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Complex Cores for New Utilities

Periodic Reporting for period 1 - CoCoNut (Complex Cores for New Utilities)

Reporting period: 2019-10-01 to 2020-12-31

The challenge in this project is to design and implement an innovative solution to make a complex composite-based part with a large trapped cavity for a critical aircraft structure, via a method compatible with high production rates and achieving the required geometry and properties for the part.

The technology of manufacture of composite landing gear components parts with complex hollow internal geometries has not yet come to mature production. In the existing manufacturing processes there is significant time wastage and part complexity associated with the use of metal cores, which need to be machined and sectioned, thus increasing the production time and the cost of the part. Improving the manufacturing process to allow for more simplified, cost-effective, and environmentally friendly serial production of these parts would be of great benefit to society.

The current process of fabricating a composite landing gear part with an internal cavity requires the use of multiple metallic internal tools. This requires design of each individual tool, integration together into an RTM tool, and then sequential extraction from the finished composite part. The proposed solution is to replace the use of multiple metallic internal tools with an internal core based on a non-metallic water-soluble material, which can be easily removed from the finished part by washing. The core material is environmentally friendly and recyclable.

The overall objective of the CoCoNut project is to demonstrate the efficacy of a water-soluble internal core material for the fabrication of the proposed composite part with internal cavity, meeting the design requirements, withstanding the required process conditions, and showing an advantage over the existing technology in terms of manufacturing time and cost.
The work performed during the period of reporting covered here was in fulfilment of the following project objectives:

Objective 1. Design of a demonstrator part, with dimensions approximating an actual carbon-fibre-reinforced polymer (CFRP) landing gear part with internal cavity
Objective 2. Design of an innovative water-soluble core (mandrel) with dimensions conforming to the complex geometry of the hollow cavity of the CFRP demonstrator part
Objective 3. Design of an appropriate Resin Transfer Moulding (RTM) production tool
Objective 4. Design of a localized internal heating concept to achieve homogeneous heating and complete curing of the CFRP demonstrator part

The following work was performed during the reporting period in fulfilment of these objectives:

Work Package 2 - Requirements Synthesis and Core Concept Generation
- A market survey was carried out to identify commercially available 'trapped tooling' options for production of hollow aircraft composite parts.
- Selection criteria were defined. General characteristics of the dry fibre, resin material, and process requirements (7 Bars RTM injection pressure, 180°C curing temperature) were agreed upon.
- Among the candidate 'trapped tooling' internal core materials investigated in the market survey, a suitable material conforming to the selection criteria was identified. "Cavuscore" (supplied by CoCoNut project participant Aero Consultants), a castable water-soluble mandrel composed of bauxite or glass beads matrix with polymer binder was selected, in particular due to its resistance to high injection pressures.
- Various options for sealing the water-soluble mandrel, to best prevent resin infiltration during RTM, were investigated.
- Integrated internal heating concepts were identified to help ensure complete curing of thicker sections of the CFRP part (See figure, "Options for localised internal heating of thick areas of part".)

Work Package 3 - Design of demonstrator part, complex cores, and RTM tool
- A CFRP demonstrator part to be manufactured by RTM was designed, based on the geometry of a cross beam landing gear part. It has an internal cavity with complex geometry that does not enable extraction of a conventional metal tool. The part will have a wall thickness variation of 6-20 mm. (See figure, "Demonstrator part design".)
- A hybrid mandrel was designed, consisting of a water-soluble "Cavuscore" mandrel mounted on an internal metal shaft, to create the hollow internal geometry of the demonstrator part. Polymer heat-shrink tube was selected as the material of choice for sealing the water-soluble mandrel. Included in the design are metallic end rings, to be removed after curing to allow water access to the water-soluble mandrel. (See figure, "Design of water-soluble mandrel".)
- An RTM tool was designed to meet the part design and process requirements. A statement of work (SoW) for the manufacture of the tool was written and submitted to candidate tool manufactures. A suitable supplier was selected and an order placed for fabrication of the tool. (See figure, "RTM tool design concept".)
- A Design Review was held, in which the design of the part, tool, and mandrels was presented to the Topic Manager and Project Officer. (See figure, "CoCoNut assembly design: part and mandrels to be used in fabrication".)
Progress beyond state-of-the art:

The technological approach of the CoCoNut project, i.e. using water-soluble mandrels instead of metal internal sections for fabrication of hollow, structural aircraft parts, and using localised internal heating for thick areas of parts, is expected to improve the Technical Readiness Level (TRL) from 3-4 to 5-6, in particular by reducing manufacturing time and cost of serial production by at least 15%.

Expected results until the end of the project:

The next steps in the CoCoNut project will involve fabrication of the internal cores (mandrels) and RTM tool. Dry fabric layup will be performed as the first step in the manufacture of the CFRP parts.
Three (3) parts are to be produced using the RTM, followed by curing, and removal of the mandrel.
- One part, using localised internal heating during the production process
- One part without the application of internal heating
- One final part, to serve as the demonstrator

Quality Assurance (QA) testing, including Non-destructive Testing (NDT) and microscopy, as well as mechanical testing will be performed on the parts.
The overall results of are expected to demonstrate the effect of the internal heating process on the final CFRP part, as well validate the proposed concept using water-soluble mandrels for producing hollow structural aircraft parts within design tolerances. It is expected that by demonstrating the repeatability of the production process, the final deliverables of the CoCoNut project may be use to indicate the suitability of the novel technological approach for serial production.

Potential impacts of the CoCoNut project:
- Lowering the tooling costs for serial production of structural CFRP aircraft components
- Simplification of the production process of these parts
- Decreasing manufacturing time
- Promotion of a recyclable and environmentally friendly production process
- Advancing technological innovation that will improve EU competitiveness
- The project innovation will be disseminated for the scientific community and general public through a few channels:
(a) a website describing the project objective and results; (b) scientific journal publications; (c) presentation of a demonstrator part in exhibitions and conferences.
RTM tool design concept
Options for localised internal heating of thick areas of part
Demonstrator part design
CoCoNut assembly design: part and mandrels to be used in fabrication
Design of water-soluble mandrel