickness coupling. When a fibre orientation change is achieved through material shearing an accompanying thickness increase is result. This thickness change thus leads directly to a mass increase whenever fibre steering is conducted by the CTS process and thus variable-angle, variable-thickness structures are produced.First a parametric variation of the available design variables is conducted as to identify a 2.63× increase in linearised buckling load, on a mass-equivalent basis, of the common problem simply supported square aspect ratio thin plate under uniaxial compression. Next, two thin plate optimisation studies are subsequently presented. Firstly, a demonstration of mass efficiency when achieving target linearised design loads is presented for a plate with a central circular cutout. Whereby the detriment to performance by this feature is favourably ameliorated by fibre steering, in which a 9% mass increase is result. Comparably a 21% mass increase is required in the case of straight fibre composites. Next, an optimisation is undertaken on a formulated representative thin-plate design problem on structures of varying load-cases and aspect ratios, here considering the nonlinear postbuckling regime. In this study mass efficiency is achieved for not all structures by CTS fibre steering. Most promisingly a square plate shows highest mass decrease of 6%. However, the ability to instead tailor the thickness of a ply by the CTS process is a research avenue of significant potential as results indicate one can reduce ply counts. Lower ply counts have real manufacturing benefits and thus deserve further structural consideration. Finally, a multi-objective optimisation is formulated to begin to uncover the trade-off between mass and imperfect buckling load increases. It was found that a fibre-steered cylinder can outperform a baseline straight fibre structure when a scaled eigenmode imperfection is seeded.
Overall, this thesis uncovers benefits of utilising fibre-steered composites manufactured by the CTS process to identify lightweight structural solutions. However from an applications perspective, a significant barrier to technological adoption is in the development of analysis tools required. This thesis proposes efficient computational modelling strategies and makes recommendations for future research efforts. The most pertinent design consideration is in the mass penalty one pays in fibre steering by the CTS process, however benefits to structural lightweighting are uncovered – especially when allowed to operate in nonlinear regimes. Thus future research efforts should embrace structural nonlinearity as a means of additional load carrying capacity and distance new methods from conventional linearised guidelines.
| Date of Award | 1 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Rainer Groh (Supervisor), Alberto Pirrera (Supervisor) & Byung Chul (Eric) Kim (Supervisor) |