In-Process Resin Transfer and Cure Monitoring for Carbon Fibre composites

翼盖、翼盒和扰流器等主要飞机结构主要由空客和波音公司通过高压灭菌工艺（AP）中的树脂转移和固化（RT&C）制造。在此过程中，碳纤维（CF）铺层在模具上进行准备，并进行真空包装，并配备端口和软管，以便将树脂转移到铺层的不同部分进行浸渍。此后，有时超过200万升容量的高压灭菌器被加压至6-7巴。施加通常为1.6-3 ℃/min的缓慢对流加热速率以避免树脂的放热失控，并且温度在180度下维持数小时直到固化循环完成。尽管对大体积进行加压和以缓慢速率加热显然有助于释放截留的挥发物并减少空隙的形成，但该方法在能量消耗和循环持续时间方面通常被认为是极其成本无效的。工厂占地面积所需的巨额资本投资以及高压灭菌器的制造和安装成本进一步加剧了这一情况，例如：SA位于Prestwick的设施耗资400万美元。AP成本和费率抑制了航空航天供应商的生产率准备和长期能力建设，以实现空客和波音为单通道生产设定的目标，因为两家公司的目标是到2025年分别达到每月60-100架和50-60架飞机。诸如“明日之翼”（Wing of Tomorrow，WoT）等项目旨在优化材料和工艺，以满足不同结构的需求组合;其中特别关注的是用于主要结构（如机翼蒙皮）的更短和更低热固化周期的非高压灭菌（Out of Autoclave，OOA）工艺。OOA工艺被认为可以实现a）50-60%的能耗节省，B）经济的高速率制造，每月60-100架飞机，潜在成本降低50%，c）到2050年实现NetZero目标的工艺优化，d）纵梁和蒙皮的近净形共固化。OOA工艺仅依赖于真空袋和1.01巴的模具压实压力，该压力通常远低于AP的压力。该过程可能涉及在真空袋模具中铺叠干燥的CF织物或纤维布。前者总是包含一些夹带的空气，这些空气不能通过施加的真空负压完全去除，后者经历溶剂浸渍过程，而溶剂可以是固化期间释放气体的来源。在固化过程中，截留的空气和产生的气体都可能严重促进空隙和干点的形成，导致较差的机械性能。过去已经研究了一系列方法，以通过引入中间停留阶段（其可以延迟生产）、通过部署微波的快速OOA固化过程以及通过使用传热流体和具有柔性膜的模具的快速步骤制造（通常具有较短的寿命）来最小化空隙形成。尽管OOA过程有多方面的好处，特别是在提高制造速度方面，但它更容易出现不希望的制造缺陷。这些缺陷主要以以下形式出现：a）预成型件的树脂灌注不足，产生干点，或最终部件中的空隙聚结，以及B）与设计不同的固化热梯度，导致部件过早脱模且刚度不足或过度固化，从而增加循环成本和残余应力。这些生产缺陷对航空航天和能源领域的高价值制造（HVM）安全关键部件构成了真实的风险，因此，开发一种非破坏性的方法来监控RT&C过程以消除与上述方法相关的不确定性至关重要。

The primary aircraft structures such as wing covers, wing boxes and spoilers are predominantly manufactured through Resin Transfer and Curing (RT&C) in Autoclave Process (AP) by Airbus and Boeing. During the process, the Carbon Fibre (CF) lay-up is prepared on a mould and vacuum-bagged with ports and hoses ready for resin to be transferred to different segments of the lay-up for impregnation. Thereafter, autoclaves with sometimes more than 2 million litres capacity are pressurised up to 6-7 bars. A slow convectional heating rate of typically 1.6-3 C/min is applied to avoid exothermic runaway of resin and the temperatures are sustained at 180 degrees for several hours until the completion of the cure cycle. Even though pressurising the large volume, and heating at slow rates evidently aids to release the entrapped volatiles and reduce the formation of voids, the process is often considered extremely cost ineffective in terms of energy consumption and cycle duration. This is further exacerbated by the immense capital investments needed in factories' footprints, and the cost of manufacturing and installation of autoclaves, for example: US$4 million for SA's facilities at Prestwick. The AP costs and rates have inhibited the aerospace suppliers' production rate readiness and long-term capacity building toward the targets set by Airbus and Boeing for single-aisle production as the companies' ambitions are to reach the 60-100 and 50-60 aircraft/month by 2025, respectively. Programmes such as Wing of Tomorrow (WoT) was launched to optimise materials and processes to meet the increased demand combinations for different structures; among which there has been a particular focus on Out of Autoclave (OOA) processes with shorter and lower-heat curing cycles for primary structures such as wing skins. The OOA process is argued to unlock a) 50-60% savings on energy consumption, b) economical high-rate manufacturing, 60-100 aircraft/per month, with potentially 50% reduced costs, c) process optimisation toward NetZero targets by 2050, d) near net-shape co-curing of stringers and skins. The OOA process solely relies on the vacuum bag and the mould tool compaction pressure of 1.01 bar which is often much lower than that of AP. The process could involve a lay-up of either dry CF fabrics or prepregs in the vacuum-bagged moulds. The former always contains some entrapped air that cannot be fully removed through the applied negative pressure of vacuum, the latter undergoes solvent dip prepregging process while the solvent can be a source of evolving gases during curing. During the curing, both the trapped air and generated gases can heavily contribute to the formation of voids and dry spots leading to inferior mechanical properties. An array of methodologies has been investigated in the past to minimise the void formation through introducing intermediate dwelling stages which can delay the production, rapid OOA curing processes by deployment of microwaves, and quick step manufacturing by using heat-transfer fluids and moulds with flexible membranes, often suffering shorter life spans. Despite the multifaceted benefits of the OOA process, particularly in terms of increased manufacturing rates, it is more prone to undesirable manufacturing defects. These defects are mainly occurring in form of a) insufficient resin infusion of the preform creating dry spots, or coalescence of voids in the final component, and b) curing thermal gradients that are different from the design resulting in either premature demoulding of components with inadequate rigidity or over curing with increased cycle costs and residual stresses. These production defects pose a real risk in safety critical High Value Manufactured (HVM) components used in aerospace and energy.Accordingly, it is crucial to develop a non-destructive means to monitor the RT&C processes to eliminate the uncertainties associated with the aforementioned approaches.