Novel Tow Termination Technology for High-Quality AFP Production of Composite Structures with Blended Ply Drop-offs

The automated fibre placement (AFP) process is a cutting-edge technology to manufacture complex composite aerospace structures, which was first developed in the early 1980s. This process lays down a set of pre-impregnated carbon fibre tapes called 'slit tape' or 'tow' following designed paths on a 3D surface. Despite increasing demands for the fibre placement technologies, the process-induced defects in fibre placement process are critical problems that need to be solved so as to guarantee the quality and structural reliability of composite aircraft components. Those defects mainly result from the limitations of the modern AFP machines.
One of the major limitations is that they always generate resin pocket defects when producing composite components with thickness variation by terminating the tows inside the laminates. The conventional guillotine tow cutting mechanism, which almost all modern AFP machines are adopting, creates stepped tow ends when cutting the tows. Although the tow thickness is quite small (~0.12 mm), empty gaps are produced when the next ply covers the ends due to the high bending stiffness of the carbon fibres. The gaps are filled with resin matrix after a curing process, and act as crack initiation points under loading. This is an example how a manufacturing limitation can increase the design complexity and manufacturing cost. This provokes complex design problems to optimise the tow termination locations considering their detrimental effects on the structural performance, which significantly delays the design process and increases the design cost.
The conventional approaches dealing with these defects have been mainly about how to quickly and accurately predict their detrimental effects on the structural performance and take them into account in the design stage. However, this project aims to provide an inventive solution to eliminate those defects by tackling the fundamental weaknesses of the concept of modern AFP machines, which is a tow scarfing mechanism that can taper the tow ends when the machine terminates the tows during the automated lay-up process. This novel mechanism, which has never existed before, will allow for eliminating the resin pocket defects in automated manufacturing of composite aircraft components, ensuring high quality and reliability of the end products. At the same time, the design process as well as the post-performance evaluation process can be shortened and simplified by removing the effort to take into account the process-induced defects. The objectives of this project are not only for developing a new mechanism but also for providing fundamental understanding of the mechanics in tow cutting process and how the geometry of the terminated tow end affects the internal load transfer and failure mechanism. This project is ultimately for establishing a design guideline supported by experimentally validated data to effectively use the advantages of the new tow termination method.

The novel tow termination technology to be developed in this research will be an innovative and disruptive technology in automated composites manufacturing, allowing for minimisation of the resin pocket defects, which is the most common and critical in modern automated fibre placement processes. This new technology will benefit the following groups in different aspects.
1) Aerospace industry
The aerospace industry will be able to remarkably improve the quality of composite components such as wings, spars, fuselages, aero-engine fan cases and blades, leading to enhancing the structural efficiency and reliability of the composite aircraft structures, saving the maintenance and repair costs, reducing the time and cost for quality inspection process and consequently increasing the overall production speed. Furthermore, this unique capability will help the UK aerospace industry to be clearly distinguished from other countries' in terms of the advance in automated fibre placement technology, which can lead the major aircraft and aerospace component manufacturers to invest in the UK aerospace sector in order to develop their high-quality future composite products. This will also strengthen the UK composites supply chain and industrial collaborations.
2) Automated Fibre Placement (AFP) machine manufacturers
The mechanism to be developed in this project can be applied to various types of modern AFP machines to improve the production quality. The UK aerospace companies such as Rolls-Royce, BAE Systems, Airbus, and GKN who are currently producing major structural components for aircraft (e.g. fan blades, wing skins, spars, and fuselage) could significantly reduce the manufacturing defects and save the inspection cost. In addition, the fundamental understanding of the structural improvement through the defect elimination, which is supported by theoretical and experimental results, will create additional momentum for innovation towards next-generation AFP technologies. A spin-off or start-up engineering company supplying customised modular devices for different types of AFP machines could benefit the UK economy and composite industry.
3) Composite designers and manufacturing engineers and researchers
The AFP process-induced defects have been the major huddle preventing the composite designers from quickly and accurately evaluating the structural performance of the designed composite components. The improved AFP process without generating the resin pocket defects will significantly shorten and simply the design and analysis process, which will help them to achieve more performance-driven designs. The new mechanism will help composite manufacturing engineers to question the conventional paradigm and come up with more challenging and innovative AFP technologies.