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ThermoformAble, repaIrable and bondable smaRt ePOXY based composites for aero structures

Periodic Reporting for period 3 - AIRPOXY (ThermoformAble, repaIrable and bondable smaRt ePOXY based composites for aero structures)

Reporting period: 2021-03-01 to 2022-08-31

The global aerospace composites market could reach USD 55 billion by the end of 2031, at a CAGR of 9% from 2021 to 2031. However, low-cost effectiveness of the manufactured composite parts is a major showstopper for the current European production for the aeronautics sector. Most parts are manufactured using autoclave processes, which are very expensive with low production rate. It is necessary to meet advances in materials, manufacturing methods and automation procedures that are aligned towards production and/or maintenance & repair cost reduction. AIRPOXY seeks to reduce the production and maintenance costs of composite parts in the aeronautic sector by introducing a novel family of thermoset composites that preserve all the advantages of conventional thermosets, but can also be easily processed and repaired, and even recycled. AIRPOXY is based on a new generation of smart epoxy composites called 3R, which are obtained by using dynamic hardeners that create reversible crosslinks in the cured thermoset resin. Once the 3R composite has been produced, the dynamic chemical bonds of the cured resin can be reshuffled under determined external stimulus such as temperature. Hence, the 3R composites are easily re-processable, repairable and recyclable. AIRPOXY had 2 main achievements. First, the development of a high performance 3R resin that meets the specific processing and performance requirements of the aeronautical sector. Second and based on the 3R resin formulated, the development of (1) a new processing technology for thermosets, 3R- Thermoforming technology, 35% cheaper than autoclave, reducing processing times by up to 87%, (2) a new 3R-Repair technology supported by Structural Health Monitoring (SHM) techniques, 55% faster than current patch techniques, and (3) 3R adhesive for 3R composites, with a similar robustness as the traditional adhesive bonds.
The 3R epoxy resin was formulated to be used in the thermoforming process and several enduring prepregs and laminates were produced with it. They were used to investigate 2 thermoforming strategies: CCM and DCM. With the support of thermoforming simulation tools adapted to the characteristics of 3R laminates, the most suitable manufacturing approaches and process parameters were defined. The conventional RTM and SQRTM processes were adapted for the use of the 3R epoxy resin to produce 3R RTM laminates and SQRTM laminates with 3R functionalization to be used for 3R bonding and repair.
2 3R adhesive films for adhesive bonding and welding were produced. A strength of 20 MPa was achieved with 3R laminates bonded with 3R adhesive film. The simulation of bonded structures was developed, and the cohesive zone modelling was selected to describe the behaviour of the adhesive in the model. Regarding 3R welding process, the SQRTM panels with 3R film on the surface were manufactured and welding parameters studied. The need to add an additional adhesive film to the joint area was observed to improve adherence and to level out the irregularities of the contact surfaces. A lap shear strength of around 15 MPa was achieved.
Regarding repair technologies, the NDE technologies were applied on various specimen geometries to detect the defects induced by mechanical testing, and in some cases, they were applied online at specimen level to simulate the in-service scenario. The knockdown effect assessment between the conventional and the 3R composites was evaluated. S HM and mechanical tests of the 3R production processes were performed and the repair efficiency evaluated. 3R composites exhibited a significant repair efficiency with variations that are dependent on the specimen geometry and production process. It was concluded that the most effective repair method is based on the application of pressure and heat in a localized way on the damaged area. Preliminary technical data sheet of the 3R materials were obtained, as well as preliminary guidelines and processing specifications.
2 demonstrators were designed to validate the technologies: A sub-component of a representative fan-cowl (FC) formed by a skin (S), a transverse stiffener (TS) and a longitudinal stiffener (LS), and a sub-component of a representative leading-edge (LE) formed by a nose (N) and two webs (W).
All demonstrators’ elements were manufactured with success: S and TS by RTM, LS and W by DCM, and N with conventional prepreg and 3R film co-cured by SQRTM. Also, a simplified omega profile was manufactured by CCM. Simulation of the thermoforming was performed to provide guidelines to optimize thermoforming set-up. Demonstrators were assembled (FC with 3R adhesive bonding and LE with 3R conduction welding) and tested to assess bonding/welding quality. Different SHM technics were tested to monitor damages in composite structures both for in-production and in-service damages. The repair capability of the 3R resin was demonstrated with success on the TS, where a large area with a delamination was repaired. The sustainability and economic performance of the solutions developed within the project were modelled and assessed through 5 case-by-case scenarios. Regarding human health risk assessment, risk calculations for the 3R technologies was also developed and compared with the conventional ones.
The results obtained were widely disseminated. Thus, to project completion date, 7 peer-reviewed articles and 2 main media articles were published. The results were disseminated at key research conferences and in the final project workshop. AIRPOXY results are attracting a lot of attention both at European & international level. Several companies approached AIRPOXY partners following the final workshop in June 2022 and information release on the web. Agreements with stakeholders are being drafted.
The 3R materials and processes developed show potential to be economically and sustainably feasible. Production costs will be reduced: manufacturing costs of composite parts by thermoforming by 37% compared with autoclave processes, assembly costs of composite parts by 34% compared with adhesive bonding, and repair costs of composite parts by 95% compared with patching process. Production rates will be multiplied compared with autoclave processes: 2.7 times for DCM and 7.5 times for CCM. Environmental benefits are clear. The results indicate that the use of 3R-resin to manufacture carbon fibre composites brings clear advantages to both production process and end-of-life, thanks to the (Re)Processability, Repairability and Recyclability. The substitution of conventional epoxy resin by 3R-resin results in significant environmental sustainability gains. Mainly as a consequence of the recyclability via solvolysis (recovering of CF and 3R-resin), the reduction of fatal production flaws as a consequence of the repairability properties of the 3R-resin, and the lower use of energy and consumables of both 3R-Welding and 3R-repair processes. AIRPOXY allows the re-use of both the resin and the fibre by simple reshaping, remoulding or solvolysis processes, so this new 3R resin will enable the full value recovery of the fibre and resin, significantly reducing the amount of waste in landfills and providing a cost-effective source of high-quality material for re-use. This will allow the reduction in the consumption of fossil raw materials in the manufacture of composites. The recycling processes will generate new companies around the treatment of the waste and recycling itself.
Transverse stiffener for Fan Cowl manufactured by RTM using 3R resin
List of acronyms used in the summary
Leading edge assembled by 3R welding
Skin for Fan Cowl manufactured by RTM using 3R resin
Nose for Leading Edge manufactured by SQRTM with conventional prepreg and 3R film
AIRPOXY concept
Longitudinal stiffener for Fan Cowl manufactured by DCM using 3R laminate
Omega profile manufactured by CCM using 3R enduring prepreg
Web for Leading Edge manufactured by DCM using 3R laminate
Fan Cowl assembled with 3R adhesive film