Multi-material 3D printed thermoplastic morphing aircraft skins: manufacturing, testing, analysis, and design space exploration

To lead the aerospace industry into a more sustainable future, greenhouse gas emissions and noise need to be reduced and aircraft manufactured from recyclable materials. Morphing wings are a promising solution to make wings aerodynamically more efficient. They allow aircraft to continuously change their shape to adapt to changing operating conditions - reducing drag and therefore fuel burned. In order to achieve these radical changes in shape in a smooth and continuous manner, morphing wings require new materials and structural design philosophies that allow for flexibility and compliance. In particular, the skins covering these devices must be flexible in the morphing direction to keep the actuation forces low while also being stiff in the out-of-plane direction to resist aerodynamic loading. Research has shown that the conflicting material properties for morphing skins can be achieved by combining a flexible membrane with a core structure which is made from a much stiffer material taking advantage of the effect of the second moment of area, effectively generating a flexible sandwich panel. Thus far the main research effort has been focused on core structures and the underlaying and load carrying morphing mechanism rather than the entire aerodynamic morphing skin.

This project will explore novel methods to develop morphing skins, taking into account the underlaying morphing technology and as well as factors such as various loading cases and aerodynamic performance, allowing the skins mechanical properties to be tailored to the specific application. Conventional core materials used in aerospace, such as the honeycomb based on a 2.5-dimensional geometry, have primarily been developed with the geometry due to the ease of manufacturing. Additive manufacturing will provide a new freedom of design where cores can be tailored to the specific requirement in the 3rd dimension. This allows for example the core member thickness to be varied through the height of the part. Using multi material additive manufacturing materials with a lower stiffness to be utilised in areas where bending is preferred and stiffer materials where rigidity is required. The multi material 3D printing has also shown that the core can be printed directly onto the outer skin membrane, where the membrane can have a variable thickness. This allows the skins to be manufactured in a single process, providing a perfect adhesion between the skin and the core. The materials used in this research are different formulations of Thermoplastic Polyurethanes which are optimised for additive manufacturing. Using thermoplastic elastomers has various advantages that makes them more sustainable than thermosets used in aircraft manufacturing. A component made from thermoplastics can be repaired in service using thermal and chemical welding, it is less likely to get damaged in service and when at the end-of-life it can relatively easily be recycled by melting the polymer.

This morphing skin concept has its significant scientific complexity. The proposed morphing skins are made from thermoplastic elastomers which have a highly non-linear behaviour when strained. Furthermore, the core structures themselves when subjected to a large in-plane deformation show a non-linear behaviour. Current models described in literature only hold true for relatively small displacements, but morphing works best with larger change in shape. In this work we aim to capture the complex interaction between the flexible core and skin membrane to develop analytical and numerical methods to determine their mechanical properties, in the in-plane as well as out-of-plane direction. These models can in return be utilised to design and optimise bespoke skin solutions for a variety of different morphing applications.

The chief impacts are twofold:

1. Supply of doctoral level engineers trained to the very highest standards in advanced composites. They will take up positions in industry as well as academia.
2. Development of next generation advanced composite materials and applications for wealth creation in the UK.

Other important impacts are:

3. Enhanced UK reputation as a world leading centre in advanced composites that attracts inward investment and export opportunity.
4. Attracting elite overseas students, enhancing the UK's global reputation for excellence in Advanced Composite materials and their applications and widening the pool of highly skilled labour for UK industry.
5. Engaging with local schools and media, to disseminate, enthuse and raise the profile of Engineering to school children and to the wider public.