Novel 3D tailored discontinuous fibre preforms for a sustainable future in composite manufacturing

纤维增强聚合物复合材料具有出色的强度、刚度和低重量，其在航空航天和汽车等各个领域的应用迅速增加。然而，长纤维增强材料显著地限制了复合材料的成形性，从而影响了复合材料的加工性能。这个基本问题可以通过将增强材料改为短纤维（即几毫米长）来解决，前提是短纤维高度对齐，以尽量减少纤维长度对最终部件机械性能的影响。虽然高度排列的短纤维复合材料及其制造工艺因此重新受到关注，但所生产的预制体是片状或胶带，这些中间材料最终被用作与长纤维增强复合材料制造方法相同的方法：切割，堆叠，成型和固化。这种传统的复合材料制造方法在生产中仍然会产生大量的浪费，即使使用排列短纤维预成型或预浸料。此外，纤维增强复合材料产品在钻孔或加工等后处理过程中容易开裂或失效。缺乏贯穿厚度方向的力学性能也是复合材料产品面临的一个关键挑战。提高纤维增强复合材料的层间剪切强度、面外刚度和断裂韧性的透厚加固概念并不新鲜。然而，这在排列短纤维复合材料的单一制造过程中尚未实现。那么，如果高度排列的短而不连续的纤维复合材料能够在工程应用中真正取代长而连续的纤维复合材料呢？难道复合材料制造业不可能创造一个更可持续的未来吗？这项研究的目的就是要回答这些问题。该研究将在增材制造技术的辅助下创新不连续纤维复合材料的制造工艺。这项新技术将实现一个自动化过程，能够在操作过程中根据最终产品所需的可成形性和结构性能的最佳纤维角度分布，控制不连续的纤维方向和预成型密度（体积分数）。它不仅可以减少一些制造步骤，如预成型、切割、放置和修剪，还可以最大限度地减少生产中的浪费。为了实现这一目标，将基于一种新的三维纤维定向机制开发一种新型短纤维定向头。独特的对准机构及其控制程序将实现面内和面外纤维取向以及纤维体积分数的高精度局部控制。然后将纤维定向头放置在CNC平台上，直接从原始短纤维中全自动生产3D定制预制体。工艺验证将通过CT扫描评估生产的预成型的3D微观结构来执行。然后，研究将量化三维结构短纤维预制体及其复合材料在改善机械和结构性能方面的效益。

Fibre reinforced polymer composites offer outstanding strength, stiffness and low weight, and their use has increased rapidly in various sectors such as aerospace and automotive. However, the long fibre reinforcements significantly limit the formability of composite materials hence the processability. This fundamental problem can be resolved by changing the reinforcements to short fibres (i.e. a few mm long) provided that the short fibres are highly aligned to minimise the fibre length effect on the mechanical properties of the final parts. Although highly aligned short fibre composites and their manufacturing processes therefore have regained attention, the produced preform is a sheet or tape, and these intermediate materials end up being used as the same way as long fibre reinforced composite fabrication methods: cutting, stacking, forming, and curing. This conventional composite fabrication method still produces a large amount of waste in production even when using aligned short fibre preforms or prepregs. Furthermore, fibre reinforced composite products are prone to crack or fail during post-processes such as drilling or machining. Lack of mechanical properties in the through-thickness direction is also a critical challenge in composite products. The through-thickness reinforcement concept to improve interlaminar shear strength, out-of-plane stiffness, and fracture toughness in fibre reinforced composite materials is not new. However, this has not been achieved yet in a single manufacturing process for aligned short fibre composites. Then, what if highly aligned short and discontinuous fibre composites can truly replace long and continuous fibre composites in engineering applications? Wouldn't it be possible to create a more sustainable future in composite manufacturing? The vision of this research is to answer these questions. The proposed research will innovate the fabrication process for discontinuous fibre composites assisted with additive manufacturing technique. The new technology will realise an automated process capable of manipulating the discontinuous fibre orientation and preform density (volume fraction) during operation following the fibre angle distribution optimal for formability and structural properties required in the final products. It will not only reduce a number of manufacturing steps such as preforming, cutting, placing, and trimming but also minimise the waste in production.To achieve the aim, a new short fibre orientation head will be developed based on a novel 3D fibre orientation mechanism. The unique alignment mechanism and its control programme will enable high-precision localised control of in-plane and out-of-plane fibre orientations as well as fibre volume fraction. The fibre orientation head will be then placed onto a CNC platform to fully automate production of 3D tailored preforms directly from the raw short fibres. The process validation will be performed by assessing the 3D microstructure of the produced preforms via CT scanning. Then the research will quantify the benefit of the 3D structured short fibre preforms and their composites for improving mechanical and structural properties.