Scientific publications

Kosarli, Maria, Georgios Foteinidis, Kyriaki Tsirka, Nerea Markaide, Alaitz Ruiz de Luzuriaga, Diego Calderón Zapatería, Stefan Weidmann, and Alkiviadis S. Paipetis. 2022. “3R Composites: Knockdown Effect Assessment and Repair Efficiency via Mechanical and NDE Testing” Applied Sciences 12, no. 14: 7269. https://doi.org/10.3390/app12147269

In this study, the mechanical properties of purposefully synthesized vitrimer repairable epoxy composites were investigated and compared to conventional, commercial systems. The purpose was to assess the knockdown effect, or the relative property deterioration, from the use of the vitrimer in several testing configurations. Mechanical tests were performed using ILSS, low-velocity impact, and compression after impact configurations. At modeled structure level, the lap strap geometry that can simulate the stiffening of a composite panel was tested. Several non-destructive evaluation techniques were utilized simultaneously with the mechanical testing in order to evaluate (i) the production quality, (ii) the damage during or after mechanical testing, and (iii) the repair efficiency. Results indicated that the new repairable composites had the same mechanical properties as the conventional aerospace-grade RTM6 composites. The electrical resistance change method proved to be a valuable technique for monitoring deformations before the initiation of the debonding and the progress of the damage with consistency and high sensitivity in real time. In terms of repair efficiency, the values ranged from 70% to 100%.

Markaide Arana, N. (2022). Design, production and characterization of high performance 3R composites based on dynamic chemistry for the aerospace industry. Materiales Compuestos, 6(2), 18-25. https://revista.aemac.org/materiales-compuestos/article/view/540

The objective of the AIRPOXY project is to reduce production and maintenance, repair & operating (MRO) costs of composite parts in aeronautics, by introducing a new family of enhanced thermoset composites that preserve all the advantages of conventional thermosets, while show unprecedented new features such as Re-processability, Repairability and Recyclability. This new generation of composites, called 3R composites, are obtained using dynamic hardeners which create reversible crosslinks in the cured epoxy resin. Once the 3R composite has been produced, the dynamic chemical bonds in the cured resin can be reshuffled under determined external stimulus, such as temperature or exposure to a specific chemical agent.

The study describes the development of a first version of 3R composite that meets the requirements of the aeronautical sector. Firstly the resin has been formulated, then the composite laminates have been produced by Resin Transfer Moulding (RTM) and finally their dynamic and mechanical properties have been studied.

Builes Cárdenas, C.; Gayraud, V.; Rodriguez, M.E.; Costa, J.; Salaberria, A.M.; Ruiz de Luzuriaga, A.; Markaide, N.; Dasan Keeryadath, P.; Calderón Zapatería, D. Study into the Mechanical Properties of a New Aeronautic-Grade Epoxy-Based Carbon-Fiber-Reinforced Vitrimer. Polymers 2022, 14, 1223. https://doi.org/10.3390/polym14061223 

The current drive for sustainability demands recyclable matrices for composite materials. Vitrimers combine thermoset properties with reprocessability, but their mechanical performance in highly loaded applications, for instance, composites for aeronautics, is still to be demonstrated. This work presents the complete mechanical characterization of a new vitrimer reinforced with carbon fiber. This vitrimer formulation consists of functional epoxy groups and a new dynamic disulfide crosslinks-based hardener. The testing campaign for the vitrimer composites encompassed tension, compression, interlaminar shear strength (ILSS), in-plane shear (IPS), open-hole tension (OHT) and compression (OHC), filled-hole compression (FHC) and interlaminar fracture toughness tests under mode I and II. Test conditions included room temperature and high temperature of 70 °C and 120 °C, respectively, after moisture saturation. Tension and flexural tests also were applied on the neat vitrimer resin. The results compared well with those obtained for current aeronautic materials manufactured by Resin Transfer Molding (RTM). The lower values observed in compression and ILSS derived from the thermoplastic veils included as a toughening material. This work demonstrates that the vitrimer formulation presented meets the requirements of current matrices for aeronautic-grade carbon-reinforced composites.

Stefan Weidmann, Petra Volk, Peter Mitschang, Nerea Markaide, Investigations on thermoforming of carbon fiber reinforced epoxy vitrimer composites, Composites Part A: Applied Science and Manufacturing, Volume 154, 2022, 106791, ISSN 1359-835X, https://doi.org/10.1016/j.compositesa.2021.106791

In contrast to thermosets, vitrimers have dynamic covalent bonds providing the potential to allow being thermoformed after stimulation by heat. However, it is unclear if the thermoforming of carbon fiber reinforced vitrimers (vitrimer-CFRPC) correlates to that of thermoplastic-CFRPC (TP-CFRPC). For this reason, their viscosity was investigated by means of a torsion clamp rheometer and tempered 3-point bending thermoforming. The results are compared with a well thermoformable TP-CFRPC. The investigated vitrimer-CFRPC base on epoxy formulations with disulfide crosslinks. The results show that the high viscosities of the vitrimers make defect-free thermoforming challenging. In the micrographs of the tempered 3-point forming of the vitrimer-CFRPCs wrinkling and no ply sliding occurred. A vitrimer-CFRPC was thermoformed to an omega profile, indicating that opposite forming radii lead to additional interlaminar shear forces promoting ply sliding. In order to approximate the thermoforming behavior of vitrimer-CFRPC, their viscosity at process temperature must be reduced.

Abderrahmen Aridhi, Jose Jorge Espi, Juan Pedro Berro, Mireia Mesas, Supporting tools to technical advance in the early stages of design, MATEC Web Conf. 304 01022 (2019), https://10.1051/matecconf/201930401022  

The development of methods and tools to support the advancement in scientific and technical solutions has emerged in recent years in order to assist decisions that allow to avoid extra efforts due to the common traditional “trial and error” approach. At the same time, new challenges in terms of environmental protection have also engaged to adopt decisions having into account that climate protection needs to remain as a primer driver in the development of the aviation sector. AIRPOXY will recover all current requirements through an integrated approach where LCA (Life Cycle Assessment), LCC (Life Cycle Cost analysis), HHRA (Human Health Risk Assessment) and numerical simulation of manufacturing processes will work together in order to demonstrate and support the development of thermoformable, repairable and bondable smart epoxy based composites for aero structures. By considering all stated before, the final aim will be double. On one hand, to be informed about technical, environmental, economic and safety requirements during key stages, in order to take informed decisions and optimise it following the Eco-design principles. On the other hand, to obtain objective data to support performance in order to increase the impact of the project and support the further implementation of the technologies as the AIRPOXY solutions reach higher TRLs.