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SANDWICH MATERIAL AND STRUCTURE

Periodic Reporting for period 1 - SMS (SANDWICH MATERIAL AND STRUCTURE)

Reporting period: 2016-01-01 to 2018-04-30

The objective of SMS project was to create break-through innovation on essential building blocks, in the field of light and stable sandwich structures, in order to make them lighter and more stable.
To achieve this main objective, very innovative Carbon Fiber Reinforced Polymers (CFRP) materials have been developed to produce drastic improvements on the two main components of sandwich structures : the core (or “honeycomb”) and the skins (or “face sheets”). At the interface between these two parts, the assembling techniques have also been at the focal point of ground-breaking developments, with the same objectives in mind : lighter and more stable. The developments have been led in a supply-chain oriented mind, taking into account, from the very start of the project, drivers such as European sourcing or ease of scale-up.
The project followed a 4 phases organization :
- Phase 1 : specification phase.
- Phase 2 : development phase
- Phase 3 : validation phase (design, manufacture and test of a breadboard)
- Phase 4 (all along the project) : scalability, as the capacity to build very large structures is one of the final objectives of this project.
The project has started with a specification activity, involving all the actors together. The specifications have been issued at two levels : Product and subsystem.

Scalability analysis has been started at the very beginning of the project, with the direct impact on the material and processes selections : the trade-offs were performed with up scaling capacity as a strong criteria.
At the end of the project, and especially after breadboard completion, this up scaling analysis has been enriched by the lessons learned during the assembly phase.
In parallel to the project, some dissemination actions have been performed via various medias (internet, publications, presentations in symposiums, etc...)
The project relies on 3 building blocks : the cyanate ester CFRP, the CFRP honeycomb, and the joining techniques.
The initial state of the art has shown that there were some available low areal weight materials based on cyanate resin, with M55J fibers, but this kind of material can not allow to reach strength and rigidity objectives of the expected CFRP material for our application. On the other hand, at the beginning of the study, there were no existing cyanate resins in a pure European formulation and supply chain. One of the objectives of this study was to develop and industrialize a European cyanate resin for space applications : this objective has been reached thanks to the development of NTPT on its cyanate resin. Furthermore, the developed material has been characterized and the measured properties are completely in line with the expectations.

Concerning the CFRP honeycomb, the state of the art at the beginning of our study has shown that there were no existing European sources of isotropic carbon fiber honeycomb; and that it was not possible to manufacture a large size honeycomb in one single part, without any variation (today large size honeycomb can be manufactured only from separate small size blocks, with identical design but different manufacturing batches, resulting in variations in their properties. The process developed by NTPT allows to manufacture large dimensions CFRP honeycomb, and to obtain an isotropic honeycomb. This process has been applied by NTPT to manufacture two breadboards : a first breadboard has been cut into samples which have been tested test (dimensions 150mm ; thickness 400 mm); the second breadboard (dimensions 600 x 600 x 400mm) was dedicated to the manufacturing of the WP5 breadboard (with other witness samples). The objective of manufacturing an isotropic honeycomb, scalable to large dimensions, has been reached and is a very important progress in this domain.
The honeycomb (like the prepreg) is almost completely European, only one component of the resin is not European.
Its performances are identical in both L and W direction. This was expected thanks to the very particular design and building technique of the honeycomb, but remains a major achievement of the project.
Concerning the scalability, it has been possible to manufacture the honeycomb of the breadboard (600 mm diameter) in one shot. Witness mechanical samples proved that the honeycomb properties of the big block were the same than for smaller ones, despite the longer wrapping duration of the composite. For very large dimensions, it is not sure that a manufacturing in one shot is the best solution : this point has been addressed in the scalability analysis (D2.2) and solutions have been identified.

Concerning the development of organic adhesives, the state of the art has shown that some solutions to bond honeycomb core to skins in ultra-light sandwich structures were existing, in particular using a film of adhesive based on epoxy or cyanate resins. EPFL has selected two potential routes, based on commercially available resins, one epoxy based, one cyanate ester based. In both cases, resin composition, cure cycle and processing method were newly developed to achieve the required specifications.
A strong limitation of the application of the existing inorganic solutions is the joining temperature. Politecnico di Torino has developed at least two interesting solutions, based on existing solutions with addition of loads, or modification of the curing cycle.
Concerning hybrid solutions, sol-gel techniques were proven to be useful for optical bonding. Nevertheless such solutions had never been applied in the case of large dimensions assemblies with a sandwich geometry. Materia Nova has identified and developed some solutions able to answer to the specifications.

Breadboard assembly proved that the selected adhesive, POLITO 10B, has a great potential for large stable structures, and allowed us to identify ways of improvement (adhesive quantity reduction and homogeneity).

Thanks to this development, the initial objectives of SMS have been reached, with a fully European sourcing for stable cfrp and honeycomb, and a solution to build large and stable structures. The competitiveness of the European space industry has been thus improved, with a significant impact on European independence on this critical topics.