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

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.