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

自动纤维放置（AFP）工艺是制造复杂复合材料航空航天结构的一项前沿技术，于20世纪80年代初首次发展起来。这个过程会在3D表面上沿着设计的路径铺设一组预先浸渍的碳纤维带，称为“狭缝带”或“拖”。尽管对纤维铺放技术的要求越来越高，但为了保证复合材料飞机部件的质量和结构可靠性，铺放过程中的工艺缺陷是需要解决的关键问题。这些缺陷主要是由于现代AFP机器的局限性造成的。其中一个主要的限制是，当生产厚度变化的复合材料部件时，通过终止层压板内部的束，它们总是产生树脂袋缺陷。传统的断头台拖曳切割机构，几乎所有现代AFP机器都采用，在切割拖曳时产生阶梯拖曳末端。虽然纤维束厚度很小（约0.12毫米），但由于碳纤维的高弯曲刚度，当下一层覆盖两端时，会产生空白间隙。经过固化处理后，缝隙被树脂基体填充，在载荷作用下成为裂纹起裂点。这是一个制造限制如何增加设计复杂性和制造成本的例子。这引发了复杂的设计问题，以优化拖曳终端位置，考虑到它们对结构性能的不利影响，这大大延迟了设计过程并增加了设计成本。处理这些缺陷的传统方法主要是如何快速准确地预测它们对结构性能的不利影响，并在设计阶段将其考虑在内。然而，该项目旨在提供一种创造性的解决方案，通过解决现代AFP机器概念的根本弱点来消除这些缺陷，这是一种拖丝缠绕机制，当机器在自动铺设过程中终止拖丝时，可以逐渐减少拖丝末端。这种以前从未存在过的新机制将消除复合材料飞机部件自动化制造中的树脂袋缺陷，确保最终产品的高质量和可靠性。同时，设计过程以及性能后评估过程可以通过消除考虑过程引起的缺陷的努力而缩短和简化。该项目的目标不仅是为了开发一种新的机制，而且还提供了对拖曳切割过程中力学的基本理解，以及终止拖曳末端的几何形状如何影响内部载荷传递和破坏机制。本课题的最终目的是建立一个由实验验证数据支持的设计准则，以有效地利用新的拖尾终止方法的优势。

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.