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

为了引领航空航天业走向更加可持续的未来，需要减少温室气体排放和噪音，并使用可回收材料制造飞机。变形机翼是一种很有前途的解决方案，可以提高机翼的空气动力学效率。它们允许飞机不断改变形状以适应不断变化的操作条件，从而减少阻力，从而减少燃料消耗。为了以平滑和连续的方式实现这些彻底的形状变化，变形机翼需要新的材料和结构设计理念，以允许灵活性和遵从性。特别是，覆盖这些设备的外皮必须在变形方向上具有柔性，以保持低驱动力，同时在面外方向上也必须具有刚性，以抵抗气动载荷。研究表明，可以通过将柔性薄膜与利用二次面积矩效应的更硬材料制成的核心结构相结合，有效地产生柔性夹层板，从而实现变形皮肤的材料特性冲突。到目前为止，主要的研究工作都集中在核心结构、底层和承载变形机理上，而不是整个气动变形表皮。该项目将探索开发变形蒙皮的新方法，考虑到底层的变形技术，以及各种负载情况和空气动力学性能等因素，使蒙皮的机械性能能够根据具体应用进行定制。航空航天中使用的传统核心材料，如基于2.5维几何形状的蜂窝，由于易于制造，主要是与几何形状一起开发的。增材制造将提供一种新的设计自由度，其中核心可以根据第三维度的特定要求进行定制。这允许例如核心成员的厚度通过部分的高度变化。使用具有较低刚度的多材料增材制造材料用于首选弯曲的区域和需要刚性的较硬材料。多材料3D打印还表明，核心可以直接打印到外层皮肤膜上，其中膜可以具有可变厚度。这使得外皮可以在一个单一的过程中制造，在外皮和核心之间提供完美的附着力。本研究中使用的材料是针对增材制造优化的热塑性聚氨酯的不同配方。使用热塑性弹性体具有各种优势，使其比飞机制造中使用的热固性弹性体更具可持续性。由热塑性塑料制成的部件可以在使用中使用热焊接和化学焊接进行修复，在使用中不太可能损坏，并且在使用寿命结束时，通过熔化聚合物可以相对容易地回收利用。这种变形皮肤的概念有其显著的科学复杂性。所提出的变形皮肤由热塑性弹性体制成，该弹性体在拉伸时具有高度非线性行为。此外，当受到较大的面内变形时，核心结构本身表现出非线性行为。目前文献中描述的模型只适用于相对较小的位移，但变形在较大的形状变化中效果最好。在这项工作中，我们的目标是捕捉柔性核心和皮肤膜之间复杂的相互作用，以开发分析和数值方法来确定它们在平面内和面外方向的机械性能。这些模型可以反过来用于设计和优化各种不同变形应用的定制皮肤解决方案。

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.